[Federal Register Volume 91, Number 4 (Wednesday, January 7, 2026)]
[Proposed Rules]
[Pages 459-490]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2026-00051]
-----------------------------------------------------------------------
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Parts 91, 121, and 129
[Docket No.: FAA-2025-5666; Notice No. 26-02]
RIN 2120-AM21
Requirements for Interference-Tolerant Radio Altimeter Systems
AGENCY: Federal Aviation Administration (FAA), Department of
Transportation (DOT).
ACTION: Notice of proposed rulemaking (NPRM).
-----------------------------------------------------------------------
SUMMARY: In July 2025, President Trump signed the One Big Beautiful
Bill Act. Section 40002 of that law re-institutes the Federal
Communications Commission's general auction authority and specifically
directs the Commission to complete a system of competitive bidding for
not less than 100 megahertz in the 3.98-4.2 gigahertz band (Upper C-
band). To ensure safe, efficient, and reliable aviation operations in
the presence of wireless signals in the Upper C-band, the Federal
Aviation Administration is proposing new regulations that would require
all radio altimeters to meet specific minimum performance requirements.
These new radio altimeters must withstand interference from wireless
signals in neighboring spectrum bands and continue to provide accurate
altitude readings to both pilots and integrated aircraft safety
systems. The minimum interference tolerance requirements proposed in
this rule reflect the best achievable interference rejection using
current technology without compromising radio altimeter system
performance. These regulations would require all aircraft equipped with
radio altimeters operating under part 121 and those aircraft with radio
altimeters operating under part 129 with 30 or more passenger seats or
a payload capacity of more than 7,500 pounds to comply with the minimum
performance requirements by the date the Federal Communications
Commission authorizes wireless services in the Upper C-band. All other
aircraft equipped with radio altimeters would be required to comply
with the same minimum performance requirements two years later. This
proposed rule is a companion to the Federal Communications Commission's
NPRM to expand the ecosystem for next-generation wireless services in
the 3.7-4.2 gigahertz band by making as much as 180, and at least 100,
megahertz of
[[Page 460]]
the Upper C-band available for terrestrial wireless flexible use via a
system of competitive bidding.
DATES: Send comments on or before March 9, 2026.
ADDRESSES: Send comments identified by docket number FAA-2025-5666
using any of the following methods:
Federal eRulemaking Portal: Go to www.regulations.gov and
follow the online instructions for sending your comments
electronically.
Mail: Send comments to Docket Operations, M-30; U.S.
Department of Transportation (DOT), 1200 New Jersey Avenue SE, Room
W12-140, West Building Ground Floor, Washington, DC 20590-0001.
Hand Delivery or Courier: Take comments to Docket
Operations in Room W12-140 of the West Building Ground Floor at 1200
New Jersey Avenue SE, Washington, DC 20590 between 9 a.m. and 5 p.m.,
Monday through Friday, except Federal holidays.
Fax: Fax comments to Docket Operations at (202) 493-2251.
Privacy: In accordance with 5 U.S.C. 553(c), DOT solicits comments
from the public to better inform its rulemaking process. DOT posts
these comments, without edit, including any personal information the
commenter provides, to www.regulations.gov, as described in the system
of records notice (DOT/ALL-14 FDMS), which can be reviewed at
www.dot.gov/privacy.
Docket: Background documents or comments received may be read at
www.regulations.gov at any time. Follow the online instructions for
accessing the docket or go to the Docket Operations in Room W12-140 of
the West Building Ground Floor at 1200 New Jersey Avenue SE,
Washington, DC 20590 between 9 a.m. and 5 p.m., Monday through Friday,
except Federal holidays.
FOR FURTHER INFORMATION CONTACT: Mark Fox, Flight Technologies and
Procedures Division, AFS-400, Federal Aviation Administration, 6500 S
MacArthur Blvd., Building 26, Suite 217, Oklahoma City, OK 73169;
telephone (847) 294-7546; email [email protected].
SUPPLEMENTARY INFORMATION:
List of Abbreviations and Acronyms Frequently Used in This Document
AC--Advisory Circular
AD--Airworthiness Directive
ADS-B--Automatic Dependent Surveillance--Broadcast
AGL--Above Ground Level
AIP--Aeronautical Information Publication
AMOC--Alternative Method of Compliance
BLS--Bureau of Labor Statistics
CAA--Civil Aviation Authority
CAT--Category (CAT II, CAT III)
C-band--3.7-4.2 GHz frequency band
CBI--Confidential Business Information
CFIT--Controlled Flight Into Terrain
CFR--Code of Federal Regulations
CMA--C-band Mitigation Airport
CPI-U-- Consumer Price Index for All Urban Consumers
dB--Decibel
dBm--Decibel-milliwatts
dBm/MHz--Decibel-milliwatts per megahertz
dBW/m2/MHz--Decibel-watts per square meter per megahertz
DOT--Department of Transportation
EA--Environmental Assessment
EFVS--Enhanced Flight Vision Systems
EGPWS--Enhanced Ground Proximity Warning System
EIRP--Effective Isotropic Radiated Power
EIS--Environmental Impact Statement
E.O.--Executive Order
EUROCAE--European Organisation for Civil Aviation Equipment
FAA--Federal Aviation Administration
FCC--Federal Communications Commission
FOIA--Freedom of Information Act
FR--Federal Register
GA--General Aviation
GHz--Gigahertz
GPS--Global Positioning System
GPWS--Ground Proximity Warning System
HAA--Helicopter Air Ambulance
HTAWS--Helicopter Terrain Awareness and Warning System
IBA--International Bureau of Aviation
ICAO--International Civil Aviation Organization
IRFA--Initial Regulatory Flexibility Analysis
ITM--Interference Tolerance Mask
Lower C-band--3.70-3.98 GHz frequency band
MHz--Megahertz
MOPS--Minimum Operating Performance Standards
MPS--Minimum Performance Standards
MSD--Minimum Separation Distance
NAICS--North American Industrial Classification System
NAS--National Airspace System
NM--Nautical Mile
NOI--Notice of Inquiry
NOTAM--Notice to Airmen
NPRM--Notice of Proposed Rulemaking
NVG--Night Vision Goggles
OCS--Obstacle Clearance Surface
OEM--Original Equipment Manufacturer
OMB--Office of Management and Budget
PFD--Power Flux Density
R & O--Report and Order
RA--Radio Altimeter (also known as Radar Altimeter)
RA Band--4.2-4.4 GHz frequency band
RF--Radio Frequency
RFA--Regulatory Flexibility Act
RFI--Radio Frequency Interference
RIA--Regulatory Impact Analysis
RMS--Root Mean Square
SA CAT--Special Authorization Category (SA CAT I, SA CAT II)
SBA--Small Business Administration
SC-239--RTCA Special Committee 239
TAWS--Terrain Awareness and Warning System
TCAS--Traffic Collision Avoidance System
TD--Touchdown
TSO--Technical Standard Order
UAS--Unmanned Aircraft Systems
UMRA--Unfunded Mandates Reform Act of 1995
Upper C-band--3.98-4.2 GHz frequency band
VSL--Value of a Statistical Life
WG-119--EUROCAE Working Group 119
Table of Contents
I. Executive Summary
A. Overview of Proposed Rule
B. Statement of the Problem
C. Summary of the Costs and Benefits
II. Authority for This Rulemaking
III. Background
IV. Discussion of the Proposal
A. Broadband Objective
B. Radio Altimeter Operation and Application
C. Current RA Limitations
D. Next Generation RA Capability
E. Proposed Regulation and Retrofit Requirements
1. Scope--Aircraft Affected
2. Schedule--Availability of Next Generation RA
3. Part 121 Air Carriers and Large Part 129 Aircraft
4. All Other Aircraft
5. Safety Analysis of the Proposed Minimum Performance
Requirements
F. Safety Analysis for Wireless Access Prior to the Initial RA
Performance Deadline
G. Lower C-Band Mitigations
H. Relationship to Airworthiness Directives and Other FAA Policy
I. Alternatives Considered
V. Regulatory Notices and Analyses
A. Regulatory Impact Analysis
1. Statement of the Need for the Proposed Action
i. Description of the Problem
ii. Need for Regulation
iii. Summary of the Proposed Regulation
2. Baseline for the Analysis
3. Benefits
4. Costs
5. Alternatives to Proposed Rule
6. Summary
B. Regulatory Flexibility Act
1. Reasons the Action Is Being Considered
2. Objectives and Legal Basis of the Proposed Rule
3. Description and Estimate of the Number of Small Entities
4. Projected Reporting, Recordkeeping, and Other Compliance
Requirements
5. All Federal Rules That May Duplicate, Overlap, or Conflict
6. Significant Alternatives Considered
C. International Trade Impact Assessment
D. Unfunded Mandates Assessment
E. Paperwork Reduction Act
F. International Compatibility
G. Environmental Analysis
H. Regulations Affecting Intrastate Aviation in Alaska
VI. E.O. Determinations
A. E.O. 13132, Federalism
B. E.O. 13175, Consultation and Coordination With Indian Tribal
Governments
C. E.O. 13211, Regulations That Significantly Affect Energy
Supply, Distribution, or Use
D. E.O. 13609, Promoting International Regulatory Cooperation
[[Page 461]]
E. E.O. 14192, Unleashing Prosperity Through Deregulation
VII. Additional Information
A. Comments Invited
B. Confidential Business Information
C. Electronic Access and Filing
D. Small Business Regulatory Enforcement Fairness Act
VIII. The Proposed Amendment
I. Executive Summary
A. Overview of Proposed Rule
Federal Aviation Administration (FAA) is proposing new regulations
that would require all radio (also known as radar) altimeter (RA)
systems \1\ on aircraft operating under title 14 of the Code of Federal
Regulations (14 CFR) part 91 in the airspace of the 48 contiguous
United States and the District of Columbia to meet minimum performance
requirements necessary to withstand interference from wireless services
in at least 100 megahertz (MHz) of the 3.98-4.2 gigahertz (GHz)
frequency band (Upper C-band), which is immediately adjacent to the RA
frequency band. FAA is proposing two compliance dates. RA systems on
aircraft operating under 14 CFR part 121, and on aircraft operating
under 14 CFR part 129 with 30 or more passenger seats or a payload
capacity of more than 7,500 pounds, would be required to meet the new
minimum performance requirements by the date the Federal Communications
Commission's (FCC) Report and Order (R&O) authorizes wireless service
in the Upper C-band. All RA systems on other aircraft operating under
part 91 in the airspace of the 48 contiguous United States and the
District of Columbia would be required to meet the new minimum
performance requirements two years after the date FCC authorizes
wireless service in the Upper C-band. As discussed in the proposal, FAA
expects the initial RA performance deadline will be achievable between
2029 and 2032, based on a variety of factors. The proposed timeline for
this retrofit is intended to reflect the urgency of expanding next-
generation wireless services in accomplishing the equipment development
and retrofit with acceptable schedule risk. The final RA system
performance deadlines, within the proposed timeframe, will be informed
by the comments to this proposal. These new regulations would require
the installation of new or upgraded RA systems for all aircraft
currently equipped with RA operating under part 121; the majority of
aircraft operating under parts 91 subpart K, 125, 129, 135, and 194;
and a minority of general aviation (GA) aircraft operating under part
91. Aircraft that are not currently equipped with an RA would not need
to replace or upgrade their RA system.
B. Statement of the Problem
RAs measure an aircraft's height above terrain and obstacles using
low-powered signals in the 4.2-4.4 GHz frequency band (RA band).
Wireless signals in the neighboring spectrum bands may interfere with
RA systems and cause inaccurate altitude readings. New RA systems must
be able to withstand interference from higher-powered wireless signals
in neighboring spectrum bands and spurious emissions from those
wireless base stations into the RA band, and continue to provide
accurate altitude readings. Accurate RA data is critical for pilots as
well as integrated automation, navigation, and safety systems,
including autoland, rotorcraft automation modes, and systems that alert
pilots of immediate hazards such as terrain, windshear, and traffic.
This is particularly critical when the pilot cannot see the runway in
low-visibility conditions. Anomalous RA inputs to these systems may
cause the aircraft to maneuver in an unexpected or hazardous manner at
a very low altitude during the final stages of approach and landing or
may prevent collision alerting technology from functioning properly.
The pilot might not be able to detect the error or adjust the flight
path in time to maintain safe flight and landing, which could result in
catastrophic outcomes, including aircraft accidents that may be fatal.
FCC issued a Notice of Inquiry (NOI) \2\ in February 2025 to signal
its intent to auction spectrum for more intensive use in the Upper C-
band, which is immediately adjacent to the RA band. This NOI also
sought comments on whether to adopt service rules similar to those in
the 3.7-3.98 GHz band (Lower C-band). The One Big Beautiful Bill Act of
2025, Public Law 119-21,\3\ signed on July 4, 2025, requires FCC to
auction at least 100 MHz in the Upper C-band by July 4, 2027. Pursuant
to this requirement, FCC has proposed to further expand the ecosystem
for next-generation wireless services in the 3.7-4.2 GHz band (C-band)
by making as much as 180, and at least 100, megahertz of the Upper C-
band available for terrestrial wireless flexible use via a system of
competitive bidding.\4\
FAA expects future wireless services in the Upper C-band aligned
with service rules in the Lower C-band to cause interference to current
RA systems. Existing RA systems are not compatible with this envisioned
use, and airworthiness directives (AD) issued by FAA in 2023 are
insufficient to address the unsafe condition that will result from
wireless services in the Upper C-band. In addition, existing RA systems
are currently operating with reduced capabilities. Several ADs
currently restrict operations to resolve the unsafe conditions caused
by wireless services in the Lower C-band. Voluntary measures were
adopted by the wireless service providers to minimize the national
economic impact of restrictions by coordinating the power level of
wireless services in the Lower C-band and ensuring airport access for
air carriers at major airports.\5\ The voluntary commitments sunset on
January 1, 2028, unless extended or reduced by mutual agreement, and
long-term compatibility between Lower C-band wireless services and RA
systems has not been resolved beyond that date. In addition to the
unsafe conditions that have been addressed through ADs, safety-
enhancing systems such as Traffic Collision Avoidance Systems (TCAS)
and Terrain Awareness and Warning Systems (TAWS) may not operate
reliably in close proximity to the Lower or Upper C-band wireless base
stations.
A single retrofit of RA systems can address long-term compatibility
with wireless in both the Lower and Upper C-band. The aviation industry
has been developing standards for next-generation RA systems for
several years. A joint industry committee, RTCA, Inc Special Committee
239 (SC-239) \6\ and the European Organisation for Civil Aviation
Equipment (EUROCAE) Working Group 119 (WG-119),\7\ is developing an
industry standard to define the maximum safely tolerable radio
frequency interference (RFI) environment for RA systems. This avionics
standard is scheduled for publication in early 2027. The wireless and
aviation industries are also engaged in ongoing discussions about how
to promote effective coexistence between RA systems and new terrestrial
wireless services in the Upper C-band.\8\
FAA is proposing new regulations that would require all aircraft
operating under part 91 in the airspace of the 48 contiguous United
States and the District of Columbia and equipped with RAs to upgrade to
RA systems that meet minimum interference tolerance requirements that
reflect the best achievable interference rejection using current
technology and without compromising the RA system performance. These
new RA systems must provide accurate altitude readings to pilots and
integrated safety systems in the presence of the defined interference
environment. The goal of these proposed regulations is to
[[Page 462]]
minimize the impact on the safety, efficiency, and reliability of
aviation operations as a result of the Presidential \9\ and
Congressional goals of increased wireless and broadband access for the
American people.
C. Summary of the Costs and Benefits
RA systems are integral to aviation safety by providing altitude
information directly to pilots and to safety systems that need accurate
information to function properly. Besides the importance of pilots
having accurate height over terrain information in low visibility
conditions, RA data is vital for the proper functioning of safety
systems such as TCAS, TAWS, and other aircraft-specific functions,
which historically have reduced the risk of airline crashes in the
United States significantly.\10\ Upgrading to new interference-tolerant
RA systems would allow RAs and their dependent safety systems to
continue to play their important role in ensuring safe aircraft
operations in the National Airspace System (NAS).
FAA is proposing two compliance dates for RA retrofits. FAA
considered several factors in proposing a staggered compliance
schedule, including the role the operations play in the economy,
expected level of safety, and the expected availability of RA units.
The initial RA performance deadline would apply to all aircraft
equipped with an RA operating under part 121 and aircraft equipped with
an RA operating under part 129 with 30 or more passenger seats or a
payload capacity of more than 7,500 pounds. FAA would require an
earlier compliance date for part 121 and 129 operations because they
constitute flights by the major domestic and international airlines and
affect the majority of the flying public, have the highest public
expectation of safety, and are the most critical to the national
economy.
Any other aircraft operating in the airspace of the 48 contiguous
United States and the District of Columbia equipped with an RA would
have two additional years from the first compliance date to retrofit
with an RA system that meets the proposed performance requirement. As
necessary, FAA would supersede the current ADs to impose operating
limitations on the use of RAs that do not meet the proposed performance
requirements until such time as the RA system is replaced. The
superseding ADs would address operators who have upgraded to a Lower C-
band interference-tolerant RA, but do not upgrade to an RA system
compliant with the proposed rule prior to the initial compliance date
(see section IV-H).
In order to properly evaluate a regulation, agencies must measure
its costs and benefits against a baseline. Office of Management and
Budget (OMB) Circular A-4 defines the ``no action'' baseline as ``the
best assessment of the way the world would look absent the proposed
action.'' FAA considers the primary baseline for this analysis to be a
no action baseline, in which FAA assumes FCC completes the auction
required by Public Law 119-21 and the voluntary commitments of the
wireless service providers lapse. Under this scenario, FAA would have
to react to the interference to prohibit all operations of certain
aircraft makes and models, as well as prohibit low-visibility
operations in all aircraft, causing significant operational impacts.
Aircraft owners would need to replace their RA systems to achieve
compatibility with the new spectrum environment. The inherent costs of
delays, cancellations, and groundings resulting from re-imposing AD
operational prohibitions under this no action baseline can be negated
by the cost of retrofitting the RA system in compliance with proposed
performance standards.
FAA also considers an alternative pre-C-band utilization baseline,
in which FAA does not account for the inherent costs of delays,
cancellations, and groundings resulting from AD operational
prohibitions that would be necessary due to the proposed Upper C-band
auction or expiration of the voluntary wireless commitments. Relative
to this baseline, FAA estimates the total undiscounted cost to retrofit
with interference-tolerant RA units is $4.49 billion, or $424 million
annualized at a 7 percent discount rate over 20 years,\11\ as shown in
Table 1.
Table 1--Cost of RA Retrofit Relative to Pre-C-Band Utilization Baseline
[Millions of 2025$]
----------------------------------------------------------------------------------------------------------------
Annualized costs \1\
Undiscounted -------------------------------
CFR operational part total cost 3% Discount 7% Discount
rate rate
----------------------------------------------------------------------------------------------------------------
Part 91......................................................... $1,589 $107 $150
Part 121........................................................ 1,363 92 129
Part 129........................................................ 891 60 84
Part 135........................................................ 651 44 61
-----------------------------------------------
Total....................................................... 4,494 302 424
----------------------------------------------------------------------------------------------------------------
Notes: Columns may not sum due to rounding.
\1\ Costs are annualized over a 20-year period, estimated to be the average remaining service life for current
fleet aircraft.
II. Authority for This Rulemaking
FAA's authority to issue rules on aviation safety is found in Title
49 of the United States Code. Subtitle I, Section 106 describes the
authority of the FAA Administrator. Subtitle VII, Aviation Programs,
describes in more detail the scope of FAA's authority. This rulemaking
is promulgated under the authority described in 49 U.S.C. 106(f), which
establishes the authority of the Administrator to promulgate and revise
regulations and rules related to aviation safety. This rulemaking is
also issued under the authority described in Subtitle VII, Part A,
Subpart III, Section 44701: General requirements. Under that section,
FAA is charged with prescribing regulations promoting safe flight of
civil aircraft in air commerce by prescribing regulations for
practices, methods, and procedures the Administrator finds necessary
for safety in air commerce.
This regulation is within the scope of this authority. This
proposed rule will ensure continued safety after completion of FCC's
auction of at least 100 MHz of spectrum in the band immediately
adjacent to the RA spectrum band, which Public Law 119-21 requires to
be completed by July 4, 2027. The requirement for an RA system retrofit
is necessary due to FCC's
[[Page 463]]
anticipated auction and is also needed to support continued safety with
respect to Lower C-band wireless services.
III. Background
Aircraft rely on RA systems to measure height above terrain and
obstacles in all phases of flight. The RA provides this information to
the pilot and to the aircraft's interconnected navigation and safety
systems to support functions such as low-visibility approaches and
landings, terrain awareness and alerting, wind shear detection and
recovery, aircraft collision avoidance, automated rotorcraft systems,
and other flight control systems. The safety and efficiency of flight
depend heavily on RAs providing accurate inputs to these systems. For
example, automatic and manual flight guidance systems on airplanes rely
on RA data to facilitate low-visibility operations such as autoland and
guidance provided for manual landing using a Head Up Display to
touchdown (TD) when conducting Category (CAT) II, CAT III, Special
Authorization (SA) CAT I, SA CAT II or Enhanced Flight Vision Systems
(EFVS) to TD operations. These inputs determine when and where the
pilot or automation system flares for landing (i.e., raising the
aircraft's nose just before touchdown to smooth touchdown), when power
reductions are made for landing, and when other control inputs are
made. On helicopters, automatic and/or manual flight guidance systems
rely on accurate RA height data to facilitate low-visibility operations
such as Category A and Category B takeoff operations.
Accurate RA readings are critical for all of these applications.
Inaccurate altitude information from an RA experiencing signal
interference from higher-powered wireless services in neighboring
frequency bands may give the pilot a false sense of the aircraft's
position in the air and can cause missing or erroneous (anomalous) RA
inputs to navigation and safety systems, potentially resulting in
catastrophic consequences. For example, automated safety systems
reading erroneous altitude information can cause the aircraft to make
unexpected or hazardous maneuvers during the final stages of approach
and landing, or prevent ground collision alerting technology from
functioning properly. Importantly, the pilot might not be able to
detect the error or adjust the flight path in time to maintain safe
flight and landing, which could result in an accident with fatalities
or injuries.
RA systems work by emitting and then detecting low-powered signals
returning from the ground or other obstacles, similar to how radar
works. The 4.2-4.4 GHz frequency band (RA band) is allocated for RA
operational use in the U.S. and worldwide. Before 2020, satellite
operators and other low-powered sources used the neighboring frequency
bands, and their low-power signals in-band and out-of-band did not
interfere with RAs. This changed when the Lower C-band was reallocated
to permit high-powered commercial wireless services.\12\ Though FCC
limits apply differently for terrestrial and satellite-based services,
as a comparison, previous low-powered satellite services were limited
such that their signals were no greater than roughly -99 decibel-
milliwatts (dBm) per MHz (dBm/MHz) at the Earth's surface, where
current Lower C-band wireless base stations can transmit up to 65 dBm/
MHz. This significant increase in signal power can interfere with the
RA's ability to receive the low-power signal reflected off the ground
or other obstacles. As a result, the RA can register incorrect data (or
no data at all) unless the RA system can block or otherwise filter out
this interference from neighboring spectrum bands and their unwanted
emissions into the RA band.
In April 2020, RTCA formed a ``5G Task Force,'' including members
from RTCA, FAA, aircraft and radio altimeter manufacturers, EUROCAE,
industry organizations, and operators, to perform ``a quantitative
evaluation of radar altimeter performance regarding RF interference
from expected 5G emissions in the 3.7-3.98 GHz band, as well as a
detailed assessment of the risk of such interference occurring and
impacting aviation safety'' \13\ that concludes there is a major risk
that C-band signals can cause harmful interference to RA on all types
of aircraft. The report further concludes that the likelihood and
severity of radio frequency interference increases for operations at
lower altitudes. That interference could cause the RA to either become
inoperable or present misleading information, as well as affect
associated systems on civil aircraft.
In late 2021, to address the unsafe conditions caused by
interference from wireless services in the Lower C-band, FAA issued ADs
prohibiting certain transport and commuter category airplane \14\ and
rotorcraft operations \15\ that require RA data. FAA also issued
airplane model-specific ADs \16\ with additional restrictions to
address unique safety issues for those airplanes. The FAA risk
assessment for these ADs included consideration of the RTCA report,
public comments to the RTCA report, and analyses from RA and aircraft
manufacturers in support of the safety risk determination. The analyses
FAA considered were consistent with RTCA's conclusions pertaining to RA
interference from C-Band emissions. Some aircraft could not operate
safely at all unless equipped with RA systems that are sufficiently
resilient to potential spectrum interference. While the ADs addressed
the unsafe conditions, the safety enhancements provided by RA systems
have been compromised where an RA experiences interference. On January
19, 2022, FAA began tracking and analyzing reports of potential
interference affecting RAs and integrated safety systems. As of August
19, 2025, FAA has received 659 reports of potential C-band
interference, and 493 of these reports were associated with RAs or
related systems. FAA has completed analysis of 625 of these reports and
identified 118 events where all other potential sources were eliminated
as likely causes and were potentially caused by C-band interference.
Most of these 118 events consist of RA display errors, including
erroneous altitude data, and/or nuisance alerts from integrated safety
systems dependent on RA data to function properly. The quantity and
details of reports received to date reflect the current spectrum
environment defined by the wireless voluntary commitments and
mitigations imposed by ADs to address the highest-risk operations.
These reports demonstrate that wireless signals disrupt radar
altimeters as predicted.
In January 2022, Verizon and AT&T (the first licensees to begin
next-generation wireless services in the Lower C-band) agreed to limit
wireless base station deployments and coordinate power levels around
certain airports with FAA until July 1, 2023. The 2022 voluntary
agreement provided the aviation industry time to find a solution to
address the immediate, critical issue of increased risk of RA
interference: to quickly develop, produce, and install modified RA
systems that were tolerant to interference caused by Lower C-band
signals. FAA worked collaboratively with RA and airframe manufacturers
throughout 2022 to develop the aviation safety case that would allow a
steady deployment of Lower C-band wireless base stations while avoiding
unsafe conditions and preventing significant disruptions for aviation
operations. Other types of operations and safety enhancements such as
TAWS, which is intended to provide ground warning away from airports,
have been disrupted
[[Page 464]]
by the current wireless deployment in the Lower C-band.
FAA conducted a series of flight tests in 2022, with cooperation
from AT&T and Verizon, to measure real-world Lower C-band signal levels
in an airspace. Each set of flights had unique goals and objectives,
with each flight furthering FAA's understanding of how to measure C-
band signals through an airspace. Lessons learned from each flight were
incrementally incorporated into subsequent flights to improve
measurement fidelity and accuracy. Flight locations were chosen
strategically to extract maximal value based on the objectives and
goals for each flight. Coordination with AT&T and Verizon preceded the
flights to ensure FAA properly understood the wireless base station
deployments relevant to each location. Technical interchanges between
FAA, AT&T, and Verizon engineers helped to ensure the measurement
procedures and equipment were properly suited for making accurate Lower
C-band signal measurements from an aircraft. After each flight,
measurement data and engineering analysis reports were shared with the
associated wireless service provider to maximize transparency. While
the primary objectives of each flight varied, FAA collected evidence
during those flights showing ambient levels of fundamental Lower C-band
signals that exceeded the interference tolerance of RA systems in use
at the time. Both the raw and processed data associated with each of
these flights were shared with AT&T and Verizon. The flight tests
measured the signal present at the aircraft at multiple locations
within the airspace and were not intended to observe real-world effects
of Lower C-band signals on the performance of any specific RA or the
test aircraft's equipped RA. These flights provided empirical evidence
that it was possible for an airborne aircraft to experience Lower C-
band signal levels that exceed the performance tolerance of unmodified
RA equipment.
As of July 1, 2023, Verizon, AT&T, and the other 19 wireless
service license holders \17\ voluntarily committed to coordinate power
levels and limit emissions into the RA band to minimize the disruption
to air carrier operations until January 1, 2028.\18\ FAA replaced its
initial ADs with a second set of ADs to address the unsafe condition in
the operating environment after July 1, 2023. AD 2023-10-02 \19\
requires transport and commuter category airplanes to have a Lower C-
band interference-tolerant RA suitable for the spectrum environment
defined in the voluntary agreement to conduct certain low-visibility
landings, and AD 2023-11-07 \20\ contains similar requirements for
rotorcraft. In addition, all airplanes operating under 14 CFR part 121
must have a Lower C-band interference-tolerant RA (or otherwise have an
FAA-approved alternative method of compliance). FAA also replaced the
existing airplane model-specific ADs with updated ADs,\21\ and issued
others where appropriate,\22\ with additional restrictions to address
issues affecting those specific airplanes. With the implementation of
the 2023 ADs and other limitations relevant to part 129 foreign air
carriers, the RAs on over 7,500 aircraft were modified to meet the
Lower C-band tolerance that was prescribed. Some operators upgraded
their RAs by adding supplemental filters, while other operators
replaced their RA with one more resilient to potential interference in
the Lower C-band.
When publishing these ADs, FAA noted they were an interim action
until a new technical standard order (TSO) for RAs is established to
incorporate new Minimum Operating Performance Standards (MOPS) that
were in development. Currently, in accordance with the provisions in
the ADs, FAA determines whether an RA is interference tolerant based on
compatibility with the power limits in the voluntary agreements with
the Lower C-band license holders, which temporarily reduces emissions
through January 2028. However, these ADs do not address future next-
generation wireless services in the Upper C-band and do not provide a
long-term resolution that would ensure safety in the presence of Lower
C-band wireless services.
A new industry standard for RA systems is being developed jointly
by U.S. and European consensus bodies through RTCA SC-239 \23\ and
EUROCAE WG-119.\24\ In 2020, RTCA/EUROCAE began developing a MOPS for
RA systems that can tolerate interference from signals in neighboring
spectrum bands. This joint industry committee has developed a draft
standard, which is being validated through testing to ensure the
proposed performance is achievable. Once the standard is validated, it
will undergo a final public comment period and is planned for
publication in March 2027. FAA has requested the committee publish the
standard by June 2026, if possible, to align with FAA's anticipated
timeline for publication of a final rule. The wireless and aviation
industries are also engaged in ongoing discussions about how to promote
effective coexistence between RA systems and new terrestrial wireless
services in the Upper C-band.\25\
When the RTCA standard is complete, FAA anticipates recognizing the
industry standard with new TSOs, which will provide a means for
obtaining an FAA design and production approval for compliant equipment
to facilitate aircraft equipage under this proposed rule. FAA will
ensure that the TSOs conform to the interference tolerance mask (ITM)
requirements in the final rule; any difference in the ITM of the
industry standard will be corrected to conform to the FAA final rule by
the implementing TSOs.
IV. Discussion of the Proposal
A. Broadband Objective To Meet Projected Spectrum Demand, Spur Economic
Growth, and Advance American Security Interests
The 3.7-4.2 GHz band (C-band) is an ideal band for many next-
generation advanced wireless services, including 5G, due to its
desirable coverage, capacity, and propagation characteristics. As a
result of previous efforts to expand access to the 3.7-3.98 GHz band,
wireless operators have extensively deployed 5G throughout the
continental United States, bringing enhanced services and increased
connectivity to countless communities, including many in rural, remote,
and underserved areas. Making additional spectrum available in the
3.98-4.2 GHz frequency range will expand on the success of these prior
efforts to help meet projected demand for advanced wireless services,
spur economic growth, and advance American security interests.
FCC issued an NOI in February 2025 to signal its intent to auction
spectrum for next-generation wireless services in the Upper C-band,
which is immediately adjacent to the RA band. While the Upper C-band
presents a unique opportunity for commercial wireless expansion, it is
even closer to the RA band than the current Lower C-band wireless
services and poses a risk of increased interference with RAs and
critical aviation systems dependent on the RA for accurate altitude
data. FCC issued an NPRM to expand the ecosystem for next generation
wireless services in the C-band by making as much as 180, and at least
100, MHz of the Upper C-band available for terrestrial wireless
flexible use via a system of competitive bidding. FAA and FCC conducted
extensive inter-agency coordination prior to the release of these
respective NPRMs, with the goal of aligning aviation and wireless
objectives
[[Page 465]]
in a way that leads to continued safe coexistence. This proposed
expansion of wireless services should occur as early as possible while
providing a high level of confidence that the proposed implementation
dates are achievable to minimize the impact on the safety, efficiency,
and reliability of aviation operations.
B. Radio Altimeter Operation and Application
The U.S. has the safest aviation system in the world, and an RA is
an essential component that contributes to this enviable safety record.
An RA measures aircraft height above terrain and obstacles in all
phases of flight for tens of thousands of commercial aircraft,
helicopters, business jets, GA aircraft, and future operations by
powered-lift. An RA operates in the frequency band 4.2-4.4 GHz (RA
band). The receiver on an RA is typically highly accurate, measuring
height to within a few feet. An RA operates like radar and must detect
faint signals reflected off the ground to measure altitude. The
receiver must be able to isolate a reflected signal as low as
approximately -120 dBm.
Automatic and manual flight guidance systems on airplanes rely on
accurate RA data to facilitate autoland and operation in low-visibility
conditions. An RA is critical equipment for conducting operations when
the cloud base is less than 200 feet above the runway, and it is
embedded within all types of CAT II, CAT III, and EFVS landing systems.
An RA determines when and where the pilot or automation systems
initiate the aircraft flare for landing, when power reductions are made
for landing, and when other control inputs are made. This is critically
important when the pilot cannot see the runway in low-visibility
conditions. Anomalous RA inputs to these systems may cause the aircraft
to maneuver in an unexpected or hazardous manner during the final
stages of approach and landing, and may not be detectable by the pilot
within sufficient time to maintain continued safe flight and landing.
This could result in catastrophic outcomes, including aircraft
accidents that may be fatal. Inaccurate RA data can also reduce pilot
confidence in their instruments, eroding the foundation of all
instrument flight training.
An RA is also integrated into several safety systems, starting with
the TAWS. TAWS is an onboard aircraft system designed to prevent
unintentional impact with the ground, commonly referred to as
controlled flight into terrain (CFIT) accidents. An operable RA is a
required element of TAWS. The accurate altitude provided by the RA is
used to trigger an alarm in the flight deck when the aircraft is too
low or there is an excessive closure rate to the ground. This system is
required to generate alerts between 30 feet and 2,500 feet above ground
level (AGL).\26\ By definition, TAWS must be able to function
everywhere, as there is no way to predict where a CFIT accident could
occur. TAWS or predecessor safety equipment, such as ground proximity
warning system (GPWS), has been required for over 50 years for many
aircraft operations. In 1974,\27\ FAA required all part 121 certificate
holders and part 135 certificate holders operating large turbojet
airplanes to install approved GPWS equipment. FAA extended the GPWS
requirement to part 135 certificate holders operating turbojet-powered
airplanes with 10 or more passenger seats in 1978,\28\ and amended this
requirement in 1992 \29\ to require GPWS equipment on all turbine-
powered airplanes (including turbo-propellor powered) with 10 or more
passenger seats. Advances in terrain mapping technology permitted the
development of enhanced GPWS (EGPWS), which provides greater
situational awareness for flight crews, and FAA adopted the broader
term TAWS to include a variety of systems that would meet improved
standards beginning in March 2000.\30\ The look-ahead feature of TAWS
provides the flight crew with an earlier aural and visual warning of
impending terrain based on Global Positioning System (GPS), forward-
looking capability, and continued operation in the landing
configuration, all of which provide more time for the flight crew to
make smoother and gradual corrective action. When GPS is not available,
such as during scheduled testing or other interference events, the GPWS
alerts are still provided to the pilots.
An RA is also used within TCAS. In 1987, Congress mandated in
Public Law 100-223 \31\ that FAA require aircraft with more than 30
seats to have TCAS. FAA issued new regulations in 1989 \32\ requiring
TCAS by December 1991 for all airplanes with 30 or more seats operating
under 14 CFR parts 121, 125, and 129, and by December 1995 for all part
129 and part 135 aircraft with 10 or more seats. The TCAS mandate was
expanded to include cargo airplanes in 2004,\33\ specifically requiring
TCAS equipment on all airplanes over 33,000 pounds, with both
requirements applicable to operations under parts 121, 125, and 129. In
2003,\34\ new regulations for fractional aircraft ownership programs
and on-demand operations included TCAS requirements for all aircraft
operating under part 91, subpart K. TCAS depends on data provided by a
properly functioning RA when below 2,350 feet AGL. If the aircraft's RA
is not functioning normally, the TCAS system may fail to issue a
collision warning to the pilot and fail to prevent a mid-air collision
and a catastrophic loss of life.
Wind shear alerting systems also require accurate RA data. Wind
shear alerting has been required for part 121 turbine-powered
commercial operations since 1991.\35\ Initial systems were only
reactive, detecting when an aircraft is in a wind shear condition by
the unexpected change in altitude, typically using the RA. Wind shear
systems have advanced with additional sensors improving performance,
and predictive wind shear systems use weather radar to improve wind
shear detection. Even in the most sophisticated systems, the pilot uses
RA callouts to diagnose the severity of the wind shear and take an
appropriate course of action. Erroneous RA altitude during a wind shear
condition could result in a failure to provide appropriate thrust to
exit the wind shear, increasing the risk of an aircraft accident and
catastrophic loss of life.
The aviation community has used RAs to improve pilot situational
awareness in a variety of visual operations, and FAA has required it
for certain helicopter operations due to the safety benefit it
provides. Public Law 112-95 \36\ requires RAs and Helicopter Terrain
Awareness and Warning Systems (HTAWS) for Helicopter Air Ambulance
(HAA) operations, which FAA implemented in 2014 \37\ in 14 CFR 135.160
and 135.605, respectively, and extended to certain powered-lift via
Sec. 194.306.\38\ While many HTAWS primarily rely on terrain maps,
barometric altitude, and position information (horizontal and vertical)
from GPS, some HTAWS do utilize RA data similar to TAWS in airplanes.
RA data is also used for vertical situational awareness in low
visibility conditions (e.g., snow and dust blown up by rotor downwash)
and as an input into several procedures and automated systems. On
helicopters, automatic and/or manual flight guidance systems rely on
accurate RA height data to facilitate low-visibility operations such as
Category A and Category B takeoff operations. Search and Rescue and
Hover autopilot modes also rely on accurate RA data to function
properly. The RA provides a precise measurement of the helicopter's
height above the ground, which is critical for safety and performance
[[Page 466]]
during low altitude and hover operations. Anomalous RA inputs to these
systems may cause the aircraft to be maneuvered in an unexpected or
hazardous manner when operating at a low altitude and may not be
detectable by the pilot in time to maintain continued safe flight and
landing.
Night Vision Goggles (NVG), the common term to describe the use of
Night Vision Imaging Systems and Night Vision Enhancement Devices, are
used in the operation of airplanes, rotorcraft, and powered-lift. When
used properly, NVGs can increase safety, enhance situational awareness,
and reduce the pilot workload and stress typically associated with
night operations. In 2009,\39\ FAA updated Sec. 91.205 by adding
paragraph (h), which established the instruments and equipment required
to be installed, functioning in a normal manner, and approved for use
by FAA to conduct NVG operations. Before 2009, RA was included as
required equipment under each design approval (type certificate or
supplemental type certificate) of an aircraft for NVG operations.
In addition to these common use cases, some aircraft designers have
integrated RA systems into other safety systems. This includes tail-
strike prevention systems, which push the nose down if the RA indicates
a tail-strike is imminent. Some aircraft use RA data to verify the
aircraft is on the ground to permit automatic throttle power reduction
as well as the safe deployment of thrust reversers and ground spoilers
after landing or during an aborted takeoff. RA data that erroneously
show the aircraft is above the ground will increase the required
stopping distance and increase the risk of overrunning the runway.
Similarly, RA data that erroneously show the aircraft is lower than the
actual position can trigger auto throttle and landing flare systems,
which reduces aircraft speed and increases the risk of landing short of
the runway if the pilots do not quickly identify and correct these
automatic control systems.
All of these applications must be preserved in the presence of
Upper C-band wireless services or restored for those that have been
degraded by wireless services in the Lower C-band. Long-term safe
coexistence between efficient aviation operations and next-generation
wireless services requires RA systems resilient to spectrum
interference from signals in neighboring spectrum bands.
C. Current RA Limitations
Historically, out-of-band emissions were not a problem for RA
because there were no high-powered signals in neighboring spectrum
bands. Current industry standards for RA such as RTCA/DO-155, Minimum
Performance Standards Airborne Low-Range Radar Altimeters,\40\ EUROCAE
ED-30, MPS (Minimum Performance Standards) for Airborne Low Range Radio
(Radar) Altimeter Equipment,\41\ and TSO-C87 \42\ which is aligned with
those industry standards, did not address this possibility when they
were published in 1974 and 1980, respectively. Before 2020, satellite
operators and other low-powered sources used the neighboring frequency
bands, and those signals did not interfere with RA systems due to their
low power. This changed when the Lower C-band was reallocated to permit
higher-powered commercial wireless services.
The voluntary commitments by the wireless service providers have
minimized the national economic impact of the AD restrictions and
ensured airport access by designating 188 major airports as C-band
Mitigation Airports (CMAs) at which Lower C-band licensees are limiting
base station power, when necessary, at the request of FAA. These 188
CMAs are the airports that would be most impacted by AD prohibitions on
specific operations due to a number of factors, such as passenger
traffic, cargo volume, very low-visibility approach procedures,
historic weather information, or a combination of these factors. Due to
extensive efforts from 2022 to 2024, the aviation industry successfully
developed, produced, and installed supplemental (in-line) filters or
replaced RA transceivers on thousands of air carrier airplanes with
other available units that were more tolerant to interference from
transmissions in the Lower C-band, and aligned with the interim
voluntary agreements from all 21 FCC license holders. However, this
work by the aviation industry to address the unsafe conditions and
quickly upgrade within the limits of existing RA system capabilities
did not provide sufficient time to develop more robust solutions that
would enable the full range of RA applications or address the potential
for additional spectrum expansion.
FAA permitted operators of approximately 26,500 aircraft to choose
to accept operational restrictions instead of upgrading their systems.
FAA analysis showed that there was not an immediate need to mandate RA
replacement for non-part 121 operators when the highest risk operations
remained prohibited by the ADs and the cumulative risk of other hazards
was found acceptable in the short-term. However, the safety
enhancements for these aircraft have been compromised, such as the
potential for erroneous alerts or no alerts from TCAS and TAWS, due to
the risk of interference causing incorrect RA altitude data. These
cumulative risks must be resolved to support long-term safe
coexistence.
The FAA requested the RA equipment manufacturers share available
data concerning the performance of their equipment to interfering
signals in the Upper C-band. All five existing manufacturers provided
proprietary data for their Lower C-band tolerant equipment (e.g., those
approved for compliance with AD 2023-10-02).\43\ The data indicate that
no existing civil equipment can tolerate wireless services aligned with
FCC's Lower C-band technical rules in the 100 MHz (or more) of the
spectrum to be auctioned above 3.98 GHz. Allocating even 20 MHz of
additional spectrum to rural or non-rural wireless services would be
incompatible with the current Lower C-band tolerant RAs and would
require more than 45% of Lower C-band tolerant RAs to be modified or
replaced. Table 2 summarizes the achievable performance of the existing
Lower C-band tolerant RAs, broken down by specific frequency ranges
within the Upper C-band. The power flux-density indicates the minimum
interference tolerance at 500 feet AGL and below, measured as a root
mean square (RMS) in decibel-watts per square meter per MHz (dBW/m\2\/
MHz).
Table 2--Frequency Ranges Within the Upper C-Band
------------------------------------------------------------------------
Power flux-density, RMS (dBW/
Frequency range (MHz) m\2\/MHz), 0-500 feet HAGL
------------------------------------------------------------------------
3980 < f < 4100........................... -40
4100 <= f < 4200.......................... -67
[[Page 467]]
4200 to 4400.............................. -105
------------------------------------------------------------------------
There are also thousands of RA systems that have not been modified
to be tolerant to Lower C-band wireless services under the current
voluntary agreement and are more susceptible to interference than shown
in Table 2.
D. Next Generation RA Capability
FAA is proposing an ITM that reflects the best achievable
interference rejection using current technology and without
compromising RA system performance. This proposal has been informed by
briefings from existing RA suppliers and by various industry forums
that have discussed performance collectively. The wireless and aviation
industries have also been engaged in ongoing discussions about how to
promote safe coexistence between expanded wireless services in the
Upper C-band and RA systems.\44\
The most substantive industry discussions concerning RA system
performance have taken place in the RTCA and EUROCAE joint committee,
which has been developing an industry consensus standard for next-
generation RA systems since 2019. These next-generation RA systems will
be responsible spectrum users, with an up-to-date design to provide the
best currently achievable performance to tolerate and reject potential
interference. RTCA SC-239 was established in 2019 and tasked with
revising RTCA/DO-155. RTCA SC-239 is working on these MOPS jointly with
EUROCAE WG-119, which will also be releasing an update to ED-30. The
joint committee has completed a draft standard that is undergoing
validation, which involves testing and analysis with prototype new
designs to ensure that the requirements are both achievable and
sufficient to meet the industry's needs. RTCA plans to publish a final
new standard in March 2027. FAA has participated in the RTCA/EUROCAE
industry standard development.
FAA has considered all available information from individual
manufacturers and the various working groups to develop the ITM
proposed in this NPRM. FAA plans to issue a TSO that references the
final industry standard and will ensure the TSO aligns with this
proposed rule, identifying differences from the final industry standard
if necessary. The TSO will enable companies to use equipment qualified
to the ITM and industry standard as a means of compliance with this
regulation. FAA is not proposing changes to the intended function or
performance requirements of RA systems, which may also include
requirements derived by the aircraft design approval holder for each RA
application. The proposed rule effectively defines an interference
environment within which the intended RA system functions and
performance are achieved.
The interference tolerance requirement would apply to the entire RA
system, comprised of the RA antenna(s), cables, and transceiver. When
defining interference tolerance close to the edge of the RA band, the
frequency selectivity of the antenna does not have an appreciable
effect due to other design constraints, such as the group delay and the
lack of available space for a separate radio frequency (RF) filter. The
achievable ITM in the near-band is driven by the transceiver
performance requirements. While it would be possible to require
additional interference rejection due to the RA antenna's ability to
reject signals far from the desired RA band, doing so would have a
significant cost and schedule effect because it would require the
requalification, and potentially replacement, of all RA antennas. The
proposed ITM does not require this additional interference rejection,
as it would not have a benefit in the potential use of the adjacent
band for next-generation wireless services. As a result, operators can
use RA transceivers that meet the ITM without requalification of an
existing RA antenna. The ITM is specified as a PFD regardless of the
angle of arrival to the RA antenna, so the maximum RA antenna gain must
be used when showing compliance. The ITM is specified for a single
polarization because the RA antennas are linearly polarized and the
orientation of the polarization of an interference source and that of
the RA antenna cannot be controlled.
FAA has developed additional guidance to address this and other
aircraft-level qualification issues in the proposed AC 20-199 Advisory
Circular (AC) for Installation of an Airborne Low-Range Radio Altimeter
System.\45\ FAA will solicit comments on the AC and update it based on
those comments and any changes to the final rule.
E. Proposed Regulation and Retrofit Requirements
FAA is proposing new regulations that would require all RAs to meet
specific minimum performance requirements for all aircraft operating
under 14 CFR part 91 that are equipped with RAs. FAA is proposing two
different compliance dates based on the safety risks associated with
the different types of aircraft operations. Aircraft operating under 14
CFR part 121, and aircraft operating under 14 CFR part 129 with 30 or
more seats or a payload capacity of more than 7,500 pounds, would be
required to meet the minimum RA performance requirements by an initial
RA performance date that would be specified in the final rule. FAA
proposes to provide an additional two years for compliance for all
other operations of aircraft operating under part 91 in the airspace of
the 48 contiguous United States and the District of Columbia and
equipped with RAs.
The initial RA performance deadline is proposed to coincide with
FCC's date authorizing the initiation of new wireless services in the
Upper C-band. FAA expects this initial RA performance deadline to be
sometime between 2029 and 2032. As addressed in section E.2, FAA is
soliciting public comments on the proposed compliance dates. In the
final rule, FAA would prescribe specific RA performance deadlines, as
informed by public comments.
To implement the new minimum performance requirements, FAA is
proposing to add Sec. 91.220 to define the minimum RA interference
tolerance necessary to address next-generation wireless in the Upper C-
band aligned with Lower C-band technical rules, subject to resolving
the spurious emissions from wireless base stations described in section
IV.E.5. FAA also proposes new sections in parts 121 and 129 to
implement the initial RA performance deadline. Specifically, Sec.
121.326 would require all aircraft operating under 14 CFR part 121, if
equipped with an RA system, to meet
[[Page 468]]
the RA system minimum performance requirements stated in Sec.
91.220(b) by the initial RA performance deadline. Section 129.16(a)
would require all aircraft with 30 or more seats or a payload capacity
of more than 7,500 pounds operating under 14 CFR part 129, if equipped
with an RA system, to meet the RA system performance requirements in
Sec. 91.220(b) by the initial RA performance deadline. Proposed Sec.
91.220(a) would impose the same RA system performance requirement by
the final RA performance deadline (two years after the initial
compliance deadline) for all other aircraft equipped with RA operating
under 14 CFR part 91, including GA, rotorcraft, other commercial
aircraft, and public aircraft. Proposed Sec. 129.16(b) would also
impose the final RA performance deadline for all other aircraft
equipped with RA operating under part 129.
FAA is proposing in Sec. 91.220(b) to specify the minimum RA
interference tolerance necessary to address wireless services in both
the Lower and Upper C-band as well as a broader range of frequencies
surrounding the RA band. Table 3 shows the proposed minimum RA system
interference tolerance requirement applicable to different frequency
ranges. The RA system would be required to operate at an altitude of 0-
500 feet above ground level in this proposed interference environment.
The interference environment is broken down by specific frequency
ranges above, in, and below the RA band as shown in Table 3. The
interference environment is specified as a PFD at the surface of the
aircraft antenna, measured as RMS in dBW/m\2\/MHz, so the RA system
compliance includes the maximum directional gain of a linearly-
polarized RA antenna. Figure 1 illustrates the interference environment
defined in Table 3.
Table 3--Proposed Minimum Requirement for RA System Interference
Tolerance
------------------------------------------------------------------------
Power flux density, single
Frequency range (MHz) polarization, RMS (dBW/m\2\/
MHz)
------------------------------------------------------------------------
3000 <= f < 4000.......................... 9.5
4000 <= f < 4100.......................... 9.5
4100 <= f < 4150.......................... 9.5
4150 <= f < 4160.......................... 6.5
4160 <= f < 4180.......................... -1
4180 <= f < 4190.......................... -17
4190 <= f < 4200.......................... -34
------------------------------------------------------------------------
4200 <= f <= 4400......................... -82
------------------------------------------------------------------------
4400 < f <= 4410.......................... -33
4410 < f <= 4430.......................... -21
4430 < f <= 4440.......................... -8
4440 < f <= 4450.......................... -1
4450 < f <= 4460.......................... 6.5
4460 < f <= 4500.......................... 9.5
4500 < f <= 4600.......................... 9.5
4600 < f <= 5600.......................... 9.5
------------------------------------------------------------------------
[[Page 469]]
Figure 1: Proposed Minimum RA System Interference Tolerance
BILLING CODE 4910-13-P
[GRAPHIC] [TIFF OMITTED] TP07JA26.001
BILLING CODE 4910-13-C
Table 4 shows the proposed CFR section additions to attain this
compliance schedule.
Table 4--Regulatory Text Changes
------------------------------------------------------------------------
CFR addition Section text
------------------------------------------------------------------------
Sec. 91.220 Radio Altimeter (a) After [two years after the
Systems. initial RA performance deadline],
no person may operate an aircraft
in the airspace of the 48
contiguous United States and the
District of Columbia with a radio
altimeter unless the radio
altimeter system meets the
performance requirements of
paragraph (b).
(b) The radio altimeter system must
operate at an altitude of 0-500
feet above ground level in the
interference environment defined in
Table 1.
Sec. 121.326 Radio Altimeter After [the initial RA performance
Systems. deadline], no person may operate an
aircraft under this part in the
airspace of the 48 contiguous
United States and the District of
Columbia with a radio altimeter
unless the radio altimeter system
meets the performance requirements
of Sec. 91.220(b) of this
chapter.
Sec. 129.16 Radio Altimeter (a) After [the initial RA
Systems. performance deadline], no person
may operate an aircraft with 30 or
more passenger seats or a payload
capacity of more than 7,500 pounds
under this part in the airspace of
the 48 contiguous United States and
the District of Columbia with a
radio altimeter unless the radio
altimeter system meets the
performance requirements of Sec.
91.220(b) of this chapter.
(b) After [two years after the
initial RA performance deadline],
no person may operate an aircraft
under this part in the airspace of
the 48 contiguous United States and
the District of Columbia with a
radio altimeter unless the radio
altimeter system meets the
performance requirements of Sec.
91.220(b) of this chapter.
------------------------------------------------------------------------
FAA considered potential changes to the current ADs that address
interference with RA systems and found that no further regulatory
action regarding those ADs needs to be taken at this time. The ADs
address unsafe conditions with wireless services in the Lower C-band,
and those conditions will continue until aircraft comply with the new
performance requirements. FAA has also granted several exemptions
providing relief from
[[Page 470]]
addressing the 14 CFR 91.205(h)(7) requirement for RA to support NVG
operations, which will continue to be necessary until all aircraft
comply with the new performance requirements. The regulations proposed
in this rule would address these issues and resolve all known
interference threats to RAs after the proposed final deadline.
1. Scope--Aircraft Affected
RA systems are used in a variety of aircraft as described
previously. To maintain the safety advantages provided by reliable,
accurate RA data, FAA proposes to require that all aircraft equipped
with RA must be equipped with an RA system that can operate in the
future interference environment. Many aircraft rely on accurate RA data
to support safety systems that are required by other regulations, and
RA systems must function properly to provide the safety benefits that
justify these equipment requirements.
There are also civil aircraft that have voluntarily been equipped
with an RA for safety and operational reasons. The intended function of
that equipment is to provide accurate altitude data, and FAA proposes
to preserve that capability in the future operating environment. For
these aircraft, there is a cost increase from the existing RA equipment
to interference-tolerant RA equipment. Some avionics companies have
proposed a class of equipment that would stop functioning by design
when exposed to adjacent band interference. Their proposal would ensure
the integrity of the RA output while exposed to the full RFI levels
specified in this proposed rule by ensuring that the RA stops
functioning rather than reporting an erroneous altitude. However, this
would prevent the RA from enhancing safety in those environments and
complicate the aircraft integration. The proposed regulation would
require all GA aircraft with an RA to upgrade their equipment to be
capable of operating in the interference environment specified in this
proposed rule. FAA recognizes that the future voluntary adoption of RA
may be negatively impacted by the increased costs of a compliant RA.
FAA proposes that these regulations apply to public aircraft
operations, including military aircraft that are equipped with RA when
operating in the airspace of the 48 contiguous United States and the
District of Columbia. The RA is important equipment for public aircraft
operations for the same reasons as civil aircraft (as discussed in
sections E.3 and E.4), and its functionality must be assured. Military
aircraft have unique use-cases for their RA systems, but the minimum
safe distance described below is expected to be sufficient for their
operations. Many military aircraft use RA technology that is different
than the civil fleet and is more robust in the presence of
interference.
The proposed rule would not address operations that are not
conducted under 14 CFR part 91 and therefore would not apply to
unmanned aircraft systems (UAS) operating under part 107, the proposed
part 108,\46\ or limited recreational operations under 49 U.S.C.
44809.\47\ RA systems are not currently integrated into these aircraft,
and integrating them is challenging due to size restrictions. Any
future use of RAs by UAS should consider the RF environment of their
operation, and the performance requirements for such equipment should
be handled through the appropriate aircraft or operational
qualification process.
FCC is proposing to preserve the status quo regarding its current
licenses outside of the contiguous United States, which would be
permitted to continue in the entire 3.7-4.2 GHz band.\48\ FCC notes
that reallocating spectrum only within the contiguous U.S. would ensure
the ongoing provision of C-band services necessary to protect life and
property outside the contiguous U.S.--including telehealth, E911, and
education services--for which C-band service may be the only option
available, such as in remote areas of Alaska. Therefore, FAA is
proposing that the RA performance requirement would not apply to
operations in the airspace over the State of Alaska, the State of
Hawaii, Puerto Rico, and other U.S. territories and possessions,
including territorial waters. Aircraft that are only operated in the
airspace where this rule does not apply would not need to equip with RA
systems that meet the proposed performance requirements. FAA
specifically requests comments on the suitability of applying the
proposed rule only in the airspace of the 48 contiguous United States
and the District of Columbia.
The proposed requirements would not extend into the airspace
overlying the waters between 3 and 12 nautical miles (nm) from the
coast of the U.S, and therefore does not propose a revision to Sec.
91.1(b). The proposed requirements would be applicable to aircraft
operating in that offshore airspace if they arrive, depart, or
otherwise operate in the airspace within 3 nm of the coast of the 48
contiguous United States as described in this proposed rule. FAA seeks
comments about the need to require specific RA performance, as
proposed, in additional offshore waters.
2. Schedule--Availability of Next Generation RA
FAA is proposing this rule to provide a permanent resolution for
next-generation wireless services in the Lower and Upper C-band, as
well as a broader range of frequencies surrounding the RA band. The
objective is to maintain aviation safety in the NAS and provide high
confidence that all aircraft equipped with RA operating under 14 CFR
part 91 will be compatible with expanded next-generation wireless
services in the Upper C-band. While FAA anticipates the initial RA
performance deadline will be between 2029 and 2032, FAA does not have
sufficient data to determine a specific date at this time. FAA will be
considering a variety of factors to help balance the urgency as a
result of expanding wireless services in the Upper C-band with the
development of the next generation RA systems with acceptable schedule
risk. FAA also asks for public comments in consideration of the factors
discussed in this section. RA performance deadlines will be prescribed
in the final rule as informed by public comments. We also seek comment
on how the timing of the aviation industry's future implementation
efforts should be aligned with FCC's statutory responsibility to
complete an auction by July, 2027.
The schedule to accomplish the retrofit is driven by several
activities and different stakeholders, so that no single stakeholder
can provide a high-confidence schedule for the retrofit. Factors to
consider in the compliance schedule include:
Requirement determination and product initiation: This proposed
rule would require new transceivers and companies would have to make
the decision to invest in detailed engineering and qualification for a
new product. New products are designed to meet specific requirements,
and without an agreement on the performance requirements for the next-
generation product, any investment is at risk that the product will not
be found acceptable. By issuing this NPRM, FAA is proposing RA
performance requirements that will be necessary for safe coexistence
between aviation operations and next-generation wireless services.
Aircraft-specific integration requirements are defined by each
aircraft's original equipment manufacturer (OEM). Completion of the
RTCA/EUROCAE industry standard may also be a factor in establishing
international industry consensus.
[[Page 471]]
Product development and certification: Companies intending to
provide next-generation RA systems would have to develop new products
to meet the ITM and market requirements. The typical product
development schedule for flight-critical avionics is two to four years.
To facilitate the demonstration of compliance with the proposed rule
and to streamline equipment certification, FAA plans to recognize the
industry standard with a new TSO for next-generation RA transceivers
and a separate TSO for RA antennas. FAA would ensure that the TSOs
conform to the ITM requirements in the final rule, identifying
differences from the final industry standard if necessary. A TSO
provides a means for obtaining FAA design and production approval based
on the applicant's statement of compliance with the TSO. FAA plans to
issue the TSOs immediately after the final RTCA MOPS.
Aircraft integration and compliance: As described previously, the
RA is integrated into a variety of other aircraft systems. An applicant
for an amended type certificate or supplemental type certificate would
be required to demonstrate that any modification to the aircraft met
FAA's airworthiness regulations, either as an amendment to the type
certificate or as a supplemental type certificate. The extent of the
engineering and associated qualification of the integrated system can
vary significantly depending on the aircraft integration, which has a
commensurate impact on the schedule to complete this work. A
significant factor for the integration of RA systems is the potential
re-use of existing RA antennas. When qualifying the RA system, the
design approval holder would be required to consider the antenna and
cable performance. Since all existing aircraft and associated RA
antennas were qualified without any specific requirements to withstand
interference from adjacent bands, there is no certification data on
antenna performance at those adjacent frequencies. Some companies have
tested the performance of in-service antennas to provide an indication
of their performance, but that data is not sufficient to address
product variability or lifecycle effects. Given this and the
considerations addressed in the next generation RA description in
section IV.D., FAA proposes an interference mask that, if met only by
the transceiver adjacent band rejection, would not require the in-
service antennas to be re-evaluated or re-qualified. FAA assumes that
aircraft integration can largely be accomplished in parallel with the
equipment compliance demonstration. Some additional time is required to
allow for testing of the integrated system, including the certified
transceiver (and antenna if applicable).
Equipment availability: RA equipment is manufactured under an FAA-
approved quality control system to ensure that every article conforms
with the approved design. The production rate for the equipment varies
by manufacturer and equipment. Changes in the production rate require
investment by the company, and planning for a surge in production that
is followed by a significant drop in production (when a retrofit is
complete) may increase costs. Replacement RAs must be manufactured for
the entire fleet of aircraft that are replacing their equipment, so the
size of the retrofit is also a factor in the time needed to complete
the fleet retrofit. FAA assumes that the production rate can increase
to equal the installation rate within months of the equipment being
approved and requests public comment on this assumption.
Aircraft alteration: The final step in accomplishing the retrofit
is to install the new equipment in aircraft. Replacing an RA
transceiver can typically be accomplished as part of overnight
maintenance, provided mounting brackets, connectors, and other physical
characteristics are compatible. Replacing an antenna and cables can
take several days to accomplish and would be scheduled to align with
other heavy maintenance activities when the aircraft would otherwise be
out of service (commonly referred to as a C-check). This type of
maintenance typically occurs every two years for transport category
aircraft. By providing a path to avoid the need for a replacement
antenna if the transceiver demonstrates the required performance, FAA
assumes that it will not be necessary to align the installation with
heavy maintenance. The general aviation fleet may require additional
time to complete the retrofit across the entire fleet due to the lack
of centralized coordination of the modification of aircraft. FAA
proposes an additional two years to demonstrate compliance with the
proposed rule to allow for the challenges in coordinating the general
aviation retrofit.
Financing and Incentive Considerations: FAA notes that FCC is
seeking comments on ways in which RA retrofits can be incentivized and
accelerated as part of the overall Upper C-band repurposing and
transition process.\49\ That includes specific proposals and mechanisms
to facilitate RA retrofits from a financial perspective. In order to
inform the deadlines for this proposed rule, FAA is seeking comments on
the schedule impacts to the proposed RA system performance requirements
resulting from such incentives.
In their terms of reference, RTCA SC-239 notes that the new MOPS
``is envisioned to be referenced by the Federal Aviation Administration
(FAA) and other civil aviation authorities (CAAs) as appropriate in
certification guidance material, including TSOs or other national
documents.'' FAA recognizes that adoption by other CAAs, as intended,
is likely to increase worldwide demand for new RA systems that meet
these performance requirements. This increased demand could result in
competition for resources to support the retrofit for civil and
military aircraft. FAA specifically requests comments about the
potential impact on schedule and cost due to early adoption by
operators who do not regularly fly to the U.S.
The aviation community has addressed a number of large-scale
equipment mandates that provide additional experience-based insight
into the schedule. For comparison, Table 5 shows the timeline for other
broad equipage mandates.
Table 5--Equipment Mandate Timelines
----------------------------------------------------------------------------------------------------------------
Equipment mandate Acronym Compliance time Related information
----------------------------------------------------------------------------------------------------------------
Ground Proximity Warning System GPWS............................... 1 year (12/1/1975). This equipment was
(14 CFR 121) (12/18/1974). subsequently
upgraded to TAWS
(add
functionality).
Terrain Awareness and Warning TAWS............................... 5 years (3/29/2005) Airplanes
System (14 CFR 121) (3/29/2000). manufactured two
or more years
after the final
rule's publication
required TAWS be
installed at time
of delivery.
Helicopter TAWS for Helicopter HTAWS (HAA)........................ 3 years (4/24/2017) ...................
Air Ambulance (2/21/2014).
[[Page 472]]
Traffic Alert and Collision TCAS I 30 seats......... 3 years (12/30/ ...................
Avoidance System (1/10/1989). TCAS I 1-30 seats.................. 1991). Extended due to
7 years (2/9/95; 12/ equipment delays.
31/95). Initially 6 years.
Automatic Dependent Surveillance- ADS-B Out.......................... 10 years (1/1/2020) Some aircraft are
Broadcast (5/28/2010). accommodated
without equipage
----------------------------------------------------------------------------------------------------------------
These schedule drivers indicate that the initial RA performance
deadline is achievable within 3 to 6 years of the final rule, or
between 2029 and 2032, depending on a variety of factors as discussed
previously. FAA intends to select compliance dates that reflect the
urgency of expanding next-generation wireless services, recognizing any
real constraints on the rapidity with which the retrofits can occur.
FAA is requesting comments from the aviation stakeholders to inform the
deadlines for inclusion in the final rule. When providing comments,
please consider the following questions:
Transceiver manufacturers: What is the status of your product
development? When do you project a next-generation RA transceiver to be
certified, and how long after certification will it take to ramp up
production? What factors could accelerate your schedule? What factors
could delay your schedule, and what affects those factors?
OEMs: What is the status of incorporating next-generation RA
systems into your aircraft designs? How long after transceiver
certification do you require to complete an amended type certificate,
and why? Are there aircraft-specific integration requirements that may
require a replacement antenna? What factors could accelerate your
schedule? What factors could delay your schedule, and what affects
those factors?
Air carriers and other operators: After a design approval is
completed for the aircraft, how long do you require to modify your
fleet? What factors could accelerate your schedule? What factors could
delay your schedule, and what affects those factors?
FAA analysis of current information indicates that these schedule
risks will be resolved as additional information is finalized before
the final rule is issued. FAA requests comments about the proposed
timeline to meet RA performance requirements, from the perspective of
RA transceiver and antenna suppliers, aircraft manufacturers, and
operators. The most valuable comments to help inform final regulations
are data-driven comments that detail capabilities, costs, benefits,
timeline impacts, and other specific information directly relevant to
the proposed regulations.
3. Part 121 Air Carriers and Large Part 129 Aircraft
FAA proposes that aircraft equipped with RA operating under part
121 and aircraft operating for foreign air carriers with 30 or more
seats or a payload capacity of more than 7,500 pounds under part 129
must retrofit their RAs by the initial RA performance deadline. This
compliance deadline is proposed to align with FCC's date authorizing
wireless services in the Upper C-band. The initial RA performance
deadline would be specified in the final rule and is anticipated to be
between 2029 and 2032. These operations are the most critical to the
national economy and have the highest expected level of safety, making
them a priority. By completing these retrofits, the U.S. would preserve
safe aviation operations while expanding the use of next-generation
wireless services in the adjacent band as addressed in section E.5.
Other actions must be taken to ensure unsafe conditions do not arise
between the sunset of the existing Lower C-band FAA-wireless voluntary
agreement and the initial RA performance deadline; this is addressed in
section G.
ICAO is planning updates to Annex 10 Volume V intended to help
protect RAs from potentially harmful in-band and adjacent band
interference caused by non-aeronautical systems operating in adjacent
frequency bands. FAA seeks comment on the proposed compliance deadline
for part 129 operators, in light of these potential updates to Annex
10.
FAA estimates that there are 8,014 aircraft operating under part
121, though some of those aircraft are temporarily or permanently
inactive. With specific fleets requiring 1 to 3 RA per aircraft, FAA
anticipates part 121 air carriers would need approximately 17,033 new
RAs to comply with this proposed rule. While part 129 foreign air
carriers operate a very large number of aircraft, not all of those
aircraft fly in U.S. airspace on a regular basis. There are
approximately 4,519 large aircraft with 30 or more seats or a payload
capacity of more than 7,500 pounds operating under 14 CFR part 129 that
fly to the U.S.,\50\ which would result in approximately 10,341 new RA
systems needed for part 129 foreign air carriers.
FAA recognizes that it may be more costly and complex to upgrade
RAs in older aircraft models due to reduced support from manufacturers
for out-of-production units and potential compatibility issues with
older integrated systems, impacting the design, development,
certification, and cost of replacement RA systems. Operators of those
airplanes will need to decide whether to upgrade to RA systems that
meet the proposed performance requirements or retire those airplanes
from contiguous U.S. operations. FAA specifically requests comments
about implementation challenges for older RAs and older aircraft and
the associated costs of retrofit or aircraft retirement for older
aircraft.
4. All Other Aircraft
FAA proposes an additional two years after the initial RA
performance deadline for all other aircraft operating under 14 CFR part
91 including GA, rotorcraft, other commercial aircraft, and public
aircraft. Some of these operators currently have AD-mandated
restrictions on their operations dependent on accurate RA data due to
the Lower C-band wireless services, and many of these operators are
accepting the risks associated with localized interference that could
disrupt TAWS, TCAS, and other RA applications. Those restrictions must
continue until a retrofit is accomplished, which would address both the
Lower and Upper C-band compatibility. Section H discusses the
relationship between the proposed rule, current ADs, and other FAA
policy.
FAA recognizes that there are potential challenges with the
proposed deadlines due to the need to complete standards, develop
prototypes, certify new RAs for multiple aircraft fleets, and install
new RAs without significantly disrupting revenue service. With the
final RA performance deadline two years after the initial RA
performance deadline, FAA seeks to reduce stress on supply chains,
manufacturing, and
[[Page 473]]
installation. This additional time accounts for unique market factors
in general aviation, including the seasonality of aircraft maintenance
in Alaska for those Alaska-based operators who also fly into the
contiguous United States. FAA estimates that approximately 31,821 new
or upgraded RA systems will be required to address helicopters,
business aviation, GA, and other aircraft equipped with RAs that are
not subject to the initial RA performance deadline.
FAA also recognizes that it may be more costly and complex to
upgrade RAs in older aircraft models. Older RA models may be more
difficult to replace due to reduced support from manufacturers for out-
of-production units and potential compatibility issues with older
integrated systems, impacting the design, development, certification,
and cost of replacement RA systems. Operators of those airplanes will
need to decide whether to upgrade to RA systems that meet the proposed
performance requirements, remove the RA system altogether, or retire
those airplanes from contiguous U.S. operations. FAA specifically
requests comments about implementation challenges for older RA and
older aircraft and the associated costs of retrofit or aircraft
retirement for older aircraft.
5. Safety Analysis of the Proposed Minimum Performance Requirements
The purpose of this proposed regulation is to achieve the full
functionality of RAs in the presence of next-generation wireless
services in the adjacent C-Band. This section summarizes FAA's
methodology to ensure the safe operation of RAs and the equipment that
relies on accurate RA data. Based on this analysis, RA systems
compliant with the proposed rule can safely operate with more than 100
MHz for next-generation wireless services in the adjacent band (up to
4160 MHz) aligned with Lower C-band technical rules, provided emissions
limits into the RA band are addressed as discussed below. This safety
analysis assumes that there are no siting constraints on the wireless
base stations.
To operate reliably, the RA system must be demonstrated for the
expected operating environment, including interference levels that may
be encountered in flight. The interference environment that will be
encountered after the initial RA performance deadline has not yet been
determined, so FAA is not able to evaluate a specific interference
proposal. In lieu of that, FAA has applied FCC's baseline proposition
that the existing 3.7 GHz Service rules would apply to new services in
the Upper C-band. FAA has found that the proposed ITM is fully
compatible with the power levels of rural next-generation wireless
services (e.g., 65 dBm/MHz Effective Isotropic Radiated Power (EIRP))
up to 4160 MHz. FAA considered minimum separation distance (MSD) and
safety margins, as discussed in this section, to determine the
allowable interference as depicted in the following formula:
PFD (in dBW/m\2\/MHz) = EIRP (per polarization, in dBm/MHz)-30-
10*log10(4*pi)-20*log10(MSD (in meters)) + SAFETY MARGIN
As long as the calculated PFD at a given frequency is less than or
equal to the ITM, the RA system will perform safely. Therefore, the ITM
levels >=+6.5 dBW/m\2\/MHz up to 4160 MHz can tolerate up to 65 dBm/MHz
total EIRP for dual-polarization base stations as shown in Table 6. The
65 dBm/MHz applies to the aggregate power of all antenna elements in
any given sector of a base station, consistent with existing FCC rules
in the Lower C-band.
The rationale for the parameters used in Table 6 are discussed
below. FAA considered MSD and 6 decibel (dB) safety margins to set
these parameters. The RA antenna gain is not shown, as the maximum RA
antenna gain is used when showing compliance to the ITM.
Table 6--Adjacent Band Compatibility Analysis
------------------------------------------------------------------------
Parameter Value
------------------------------------------------------------------------
ITM (4150-4160 MHz)..................... +6.5 dBW/m\2\/MHz.
Minimum Separation Distance (MSD) (loss) 35 ft. (-31.6 dB).
Safety margin........................... 6 dB.
Safe level of wireless emission (EIRP).. 62 dBm/MHz.
Safe level of wireless emission (dual- 65 dBm/MHz.
pol EIRP).
------------------------------------------------------------------------
Due to the wide range of applications for the RA system and the
variety of aircraft equipped with RAs, FAA proposes that the RA must
function reliably at 35 feet MSD from any wireless base station when
the aircraft is 500 feet AGL or lower. MSD is defined as a sphere with
a 35-foot radius, originating at the wireless base station antenna
phase center, for an aircraft at 500 feet AGL and lower. The smallest
transport category airplanes certificated under part 25 have wingspans
greater than 35 feet (and half-wingspans of approximately 35 feet), and
most helicopters required to be equipped with RA have an overall length
of 35 feet or more. The proposed MSD supports the continued safe
function of the RA and integrated safety systems in all normal, off-
nominal, and emergency operations unless the aircraft is so close to a
wireless base station or the structure where it is mounted that the
catastrophic risk of collision is greater than the risk of
interference.
Thirty-five feet of vertical clearance is less than the closest
expected distance during normal and off-nominal operation for aircraft
equipped with RA systems. Aircraft have significantly greater
separation from obstacles during normal operations due to the minimum
safe altitude requirements in Sec. 91.119, obstacle clearance criteria
for instrument procedures and routes, and requirements for obstacle-
free areas surrounding runways, including in the approach and departure
area to protect low altitude operations and ensure approach light
systems are not obscured. FAA heliport criteria \51\ also define
obstacle-free areas based on the largest helicopter supported and
greater than 35 feet for the final approach and takeoff area, with an
additional obstacle buffer in the safety area and under the recommended
approach and departure paths. When there is sufficient visibility,
pilots see and avoid obstacles to ensure safe minimum separation. Below
500 feet AGL, helicopters must be operated without hazard to persons or
property on the surface, and helicopter operations away from airports
or heliports must be performed with sufficient flight visibility to
ensure safe separation from antenna structures, aligned with the MSD
assumptions. In normal instrument approach operations and at a 200-foot
AGL decision height, the airplane must descend almost twice as much as
a full-scale low indication on the glide slope to get within 35 feet
[[Page 474]]
vertically of the obstacle clearance surface.
The MSD also considered off-nominal operations and emergency
operations. One engine inoperative obstacle clearance requirements in
Sec. Sec. 121.189, 135.379, and 135.398 require 35 feet of vertical
clearance. The most demanding alerting function is the ground proximity
warning of TAWS, which must properly analyze and alert pilots of
hazards as low as 30 feet AGL. The 35-foot MSD provides assurance that
GPWS will operate in all but the most severe terrain scenarios.
Predictive windshear alerting systems must also be able to function
properly at a very low altitude due to the potentially catastrophic
risks of microbursts, downdrafts, and similar wind shifts that cause
the aircraft to lose altitude and approach the bottom of the normal
approach obstacle clearance surface (OCS). The RA must function
properly, even when very close to a wireless base station, to ensure
that the RA does not report an erroneous low altitude, which could
cause TCAS to fail to provide resolution advisory guidance if a nearby
aircraft is on a collision course.
RA performance requirements for operations above 500 feet AGL are
not specifically addressed in the proposed regulations. When an
aircraft is above 500 feet AGL, interference that prevents the RA
system from operating normally is less likely, and the consequence is
also reduced as there is more time to recover after interference.
Stricter obstacle clearance rules apply for all operations above 500
feet AGL. Minimum safe altitude requirements in Sec. 91.119 define
clearance from terrain and obstacles, such as the requirement to be at
an altitude of 1,000 feet above the highest obstacle within a
horizontal radius of 2,000 feet of the aircraft when operating over
congested areas; the requirement to be at an altitude of 500 feet above
the surface when operating over other than congested areas; and the
requirement to be no closer than 500 feet to any person, vessel,
vehicle, or structure when operating over water or sparsely populated
areas. Under instrument flight rules, separation from obstacles
increases at higher altitudes due to increases in required obstacle
clearance for routes at higher altitudes and greater separation
distances provided by sloping OCS when the aircraft is further from the
runway and at a higher altitude. Given the larger MSD in operation, the
RA system is expected to operate normally above 500 feet AGL as the
amount of interference received by the RA antenna decreases with the
increasing path loss.
For safety applications, the aviation community applies a minimum 6
dB safety margin above the expected interference environment to account
for unknown issues that could impact the safe operation of the RA. The
equipment is required to operate normally when the actual interference
level is 6 dB above the expected interference level. For spectrum
compatibility, this accounts for uncertainties in the design and
implementation of adjacent-band RF base stations, which do not have to
meet aviation safety standards. The safety margin also addresses the
risk from unforeseen factors. This is consistent with ICAO
recommendations in ICAO Doc 9718, the Handbook on Radio Frequency
Spectrum Requirements for Civil Aviation, which indicates that a safety
margin of 6-10 dB is to be applied for aeronautical safety systems.
FAA also evaluated the safe compatibility with respect to
interference into the RA band. Emissions into the RA band are a result
of base station out-of-band spurious emissions. The RA system must
operate with the interference from all emissions sources into the RA
band, including, but not limited to, the interference from Lower and
Upper C-band wireless service. The total aggregate in-band interference
depends on the number and the relative position of all other
interference sources to the RA system antenna. To simplify that
analysis, FAA considered the out-of-band emissions from a dominant
source. A dominant source would have the same MSD as the adjacent band
case (35 feet), for the reasons described previously above. Wireless
base stations may be housed on the same antenna structure operating at
different frequencies. An upper limit of three base stations is
assumed, with the effective aggregate interference of all other base
stations and mobile units no greater than that of a single base station
at the MSD. This limiting case has an aggregate interference that is 6
dB higher than a single base station. Table 7 summarizes the parameters
that are used to determine in-band compatibility.
PFD (in dBW/m\2\/MHz) = EIRP (per polarization, in dBm/MHz)-30-
10*log10(4*pi)-20*log10(MSD (in meters)) + SAFETY MARGIN + AGGREGATION
Table 7--In-Band Compatibility Analysis
------------------------------------------------------------------------
Parameter Value
------------------------------------------------------------------------
ITM (In-Band tolerance)................. -82 dBW/m\2\/MHz.
Minimum Separation Distance (MSD) (loss) 35 ft. (-31.6 dB).
Safety margin........................... 6 dB.
Emitter Aggregation..................... 6 dB.
Safe level of wireless emission into RA -33dBm/MHz.
band (EIRP per polarization).
Safe level of wireless emission into RA -30 dBm/MHz.
band (EIRP for dual polarization).
------------------------------------------------------------------------
The RA system can operate safely if the aggregate in-band
interference from external sources is less than the in-band
interference limit of -82 dBW/m\2\/MHz. Therefore, the RA system can
operate safely with an EIRP from each base station of -33 dBm/MHz per
polarization (or -30 dBm/MHz for equal dual-polarized signals). When
FAA completed the safety analysis for the Lower C-band, FAA accepted
maximum antenna coupling between the RA antenna and the wireless base
station of 10 to 12 dBi. The coupling is the sum of the RA antenna gain
and the base station gain. Under this proposal, the RA antenna gain is
accounted for within the ITM requirement. With the base station tuned
to a closer frequency to the edge of the 4.2-4.4 GHz band, FAA is
seeking comment on base station antenna gain characteristics between
4.2 and 4.4 GHz so FAA can finalize the safety analysis. As a point of
comparison, the voluntary commitment for the Lower C-Band specifies a
maximum of -48 dBm/MHz conducted emissions in the 4.2-4.4 GHz band,
which would be safe with up to 18 dBi of base station gain.
FAA is also seeking comments on the overall safety analysis
presented in this section. The factors in the safety link analysis have
many variables. Due to the potentially catastrophic severity of
interference, FAA has adopted values
[[Page 475]]
that reflect a very low likelihood of occurrence. The typical
interference will be considerably less. For example, the base station
spot-beam is frequently pointed away from the aircraft when the
aircraft is overhead, and the RA antenna would typically have low gain
towards the base station when the aircraft is adjacent to a wireless
base station. Multipath can increase or decrease the received signal
strength, though not typically within the maximum antenna spot beam.
While the duty cycle of the base station is limited based on the next-
generation wireless technology, FAA seeks to adopt an RA system
requirement regardless of the wireless service technology to be used.
The motion of the aircraft, as compared to a fixed wireless base
station, can also affect the tolerable interference in the integrated
aircraft systems. When considering the in-band interference, the
spurious emissions would typically be decorrelated across multiple
wireless base stations and not add constructively. Finally, the
aggregate interference would typically be the sum of one or two
collocated base stations, a large number of mobile units, and a few
other base stations at different distances. FAA's analysis
intentionally provides a very high assurance that interference will not
occur, thus averting a catastrophic outcome. Comments on these factors
should address the likelihood of the various conditions, so FAA can
ensure that the likelihood of interference that could lead to a
catastrophic outcome is sufficiently low.
F. Safety Analysis for Wireless Access Prior to the Initial RA
Performance Deadline
Existing RA systems cannot accommodate wireless signals above 3.98
GHz aligned with Lower C-band technical rules without constraints on
wireless base station location and power levels. While FAA and wireless
service providers have agreed to similar constraints in the short term
for the Lower C-band, FAA does not plan to expand that analysis to the
Upper C-band. Instead, FAA proposes to require the RA retrofit to be
completed in the most critical aircraft by the time FCC authorizes new
wireless services in the Upper C-band. The safety analysis presented
previously provides a template for MSD from next-generation wireless
services in the 3.98-4.2 GHz band, accounting for the more sensitive RA
performance described in section C, Current RA Limitations.
G. Lower C-Band Mitigations
The suitability of a new RA cannot be assured without also
addressing the potential for interference from wireless base stations
in the Lower C-band (3.7-3.98 GHz). The twenty-one wireless licensees
have filed a voluntary commitment with FCC to ensure their signals do
not cause an unsafe condition, as determined by FAA, and that the most
critical aircraft operations for commerce can continue without
disruption.\52\ The voluntary commitment runs through January 2028,
unless extended or reduced by mutual agreement. FAA intends to seek an
extension of the terms of the voluntary commitment until the initial RA
performance deadline.
In the end state, after the RA retrofit proposed by this rule is
complete, the updated RA systems will operate safely, assuming the
final Lower and Upper C-band wireless transmissions into the RA band
are harmonized.
H. Relationship to Airworthiness Directives and Other FAA Policy
There are a number of ADs that address the unsafe conditions posed
by interference from the Lower C-band wireless services, as discussed
previously in section III. The RA system performance requirements
proposed by this rule would provide sufficient tolerance to Lower C-
band wireless services to prevent the unsafe conditions identified and
addressed in the current ADs, subject to resolving the spurious
emissions issue described in section IV.E.5.
Under the wireless voluntary agreement, the wireless signals near
188 airports are limited to lower levels to allow certain aircraft to
conduct unrestricted operations. Those aircraft were modified in 2022-
2024 with RA systems that are tolerant to the Lower C-band wireless
signals. The next generation RA systems proposed in Sec. 91.220 would
ensure continued unrestricted operations after the initial RA
compliance deadline without any airport-specific wireless power
limitations. After that date, if necessary, FAA would supersede the
current ADs to impose operating limitations on the use of RAs that do
not meet the proposed performance requirements until such time as the
RA system is replaced. The superseding ADs would address operators who
have upgraded to a Lower C-band interference-tolerant RA, but do not
upgrade to an RA system compliant with the proposed rule prior to the
initial compliance date.
FAA plans to recognize an aircraft's compliance with the proposed
14 CFR 91.220(b) as an AMOC with all existing ADs and any superseded
ADs that may be necessary, to permit operation without limitations for
those aircraft once they are equipped. FAA also plans to authorize a
foreign operator to operate without additional limitations in the U.S.
if the aircraft complies with this retrofit requirement.
Most airplanes operating under part 121 and large airplanes
operating under part 129 are equipped with RA systems that comply with
FAA policy statement PS-AIR-600-39-01,\53\ which provides guidance for
operators and manufacturers to demonstrate that an aircraft is equipped
with an interference-tolerant RA that meets the performance
requirements in the current ADs. FAA has assessed the risk for these
aircraft until a hypothetical initial RA performance deadline as late
as 2032 and determined that the conditions of policy statement PS-AIR-
600-39-01 will provide an acceptable risk mitigation, provided the
terms and conditions of the voluntary commitments for the Lower C-band
are extended to the initial RA performance deadline. An earlier
compliance date would reduce the risk. As addressed in the Schedule
section E.2, FAA is soliciting comments on the achievable initial and
final RA performance deadline.
There are also a small number of transport category airplanes
operating under the restrictions prescribed in the current ADs.\54\ FAA
has assessed the risk for operators of those airplanes and determined
that the existing operating limitations are sufficient until the final
RA performance deadline to address the additional sources of
interference that may arise from Upper C-band wireless services aligned
with Lower C-band technical rules.
FAA also issued ADs applicable to helicopters,\55\ where the
interference from Lower C-band wireless services posed an unsafe
condition. FAA has evaluated the additional risk to helicopter
operators from Upper C-band wireless services aligned with Lower C-band
technical rules and determined that the scope and conditions of the
current helicopter AD are adequate until the final RA performance
deadline. NVG operations under Sec. 91.205 will continue to require an
FAA exemption for aircraft not equipped with RA systems that meet the
new performance requirements.
Finally, FAA had identified a number of aircraft systems that could
be affected by erroneous RA data and issued SAFO 21007 \56\ to advise
operators of the potential for erroneous or degraded RA output as it
relates to those operations. The SAFO would remain in effect until
[[Page 476]]
aircraft comply with the proposed RA system requirements.
As FAA would end the accommodation of Lower C-band interference-
tolerant RA systems at the initial RA performance deadline, several
policies would end at that time. A current Flight Standards policy memo
\57\ would be canceled at the initial RA performance deadline. This
policy memo requires an additional C-band assessment for specific new
or amended CAT II/III and SA CAT I/II instrument approach procedures,
primarily impacting the development of new procedures at airports that
are not on the list of 188 CMAs. These additional requirements would no
longer be necessary to support safe operations after the initial RA
performance deadline.
FAA would withdraw PS-AIR-600-39-01 after the initial RA
performance deadline. If there were aircraft that upgraded to Lower C-
band tolerant equipment but did not subsequently upgrade to comply with
the proposed 91.220(b), the relevant AD would need to be updated to
restore the original operating limitations to reflect the sunset of the
Lower C-band commitments and the onset of Upper C-band emissions.
After the final RA performance deadline, FAA may elect to remove
the ADs as they would be made obsolete by the proposed RA requirements.
FAA will also be evaluating if any frequencies in the Lower and
Upper C-band should be added to the Colo Void Policy \58\ after the
final RA performance deadline. The Colo Void Policy identifies
frequencies that do not need to provide notice to FAA for a
construction or alteration under 14 CFR part 77 because FAA has studied
any potential impacts and found that the frequency is not a hazard to
aviation safety. C-band frequencies cannot be added to the list of
exempted frequencies until after the final RA performance deadline
because wireless base station locations would still be required to
support aircraft-specific AMOCs after the initial RA performance
deadline.
I. Alternatives Considered
An alternative to this retrofit requirement would be for FAA to
evaluate whether an unsafe condition is created by changes in the RF
environment and issue additional ADs as appropriate. That alternative
would not regain the full safety benefits of RA systems, would have a
significant impact on aircraft operational capability by imposing new
limitations for aircraft with RA systems that are currently compliant
(limiting some aircraft from operating at all airports where C-band
wireless base stations transmit and limiting low-visibility access for
all aircraft), and would create market instability both for the
aviation and wireless industries. Because ADs would be issued after FCC
finalized service rules, ADs would impose severe operational
limitations until the aviation industry has sufficient time to dedicate
the necessary capital and resources to the appropriate RA upgrades and
replacements. In addition, ADs would require an extension to the
current voluntary wireless agreements or amendments to the current FCC
R&O necessary to ensure long-term safe coexistence with Lower C-band
wireless service, potentially exposing more severe operating conditions
if the wireless service providers do not agree to indefinitely extend
the voluntary agreements. Because ADs are not applicable to non-U.S.
registered aircraft, other methods would also be required to ensure
safety for part 129 foreign air carriers, such as issuing notices to
airmen (NOTAM) and amending the U.S. Aeronautical Information
Publication (AIP) to address changes in the spectrum environment. In
addition, FCC would have to determine the new Upper C-band wireless
environment without a compatible RA standard. This may result in
wireless interference that cannot be safely accommodated even with new
RA systems, which would indefinitely prohibit certain aircraft from
operating in the U.S. and prohibit all low-visibility approach and
landing operations. The absence of a compatible standard could also
result in FCC authorizing less spectrum than could otherwise be safely
accommodated, such as if only 100 MHz were authorized. In the best
case, FCC would define the Upper C-band wireless environment that is
aligned with the feasible RA performance. However, this would not
ensure that aircraft upgrade to suitable RA systems in time to avoid
severe operational disruption.
Similarly, an alternative where FAA delays the proposed performance
requirement until completion of the new RTCA/EUROCAE standards would
introduce the same costs, limitations, and risks.
Another alternative, where FAA does not evaluate and address any
unsafe conditions that would be created by changes in the RF
environment, would create unacceptable catastrophic risks and would not
address FAA's statutory mandate to ensure safe operations in the NAS.
FAA risk assessments in support of the ADs issued to date \59\ found
the most significant risks are to operations in very low visibility and
aircraft-specific risks with dependent safety systems. FAA has
previously determined that training, service bulletins, and guidance
would not be sufficient to overcome the high likelihood of hazardously
misleading or missing RA information impacting multiple aircraft safety
systems, some of which are required by legislation and regulations in
large part due to fatal accidents in the past.
FAA also considered a two-phase implementation, with the goal of
enabling earlier access to less than 100 MHz as soon as possible and
transitioning to the next generation RA as a second phase. However, due
to the existing RA performance (see Section IV.C), any early wireless
access that requires an interim retrofit for safe operations would
impose a significant additional cost on the aviation industry,
requiring operators to procure and install interim RA solutions
available now that are not likely to meet these proposed RA performance
requirements. Increased demand and manufacturing requirements for an
interim retrofit would also significantly extend the timeline for all
operators to equip with RA systems that meet these proposed
requirements, duplicating the requirements and efforts needed and
diverting aviation manufacturers' resources and personnel who are
working towards the development and certification of new RA systems
that will meet the proposed RA performance requirements. Also, it would
not result in the full 100 MHz being available to next-generation
wireless services, requiring extensive and ongoing coordination,
reduced power level, and constraints on wireless base station antenna
height/elevation masks.
V. Regulatory Notices and Analyses
A. Regulatory Impact Analysis (RIA)
E.O. 12866 (``Regulatory Planning and Review'') and E.O. 13563
(``Improving Regulation and Regulatory Review'') require agencies to
regulate in the ``most cost-effective manner,'' to make a ``reasoned
determination that the benefits of the intended regulation justify its
costs,'' and to develop regulations that ``impose the least burden on
society.'' The Office of Management and Budget has determined this
proposed rulemaking is a significant regulatory action under section
3(f) of E.O. 12866.
[[Page 477]]
1. Statement of the Need for the Proposed Action
i. Description of Problem
Radio or Radar Altimeters (RAs) are devices that measure an
aircraft's current height above terrain by sending out low-powered
radar waves in the 4.2 to 4.4 GHz spectrum and measuring their return
against the ground or other obstacles. The accurate height data RAs
provide is crucial to a variety of automatic safety systems and is used
by pilots in low-visibility situations. Since RAs utilize relatively
low-powered transmissions, there is a risk that wireless signals, such
as those emitted by next-generation wireless base stations utilizing
adjacent spectrum bands, can interfere with or overpower the RA signal
and result in missing or erroneous data. As was discussed in more
detail in the preamble to the NPRM, the coming expiration of current
voluntary commitments by wireless license holders to limit base station
power level and out-of-band emissions in the Lower C-band spectrum
(3.7-3.98 GHz) in 2028 and the upcoming FCC auction reallocating some
or all of the Upper C-band spectrum (3.98-4.2 GHz) directly adjacent to
the RA band are expected to exceed the ability of current avionics
technology to mitigate the risk of spectrum interference and will
create unacceptable risk to the NAS.
ii. Need for Regulation
Public Law 119-21 requires FCC to complete an auction of at least
100 MHz in the Upper C-band, and FAA has found that the associated
authorization would cause existing RAs to experience interference and
cause unsafe conditions. The upcoming auction would create an
externality, defined as a market failure in OMB Circular A-4 occurring
when one party's actions impose uncompensated benefits or costs on
another party.\60\ The proposed utilization of Upper C-band spectrum
directly imposes uncompensated safety costs (increased risk of
accidents) and fiscal costs (replacing RA systems to redress safety
costs) to aircraft operators and the flying public.
iii. Summary of the Proposed Regulation
To address this risk, FAA proposes requiring the replacement of all
existing RA systems with ones that meet the new interference tolerance
performance standards for aircraft operating under part 91 in the
airspace of the 48 contiguous United States and the District of
Columbia. FAA is proposing RA performance requirements that reflect the
best achievable interference rejection and without compromising the RA
system performance. These requirements would apply first to all
aircraft with an RA operating under 14 CFR part 121 and all aircraft
with an RA operating under 14 CFR part 129 with 30 or more passenger
seats or a payload capacity of more than 7,500 pounds, which have the
highest expected level of safety and are the most critical to the
national economy. All other aircraft operating under part 91 in the
airspace of the 48 contiguous United States and the District of
Columbia and equipped with RAs would have two additional years from the
initial RA performance deadline to replace any RAs with units that meet
the proposed performance requirement.
2. Baselines for the Analysis
To properly evaluate regulations, agencies must weigh the costs and
benefits against a baseline. OMB Circular A-4 defines the ``no action''
baseline as ``the best assessment of the way the world would look
absent the proposed action.'' It also specifies that the baseline
``should incorporate the agency's best forecast of how the world will
change in the future,'' absent the regulation. FAA considers the
primary baseline for this analysis to be a no action baseline, in which
FAA assumes FCC completes the auction required by Public Law 119-21 and
the voluntary commitments of the wireless service providers lapse.
Under this scenario, FAA would have to react to the interference to
prohibit all operations of certain aircraft makes and models and
prohibit low-visibility operations in all aircraft, causing significant
operational impacts. Aircraft owners would need to replace their RA
systems to achieve compatibility with the new spectrum environment, if
it is possible to do so. The inherent costs of delays, cancellations,
and groundings resulting from re-imposing AD operational prohibitions
under this no action baseline can be negated by the cost of
retrofitting the RA system in compliance with proposed performance
standards. FAA could also seek voluntary constraints from the wireless
carriers in order to mitigate these aviation impacts. There is no
assurance that an agreement could be reached, and that scenario could
impact FCC as the constraints would not be known at the time of the
auction.
These costs are based on the prior expansion of next-generation
wireless services in the Lower C-band, where FAA issued 14 ADs for
aircraft equipped with RAs. These ADs maintained the safety environment
of the NAS by prohibiting operations when spectrum interference affects
the accuracy of RA data critical for safe operation of the aircraft. To
accomplish this goal, the ADs prohibited transport and commuter
category airplanes without an upgraded RA from flying in very low
visibility conditions (CAT II/III and other operations), prohibited
rotorcraft without an upgraded RA from flying in specific automation
modes dependent on RA data, and imposed additional operating
restrictions on specific model airplanes with vital safety systems
heavily tied to RA data. The airplane model-specific ADs cover Boeing,
McDonnell Douglas, MHI RJ, and Airbus 220/Bombardier 500 models. All
combined, these aircraft make up around 52 percent of the U.S.
commercial fleet based on MITRE fleet data.\61\ These ADs are still in
effect, but do not significantly restrict operations currently due to
operator compliance with lower C-band interference mitigation,
including RA retrofits or other measures as specified in the ADs. If
the spectrum environment changes due to the expiration of the voluntary
commitments in 2028 or the utilization of the Upper C-band after FCC
auction, the current AD compliance requirements would not be sufficient
to mitigate the unsafe condition caused by interference with the RA. To
maintain safety in the NAS, FAA would supersede the ADs along the same
restrictions, with the potential for issuing additional ADs covering
other operations or aircraft models as required, resulting in
significant operational impact and baseline costs.
Along with the aircraft specific ADs, FAA would have additional ADs
restricting operations in low visibility CAT II or III conditions,
which would impact air travel in the NAS. In 2019, these conditions
ranged from 0 to 1.14 percent of hours at the core 30 airports,\62\
overall averaging 0.24 percent.\63\ With over 56.5 million operations
at towered airports in 2024,\64\ AD limitations on flying in CAT II/III
conditions would disrupt an average of 135,600 takeoffs and landings
per year, inducing recurring delay, diversion, and cancellation costs
to aircraft operators and the flying public until emission interference
mitigation of the Upper C-band is achieved. These baseline costs from
any effective reduction in NAS capacity due to the aircraft model and
low-visibility weather ADs can be significant. For example, regarding
similar limitations due to air traffic controller staffing constraints
when FAA issued Emergency Order Establishing Operating Limitations on
the Use of Navigable Airspace (90 FR 50884, Nov. 12, 2025),\65\
Airlines for
[[Page 478]]
America (A4A) stated, ``When the FAA flight-reduction order reaches 10%
on Nov. 14, A4A estimates a daily average U.S. economic impact of $285
[million]-$580 [million], depending on the degree to which airlines can
reaccommodate cancellation-disrupted passengers on the remaining
flights.'' \66\
Air carriers may choose to voluntarily upgrade their RA units to
address potential interference concerns associated with the use of the
Upper C-band spectrum, either to directly address the related safety
risks to their aircraft or as a method of compliance with the new ADs
to avoid the cost of capacity disruption. This action would limit both
the operational impacts of the ADs and any impacts on the wireless
industry's use of the spectrum. However, without the proposed rule, FAA
is unable to assume the availability of Upper C-band compliant units or
the extent and timeline of voluntary compliance.
FAA also considers an alternative pre-C-band utilization baseline,
in which FAA avoids the prohibition of certain operations by achieving
full fleet retrofit of RA systems to the proposed performance standards
before any change in the spectrum environment. With no need for new ADs
in this alternative baseline, only the costs of RA retrofit are
considered in the current environment prior to the auction mandated by
Public Law 119-21. With the pre-C-band utilization baseline
representing a world where FAA considers mandating equipage of RAs that
are tolerant to the Upper C-band spectrum and aircraft operators
continue being able to fly without restrictions, baseline costs are $0.
As discussed in the preamble, RAs are not directly required by
regulation for most aircraft, except for NVG operations under Sec.
91.205(h)(7) and for rotorcraft operations under Sec. 135.160, but are
still carried on nearly all commercial and many noncommercial aircraft
due to the vital role they play in the safety of aircraft operations by
providing critical information directly to pilots and for mandated
safety systems such as TCAS, TAWS, and other functions like autoland.
Some aircraft may only need one RA unit, but given how vital the
information is to safe operation, many commercial aircraft use two or
more RA units to ensure accurate data. Using April 2025 data from
MITRE, FAA estimates that there are roughly 58,579 RA units across
40,871 aircraft in the current operating civilian fleet (including many
State-owned aircraft) that would be affected by the proposed rule.\67\
This estimate is likely an overcount as FAA currently lacks data to
specify which U.S. aircraft operate solely in Alaska or Hawaii, which
would not be subject to this proposed rule. Conversely, though the
proposed performance requirements would apply to all aircraft equipped
with an RA operating in the airspace of the 48 contiguous United States
and the District of Columbia, military and Federal law enforcement-
owned aircraft are not included in the estimates as FAA lacks data on
RA-equipped aircraft totals and the costs to purchase and replace
military RA units. The breakout of RAs by 14 CFR part operation and
aircraft type can be found in Table 8:
Table 8--Number of Aircraft and RA Units by CFR Operation
----------------------------------------------------------------------------------------------------------------
Count of Count of RA
CFR operational part Aircraft type aircraft units
----------------------------------------------------------------------------------------------------------------
Part 91....................................... Airplane........................ 16,657 18,452
Rotorcraft...................... 2,818 2,819
Part 121...................................... Airplane........................ 8,014 17,033
Rotorcraft...................... .............. ..............
Part 129 \1\.................................. Airplane........................ 5,050 11,127
Rotorcraft...................... 18 27
Part 135...................................... Airplane........................ 6,385 7,151
Rotorcraft...................... 1,929 1,970
Total......................................... Airplane........................ 36,106 53,763
Rotorcraft...................... 4,765 4,816
Total........................... 40,871 58,579
----------------------------------------------------------------------------------------------------------------
\1\ Part 129 totals only include aircraft that had at least one U.S. operation in the 17-month period from 04/01/
2024 to 09/01/2025.
From the same MITRE data, Table 9 below shows the estimated number
of operators of affected RA-equipped aircraft operating under the rules
of each CFR part.
Table 9--Operators of RA Equipped Aircraft
------------------------------------------------------------------------
Number of
CFR operational part operators
------------------------------------------------------------------------
91...................................................... 12,365
121..................................................... 60
129..................................................... 330
135..................................................... 1,131
---------------
Total............................................... 13,886
------------------------------------------------------------------------
FAA requests comment, with supporting documentation, on the no
action and pre-C-band utilization baseline estimates and assumptions.
3. Benefits
The benefits of this proposed rule stem from maintaining the safety
benefits of RAs and preventing operational restrictions due to the high
risk to aviation safety when utilizing current generation RA systems
that are unable to filter out wireless signals (e.g., Upper C-band
wireless services aligned with Lower C-band technical rules, if
allocated as proposed by FCC). Installing RA systems that meet the
requirements of this proposed rule would limit the risk of inaccurate
or missing height above terrain data, allowing air transportation
operations to continue at their current tempo and safety environment.
At the immediate safety level, having accurate data provided by the RA
is essential information for pilots, especially in low-visibility
airport operations that can affect, on average, 135,600 takeoffs and
landings each year.
Beyond data provided directly to pilots, RA information is used by
several mandated systems whose safety benefits this proposed rule aims
to preserve. Systems such as TCAS and TAWS, which rely on accurate RA
altitude data, provide pilots vital safety enhancements for collision
avoidance. Since implementation, these systems have played a large role
in significantly reducing mid-air collisions or CFIT accidents on
equipped aircraft in the United States.\68\ Additional aircraft systems
that rely on RA information, such as autoflight functions, wind shear
[[Page 479]]
protection, and other aircraft-specific features, provide further
unquantified safety benefits by aiding pilots in operating the aircraft
and avoiding unsafe conditions.
Should interference-tolerant RAs not be available or mandated, FAA
would supersede the current ADs to maintain the safety environment,
with the potential to issue additional ADs covering other operations or
aircraft models as needed. These ADs would maintain the appropriate
level of safety in the NAS by preventing the operation of certain
aircraft or in conditions where accurate RA data is vital to the safe
operation, but do not retain the additional safety benefits generated
by RAs and their dependent safety systems. There also would be further
loss of economic benefits from the resulting groundings, cancellations,
and delays of operational restrictions affecting the efficiency of air
travel in the NAS. FAA currently lacks data to assess the estimated
potential effects and requests comment with supporting documentation on
the expected economic impact or on any other benefit assumption or
estimate in this analysis.
4. Costs
Under the proposed rule, airlines and other operators would incur
costs to retrofit their RA equipped aircraft with systems meeting the
proposed RA interference tolerance standards. When issuing ADs in 2023
for transport and commuter category airplanes and for rotorcraft to
mitigate interference from Lower C-band wireless services, FAA
estimated that replacement of the RA transceiver unit for a civil
aircraft would cost up to $80,000 for an airplane \69\ and $40,000 for
a rotorcraft,\70\ inclusive of parts and labor. FAA acknowledges that
the unit cost of the new and more complex RA units required by this
rule may be greater and would result in an underestimation, but does
not have any alternative estimates at this time since the new products
are not yet available, and thus for purposes of this analysis uses
estimates based on the current unit cost. These values assume
replacement of just the RA transceiver unit, which for most aircraft is
expected to be a ``plug-and-play'' operation requiring minimal labor
hours, aircraft downtime, or time out of service, such as during
regular maintenance. Retrofitting just the transceiver unit is expected
to solve the spectrum interference issue and would not require changing
out the RA antenna or wiring, which would greatly increase completion
time and costs. Once installed, there are not any expected notable
operational differences between the current RAs and the new units, so
there are no estimated recurring costs associated with the new units
after replacement. In addition, as this analysis uses current prices
for RA units, there is no estimated price delta and therefore costs for
future built aircraft using an Upper C-band compliant system. FAA
requests comments, with supporting documentation, on the expected RA
unit price difference, estimated future annual production of units, and
any other cost assumptions or estimates presented in this analysis.
FAA proposes the compliance timeline to complete the retrofitting
as two tranches. For the first, all aircraft with RAs operating under
14 CFR part 121, and those aircraft operating under 14 CFR part 129
with 30 or more passenger seats or a payload capacity of more than
7,500 pounds, would have to retrofit with RA systems meeting the new
performance requirements by the initial RA performance deadline. These
operations are the most critical to the national economy and have the
highest expected level of safety, making them a priority. FAA proposes
that this initial RA performance deadline be between 2029 and 2032.
Based on the fleet data from MITRE, FAA estimates there are roughly
27,374 RA units on aircraft subject to the first deadline: 17,033 used
by domestic part 121 operators and 10,341 used by foreign part 129
operators.\71\ Applying the $80,000 cost to the RA totals yields a
total retrofit expense of $1.36 billion for part 121 operators and $827
million for part 129 operators, yielding a total undiscounted cost of
$2.19 billion for the first group. FAA requests comment on the expected
schedule of replacement or retrofit of RA units to Upper C-band
tolerant systems to develop discounted cost estimates.
The second tranche includes any other aircraft operating under part
91 in the airspace of the 48 contiguous United States and the District
of Columbia and equipped with an RA; they would have an additional two
years after the initial RA performance deadline to complete the
retrofit. FAA currently estimates that there are 31,205 RA units across
this category, covering both airplanes and rotorcraft.\72\ Applying the
respective cost for airplanes and rotorcraft to the populations, FAA
estimates an undiscounted cost of $2.30 billion to retrofit the
remaining RA units in the second group. FAA requests comment on the
expected Upper C-band tolerant RA adoption curve for this group of
aircraft to develop a discounted cost total.
Combining the estimates from both groups, the expected undiscounted
total cost of retrofitting RAs across the civil fleet is $4.49 billion.
Table 10 shows the total and annualized costs, broken out by type of
CFR operation and annualized discount rate.
FCC's NPRM section 3.D also discusses exploring options for
potential incentivization or reimbursement of RA retrofits. This action
would be considered a transfer of costs under OMB Circular A-4
accounting, reducing or eliminating the burden of RA system retrofit
for aircraft operators. The availability of incentive or reimbursement
payments could affect the rate at which RAs are replaced in response to
the requirements of this proposed rule.
FAA requests comment with supporting documentation on the estimated
costs.
Table 10--Costs of RA Replacement
[Millions of 2025$]
----------------------------------------------------------------------------------------------------------------
Annualized costs \1\
Undiscounted -------------------------------
CFR operational part total cost 3% Discount 7% Discount
rate rate
----------------------------------------------------------------------------------------------------------------
Part 91......................................................... $1,589 $107 $150
Part 121........................................................ 1,363 92 129
Part 129........................................................ 891 60 84
Part 135........................................................ 651 44 61
-----------------------------------------------
Total....................................................... 4,494 302 424
----------------------------------------------------------------------------------------------------------------
Notes: Columns may not sum due to rounding.
[[Page 480]]
\1\ Costs are annualized over a 20-year period, estimated to be the average remaining service life for current
fleet aircraft.
5. Alternatives to Proposed Rule
Scenario 1: AD Operational Restrictions With No Retrofit Requirement
(Baseline)
FAA considers this scenario as an alternative to the Pre-C-band
Utilization Baseline. Without the availability of new interference-
tolerant RAs, either due to failure to certify the new product in time,
uncertainty regarding supply within the compliance timeframe, or not
issuing the proposed regulations on RA performance, FAA would follow
the actions presented in the baseline section and supersede the ADs
covering Lower C-band interference based on changes in the spectrum
environment to maintain current safety levels. Expiration of the
wireless agreements in 2028 and expansion into frequencies closer to
the RA band from the upcoming FCC auction would likely require
prohibiting specific operations and grounding aircraft that cannot
operate safely without interference-resistant RAs. These ADs would not
be applicable to non-U.S. registered aircraft, so other methods would
be required to ensure safety, such as issuing NOTAMs and amending the
U.S. AIP to address changes in the spectrum environment.
The method by which the ADs would maintain the safety of the NAS is
by prohibiting flights in low visibility conditions for aircraft that
are heavily dependent on RA data for their safety systems. In doing so,
safety is maintained by preventing scenarios where there is an
unacceptable risk of incorrect RA data causing a catastrophic accident;
however, this also comes with the loss of the additional safety
benefits RAs and their dependent systems provide. The cost of these ADs
would be flight delays and cancellations by operators, with spillover
effects for the flying public.
FAA compares these grounding costs that may be incurred by aircraft
operators to the costs within the pre-C-band utilization baseline to
further consider this scenario. The International Bureau of Aviation
(IBA) estimated in 2019 that the direct costs for an operator to ground
a passenger jet like the Boeing 737 Max could be up to $150,000 per
day.\73\ Based on that value, grounding the 8,014 aircraft in part 121
under the weather and model restrictions of the ADs for just 4 days
would cost operators $4.8 billion, exceeding the undiscounted cost of
$4.49 billion to retrofit RAs for the entire civil fleet. Beyond the
costs to operators of the aircraft, as a representation of how
expensive airline delays and cancellations are to the economy, a 2010
FAA-commissioned study found the total delay impact of flight delays in
2007 cost the U.S. $32.9 billion between airline operators, passengers,
and general economic welfare losses.\74\ Adjusted forward using the
Bureau of Labor Statistics (BLS) Consumer Price Index for All Urban
Consumers (CPI-U), this equates to $51.2 billion in 2025 dollars.\75\
If FAA has to issue new ADs and NOTAMs to maintain safety due to
changes in the Upper C-band spectrum environment, approximately 4
percent of the part 121 fleet and 22 percent of the part 129 airplane
fleet would not be able to operate in the contiguous U.S.,\76\ and the
majority of the part 121 and part 129 fleets would experience delays
due to prohibiting operations in low visibility conditions. The
resulting economic consequences of shutting down portions of major
domestic and international air carrier operations due to AD
restrictions under this baseline would likely exceed the cost of the
proposed rule well within the compliance period and incur additional
recurring costs until the interference issue is addressed.
In this environment, industry would likely turn to the upcoming new
RTCA/EUROCAE standards to guide development of Upper C-band tolerant
RAs. However, due to the timeline mandated by Public Law 119-21, FCC
would have to determine the new Upper C-band wireless environment prior
to standards publication. This may result in FCC establishing an
environment where wireless interference cannot be safely accommodated,
even with new RA systems, which would have significant economic costs
as FAA would indefinitely prohibit certain aircraft from operating in
the U.S. and prohibit all low-visibility approach and landing
operations. In the best case, FCC would define the Upper C-band
wireless environment that is aligned to the feasible RA performance.
Even then, awaiting the international standards to be published would
delay the design and production of RAs that can accommodate the new
spectrum environment, requiring FAA to use the more costly ADs to cover
the safety gap until the fleet is fully equipped.
This scenario would also require an extension of the current
voluntary wireless agreements to continue safe coexistence with Lower
C-band wireless service and continue to mitigate operational
limitations in the current ADs. FAA lacks the authority to compel
wireless licensees to extend the voluntary agreements, and notes that,
even if extended, new voluntary emission limits for safe RA use are not
commercially viable for the Upper C-band wireless services (see section
IV.C for discussion). The additional uncertainty and timeline pressure
of interference tolerant RA availability would continue to inhibit
wireless companies' usage of the C-band and would severely limit
realizing the full value of the FCC spectrum auction and the general
economic benefits of expanding spectrum usage compared to the proposed
rule.
Scenario 2: No AD Operational Restrictions or Retrofit Requirement
If new interference-tolerant RA units are not developed or
available, and the current ADs are withdrawn, FAA would be maintaining
the current tempo of air operations, but would be accepting the risk of
Upper C-band interference on the RA and all dependent aircraft safety
systems. The most recent FAA risk assessments rated these risks from
minor to catastrophic, with the most significant risks to operations in
very low visibility (e.g., CAT II/III, use of EFVS to touchdown,
Autoland). In addition, aircraft with dependent safety systems may
react incorrectly and catastrophically at low altitude due to erroneous
or missing RA data. Training, service bulletins, and guidance will not
be sufficient to overcome the high likelihood of hazardously misleading
or missing RA information impacting multiple aircraft safety systems,
some of which are required by legislation and regulations based on
previous fatal accidents. In comparison to the no-action baseline, this
scenario would retain economic benefits from maintaining the pace of
air operations but is considered unacceptable, as FAA has a statutory
responsibility to protect the safety of the NAS from the high level of
risk this option creates.
6. Summary
This proposed rule aims to address a critical safety issue in the
NAS, with the upcoming auction and proposed reallocation of the Upper
C-band spectrum for next-generation wireless services posing a serious
risk to the accuracy and usability of RAs. RAs provide height above
terrain information, and the accuracy of its data is critical for low
visibility operations and use in numerous mandated automatic safety
systems. Without the ability to filter out neighboring C-band signals
and prevent inaccurate or
[[Page 481]]
missing RA data, and absent the extension or modification of voluntary
agreements from Upper C-band spectrum holders, FAA would have to issue
ADs prohibiting the operation of certain aircraft and prohibiting
specific operations in low visibility conditions to maintain the safety
of the NAS.
To prevent this disruption to air operations and maintain high
levels of aviation safety, FAA is proposing new regulations to require
all RA systems meet specific minimum performance requirements on
aircraft operating in the airspace of the 48 contiguous United States
and the District of Columbia by an initial performance deadline between
2029 and 2032 for all aircraft operating under 14 CFR part 121 and
aircraft operating under part 129 with 30 or more passenger seats or a
payload capacity of more than 7,500 pounds. All other aircraft
operating under part 91 in the airspace of the 48 contiguous United
States and the District of Columbia and equipped with an RA will have
an additional two years after the initial performance deadline to use a
unit that meets the proposed performance standard. These new RA systems
must be resilient to interference from signals in neighboring spectrum
bands and continue to provide accurate altitude readings to pilots and
integrated aircraft safety systems.
FAA estimates the undiscounted total cost to retrofit all RAs in
the civil fleet is $4.49 billion, or $424 million annualized at a 7
percent discount rate over a 20-year average remaining aircraft service
life compared to the pre-C-band utilization baseline. Compared to the
no-action baseline, FAA assumes relative cost savings for operators of
RA equipped aircraft to retrofit to units that meet the new
interference tolerance standards and therefore not be subject to the
operating restrictions of the current ADs, which would also be required
in future ADs. FAA requests comments, with supporting documentation, on
the assumptions and estimates made in this RIA. As the estimated cost
exceeds the threshold for an economically significant rule under
section 3(f)(1) of E.O. 12866, FAA prepared the required OMB Circular
A-4 accounting statements below.
Table 11--OMB Circular A-4 Accounting Statement, No-Action Baseline, U.S. and Non-U.S. Parties
[Millions of 2025$]
----------------------------------------------------------------------------------------------------------------
Source citation (RIA.
Category 3-Percent discount rate 7-Percent discount rate Preamble, etc.)
----------------------------------------------------------------------------------------------------------------
BENEFITS
----------------------------------------------------------------------------------------------------------------
Annualized Monetized $millions/year.. N/A.................... N/A.................... N/A.
Annualized Quantified................ N/A.................... N/A.................... N/A.
----------------------------------------------------------------------------------------------------------------
Qualitative.......................... ADs maintain baseline safety in the NAS Preamble, RIA Section
by prohibiting operations where RA interference A.2.
presents unacceptable risk.
Use of interference-tolerant RA units
allows operators to keep safety benefits of RAs
and their dependent systems.
Permits airlines with a retrofitted RA
to maintain current schedule efficiency and
reliability.
Allows FAA to remove ADs associated
with RA interference once the fleet has fully
equipped to upgraded RA systems.
----------------------------------------------------------------------------------------------------------------
COSTS
----------------------------------------------------------------------------------------------------------------
Annualized Monetized $millions/year.. N/A.................... N/A.................... Preamble RIA Section
A.2.
Annualized Quantified................ N/A.................... N/A.................... N/A.
----------------------------------------------------------------------------------------------------------------
Qualitative.......................... The baseline cost to aircraft operators Preamble, RIA Sections
includes recurring delays, cancellations, and A.2 and A.4.
groundings due to model and visibility
operating restrictions covered by the ADs.
These baseline costs can be negated by the cost
of retrofitting RAs to be in compliance with
the ADs.
Retrofit costs include purchasing new
RA transceiver units, installation is expected
to be simple and done during regular
maintenance cycles.
Split between two groups, first
compliance requirement is all part 121 and 30+
seat or 7,500+ pounds payload capacity part 129
aircraft, all others will have two additional
years to retrofit.
No expected operational or recurring
cost differences between current and potential
future RAs.
----------------------------------------------------------------------------------------------------------------
TRANSFERS
----------------------------------------------------------------------------------------------------------------
Annualized Monetized $millions/year.. N/A.................... N/A.................... N/A.
Annualized Quantified................ N/A.................... N/A.................... N/A.
----------------------------------------------------------------------------------------------------------------
Qualitative.......................... FCC is requesting proposals to N/A.
facilitate these retrofits from a financial
perspective.
Would allow full utilization of
auctioned wireless spectrum.
----------------------------------------------------------------------------------------------------------------
State, Local, and/or Tribal Any state, local, or tribal governments Preamble, RIA Section
Government. that utilize aircraft with an RA onboard, such A.4.
as police or search and rescue rotorcraft, will
have restrictions on operating in conditions
specified in the ADs.
[[Page 482]]
The cost of not being able to utilize
some aircraft under such conditions may be
greater than the cost of retrofitting with an
RA unit meeting the new performance standards.
----------------------------------------------------------------------------------------------------------------
Small Business....................... Small businesses utilizing RA-equipped Initial Regulatory
aircraft will be subject to restrictions of the Flexibility Analysis.
ADs.
Lost revenue and other expenses from
groundings, delays, and cancellations stemming
from the ADs are likely greater than the cost
to retrofit RAs per the proposed standards of
the rule.
Total cost to an entity is dependent on
the size of their fleet.
----------------------------------------------------------------------------------------------------------------
Wages................................ N/A. N/A.
Growth............................... N/A. N/A.
----------------------------------------------------------------------------------------------------------------
Table 12--OMB Circular A-4 Accounting Statement, Pre-C-Band Utilization Baseline, Retrofit Cost to U.S. and Non-
U.S. Parties
[Millions of 2025$]
----------------------------------------------------------------------------------------------------------------
Source citation (RIA.
Category 3-Percent discount rate 7-Percent discount rate Preamble, etc.)
----------------------------------------------------------------------------------------------------------------
BENEFITS
----------------------------------------------------------------------------------------------------------------
Annualized Monetized $millions/year.. N/A.................... N/A.................... N/A.
Annualized Quantified................ N/A.................... N/A.................... N/A.
----------------------------------------------------------------------------------------------------------------
Qualitative.......................... Use of interference-tolerant units Preamble, RIA Section
allows operators to keep safety benefits of RAs A.3.
and their dependent systems.
Permits airlines to maintain the
current schedule efficiency and reliability.
Allows FAA to remove ADs associated
with RA interference once the fleet has fully
equipped to upgraded RA systems.
----------------------------------------------------------------------------------------------------------------
COSTS
----------------------------------------------------------------------------------------------------------------
Annualized Monetized $millions/year.. $302................... $424................... Preamble RIA Section
A.4.
Annualized Quantified................ N/A.................... N/A.................... N/A.
----------------------------------------------------------------------------------------------------------------
Qualitative.......................... Burden on operators of RA carrying Preamble, RIA Section
aircraft to replace or retrofit to units that A.4.
meet the new interference tolerance standards.
Split between two groups, first
compliance requirement is all 14 CFR part 121
and 30+ seat or 7,500+ pounds payload capacity
part 129 aircraft, all others will have two
additional years to retrofit.
Primary cost is purchasing new RA
transceiver units, installation is expected to
be simple and done during regular maintenance
cycles.
No expected operational or recurring
cost differences between current and potential
future RAs.
----------------------------------------------------------------------------------------------------------------
TRANSFERS
----------------------------------------------------------------------------------------------------------------
Annualized Monetized $millions/year.. N/A.................... N/A.................... N/A.
Annualized Quantified................ N/A.................... N/A.................... N/A.
----------------------------------------------------------------------------------------------------------------
Qualitative.......................... FCC is requesting proposals to N/A.
facilitate these retrofits from a financial
perspective.
Would allow full utilization of
auctioned wireless spectrum.
----------------------------------------------------------------------------------------------------------------
State, Local, and/or Tribal Any state, local, or tribal governments that Preamble, RIA Section
Government. utilize aircraft with an RA onboard, such as A.4.
police or search and rescue rotorcraft, will
incur costs to replace the unit with an
interference-tolerant version.
----------------------------------------------------------------------------------------------------------------
Small Business....................... Small businesses will incur $40k to $80k in Initial Regulatory
costs per aircraft to retrofit with an RA that Flexibility Analysis.
meets the proposed performance requirement.
Total cost to an entity is dependent on the
size of their fleet.
----------------------------------------------------------------------------------------------------------------
[[Page 483]]
Wages................................ N/A. N/A.
Growth............................... N/A. N/A.
----------------------------------------------------------------------------------------------------------------
B. Regulatory Flexibility Act
The Regulatory Flexibility Act (RFA) of 1980, Public Law 96-354, 94
Stat. 1164 (5 U.S.C. 601-612), as amended by the Small Business
Regulatory Enforcement Fairness Act of 1996 (Pub. L. 104-121, 110 Stat.
857, Mar. 29, 1996) and the Small Business Jobs Act of 2010 (Pub. L.
111-240, 124 Stat. 2504 Sept. 27, 2010), requires Federal agencies to
consider the effects of the regulatory action on small business and
other small entities and to minimize any significant economic impact.
The term ``small entities'' comprises small businesses and not-for-
profit organizations that are independently owned and operated and are
not dominant in their fields, and governmental jurisdictions with
populations of less than 50,000.
FAA is publishing this Initial Regulatory Flexibility Analysis
(IRFA) to aid the public in commenting on the potential impacts to
small entities from this proposal. FAA invites interested parties to
submit data and information regarding the potential economic impact
that would result from the proposal. FAA will consider comments when
making a determination or when completing a Final Regulatory
Flexibility Analysis.
Under Section 603(b) and (c) of the RFA, an IRFA must contain the
following:
(1) A description of the reasons why the action by the agency is
being considered;
(2) A succinct statement of the objective of, and legal basis for,
the proposed rule;
(3) A description of and, where feasible, an estimate of the number
of small entities to which the proposed rule will apply;
(4) A description of the projected reporting, recordkeeping, and
other compliance requirements of the proposed rule, including an
estimate of the classes of small entities which will be subject to the
requirement and the type of professional skills necessary for
preparation of the report or record;
(5) An identification, to the extent practicable, of all relevant
Federal rules that may duplicate, overlap, or conflict with the
proposed rule; and
(6) A description of any significant alternatives to the proposed
rule which accomplish the stated objectives of applicable statutes and
which minimize any significant economic impact of the proposed rule on
small entities.
1. Reasons the Action Is Being Considered
This rule is being proposed to address a critical safety issue with
RAs. RAs are dependent on receiving faint waves in the 4.2 to 4.4 GHz
spectrum reflected by terrain and obstacles to determine the aircraft's
height above the terrain. Higher power signals in neighboring spectrum
bands, such as those emitted by next-generation wireless services, can
interfere with the RA waves and cause the unit to indicate missing or
erroneous data. In turn, the lack of accurate height above terrain data
presents a significant safety risk for pilots operating in low-
visibility conditions and affects numerous safety systems that are
dependent on RA information. These issues have been previously
mitigated with wireless companies voluntarily agreeing to limit base
station power level and out-of-band emissions in the Lower C-band (3.7
to 3.98 GHz) and operators making changes to their RA units to improve
interference tolerance. However, with the voluntary agreements expiring
in 2028, and the mandate for FCC to auction off the Upper C-band
spectrum (3.98 to 4.2 GHz) adjacent to the RA band, these measures will
no longer be adequate to prevent RA interference and associated
catastrophic risk to air operations.
2. Objectives and Legal Basis of the Proposed Rule
To address the safety issue from wireless interference, this rule
proposes that all RA units on aircraft operating under part 91 in the
airspace of the 48 contiguous United States and the District of
Columbia must be replaced by new RA systems that meet the proposed
interference tolerance requirements. RA systems that meet the new
requirements will continue to function properly when the Lower and
Upper C-band wireless services become active following FCC auction and
expiration of the voluntary Lower C-band wireless agreements.
Installing these interference-tolerant RAs in the fleet would allow air
operations to continue at their current tempo and preserve safety
levels provided by the benefits of accurate RA data and its use in
numerous dependent safety systems. In the absence of requiring
interference-tolerant RAs, FAA would issue ADs to maintain the safety
environment, which would cost operators more over time due to
groundings, delays, and cancellations of aircraft operations.
The legal basis for this action lies in FAA's authority to issue
rules on aviation safety found in Title 49 of the United States Code.
Subtitle I, Section 106 describes the authority of the FAA
Administrator. Subtitle VII, Aviation Programs, describes in more
detail the scope of FAA's authority. This rulemaking is promulgated
under the authority described in 49 U.S.C. 106(f), which establishes
the authority of the Administrator to promulgate and revise regulations
and rules related to aviation safety. This rulemaking is also issued
under the authority described in Subtitle VII, Part A, Subpart III,
Section 44701: General requirements. Under that section, FAA is charged
with prescribing regulations promoting safe flight of civil aircraft in
air commerce by prescribing regulations for practices, methods, and
procedures the Administrator finds necessary for safety in air
commerce.
3. Description and Estimate of the Number of Small Entities
FAA used the definition of small entities in the RFA for this
analysis. The RFA defines small entities as small businesses, small
governmental jurisdictions, or small organizations. In 5 U.S.C. 601(3),
the RFA defines ``small business'' to have the same meaning as ``small
business concern'' under section 3 of the Small Business Act. The Small
Business Act authorizes the Small Business Administration (SBA) to
define ``small business'' by issuing regulations.
SBA (2023) has established size standards for various types of
economic
[[Page 484]]
activities, or industries, under the North American Industry
Classification System (NAICS).\77\ These size standards generally
define small businesses based on the number of employees or annual
receipts. Table 13 shows the SBA size standards for airlines as an
example. Note that the SBA definition of a small business applies to
the parent company and all affiliates as a single entity.
Table 13--Small Business Size Standards: Air Transportation
------------------------------------------------------------------------
NAICS code Description Size standard
------------------------------------------------------------------------
481111........................ Scheduled Passenger 1,500 employees.
Air Transportation.
481112........................ Scheduled Freight Air 1,500 employees.
Transportation.
481211........................ Nonscheduled Chartered 1,500 employees.
Passenger Air
Transportation.
481212........................ Nonscheduled Chartered 1,500 employees.
Freight Air
Transportation.
481219........................ Other Nonscheduled Air $25.0 million.
Transportation.
------------------------------------------------------------------------
Source: SBA (2023).
NAICS = North American Industrial Classification System.
SBA = Small Business Administration.
To identify small entities, FAA first identified the primary NAICS
of the airline or parent company and then used data from different
sources (e.g., company annual reports, FAA operator data, Bureau of
Transportation Statistics, D&B Hoovers) to determine whether the
airline meets the applicable size standard. Table 14 provides a summary
of the results.
Table 14--Estimated Number of Small Entities
----------------------------------------------------------------------------------------------------------------
Number of Number small Percent small
CFR operational part entities entities entities
----------------------------------------------------------------------------------------------------------------
Part 91 \1\..................................................... 12,365 11,371 91.95
Part 121........................................................ 60 35 58.3
Part 135........................................................ 1,131 1,114 98.5
-----------------------------------------------
Total....................................................... 13,535 12,520 92.5
----------------------------------------------------------------------------------------------------------------
\1\ The percent of part 91 small entities, and resulting total number of small entities, is based on a random
sample of 373 operators. This estimate is likely an overcount as FAA is unable to remove private/GA aircraft
owners from its dataset.
In general, entities classified as scheduled air transportation
(NAICS 481111 and 481112) operate under part 121, and entities engaged
in nonscheduled air transportation (NAICS 481211 and 481212) operate
under part 135. Part 91 operations include entities under NAICS 481219,
such as air clubs and sightseeing operations, as well as entities in
any other non-air transportation NAICS code that own and operate
aircraft for private use or internal company transportation.
4. Projected Reporting, Recordkeeping, and Other Compliance
Requirements
In the absence of cost data on a future product, FAA assumes the
cost to retrofit an interference-tolerant RA in accordance with this
proposed rule is up to $80,000 for an airplane and $40,000 for a
rotorcraft, based on the 2023 ADs concerning Lower C-band interference
mitigation.\78\ Therefore, the cost to each entity is based on how many
aircraft are in their fleet, which induces higher costs to larger
operators that have larger fleets. However, since operations and
resulting revenue scale with fleet size as well, larger firms may be
able to better absorb those increased costs compared to small entities
with only one or two aircraft. By applying these equipment costs to the
average number of aircraft for a small entity based on its size
category, FAA estimates the average one-time RA replacement cost per
small entity. These costs are then weighed against the average annual
revenue per small entity data from the 2022 U.S. Census Statistics of
U.S. Businesses (SUSB),\79\ displayed in table 15 for part 121
operators and table 16 for part 135 operators.
Table 15--Part 121 Cost of Compliance
[Thousands of 2025$]
----------------------------------------------------------------------------------------------------------------
Average one- Average
Number of Average time RA cost annual Average cost/
Number of employees small number of per entity revenues per annual
entities aircraft \1\ entity \2\ revenue (%)
----------------------------------------------------------------------------------------------------------------
20-99 employees............................. 9 4 $356 $69,356 0.5
100-499 employees........................... 18 13 1,031 246,082 0.4
500+ employees.............................. 8 29 2,310 5,075,566 0.0
----------------------------------------------------------------------------------------------------------------
Notes: Rows may not sum due to rounding.
\1\ Based on a unit and labor cost of $80,000 for a new RA.
\2\ FAA estimates receipts per entity using 2022 SUSB data on NAICS 48111 firm counts and receipts. Receipts are
adjusted to 2025 dollars using the BLS Consumer Price Index for all Urban Consumers (Series ID: CUUR0000SA0).
[[Page 485]]
Table 16--Part 135 Cost of Compliance
[Thousands of 2025$]
----------------------------------------------------------------------------------------------------------------
Average one- Average
Number of Average time RA cost annual Average cost/
Number of employees small number of per entity revenues per annual
entities aircraft \1\ entity \2\ revenue (%)
----------------------------------------------------------------------------------------------------------------
1-19 employees.............................. 640 2 $155 $2,906 6.8
20-99 employees............................. 376 7 469 21,400 2.8
100-499 employees........................... 76 20 1,402 84,939 2.1
500+ employees.............................. 22 75 5,301 250,641 2.7
----------------------------------------------------------------------------------------------------------------
Notes: Rows may not sum due to rounding.
\1\ Based on RA unit and labor cost of $80,000 for aircraft and $40,000 rotorcraft, applied by the ratio of each
type within part 135.
\2\ FAA estimates receipts per entity using 2022 SUSB data on NAICS 48112 firm counts and receipts. Receipts are
adjusted to 2025 dollars using the BLS Consumer Price Index for all Urban Consumers (Series ID: CUUR0000SA0).
FAA does not estimate the per entity costs for part 91 operators,
as companies operating under this section are generally not engaged in
commercial air transportation services. While there are some operators
for sightseeing services or aviation club activities under NAICS
481219, the vast majority of these aircraft are used by private
operators or entities for personal transportation across many different
industries (i.e. corporate jets). This is reflected in the fleet data
FAA used, as roughly 90 percent of operators under part 91 only have
one aircraft, and another eight percent operate just two. Depending on
whether the RA unit is used in automated aircraft safety systems or
not, some part 91 operators may even have the choice to simply remove
their RA after the proposed rule takes effect to avoid the replacement
cost, though they would not retain the safety benefits RAs provide as
discussed in section V.3. Entities that choose to replace the RA may
also have access to noncommercial use units at lower cost than the
estimated $40,000-$80,000. However, without information on what models
manufacturers will provide in the future, FAA is unable to determine a
potential reduction in burden.
5. All Federal Rules That May Duplicate, Overlap, or Conflict
There are no relevant Federal rules that may duplicate, overlap, or
conflict with the proposed rule.
6. Significant Alternatives Considered
As discussed in Section V.A.5 of the preamble, the alternative to
not requiring the use of interference-tolerant RAs would be for FAA to
supersede the current ADs to impose new requirements curtailing
operations where inaccurate RA data poses a catastrophic risk to air
safety. These ADs would cover commuter and transport category
airplanes, rotorcraft, and some specific large passenger aircraft, with
potential as needed for FAA to issue additional ADs based on changes in
the C-band spectrum environment. The cost of these ADs is likely to
outweigh the cost of retrofitting with an interference-tolerant RA in
expenses incurred from resulting groundings, cancellations, and delays.
The option of not controlling the risk of spectrum interference with
ADs or requiring interference-tolerant RAs is not considered acceptable
as FAA has a statutory responsibility to protect the safety of the NAS.
FAA requests comments on alternatives to the proposed rule that
accomplish the stated objectives of the applicable statutes, and that
minimize impact of the proposed rule on small entities.
C. International Trade Impact Assessment
The Trade Agreements Act of 1979 (Pub. L. 96-39), as amended by the
Uruguay Round Agreements Act (Pub. L. 103-465), prohibits Federal
agencies from establishing standards or engaging in related activities
that create unnecessary obstacles to the foreign commerce of the United
States. Pursuant to these Acts, the establishment of standards is not
considered an unnecessary obstacle to the foreign commerce of the
United States, so long as the standard has a legitimate domestic
objective, such as the protection of safety, and does not operate in a
manner that excludes imports that meet this objective. The statute also
requires consideration of international standards and, where
appropriate, they be the basis for U.S. standards.
FAA has assessed the potential effect of this proposed rule and
determined that it ensures the safety of the American public. If this
proposed rule is not implemented, there would be no cost savings and no
significant differences in the potential impacts to foreign commerce.
In the absence of new regulations, FAA will have to issue new or
amended ADs to address U.S. registered aircraft, as well as other
necessary policy changes directly relevant to foreign air carriers to
prevent catastrophic risk to aviation safety due to future changes in
the spectrum environment. The cost of compliance with the ADs would
likely be higher than the cost of compliance with the proposed rule, as
a lack of RA retrofit compliance would result in significant impacts to
domestic and foreign air carrier capacity, efficiency, and schedule
reliability. As a result, FAA does not consider this proposed rule as
creating an unnecessary obstacle to foreign commerce and welcomes
comment on this assessment.
D. Unfunded Mandates Assessment
Section 202 of the Unfunded Mandates Reform Act of 1995 (UMRA)
requires Federal agencies to prepare a written assessment of the costs,
benefits, and other effects of proposed or final rules that include a
Federal mandate likely to result in the expenditure by State, local or
tribal governments, in the aggregate, or by the private sector, of more
than $100 million in any one year (adjusted for inflation with base
year of 1995). The value equivalent of $100 million in 1995 adjusted
for inflation using the most current Implicit Price Deflator for the
Gross Domestic Product is $187 million. Before promulgating a rule for
which a written statement is needed, section 205 of the UMRA generally
requires FAA to identify and consider a reasonable number of regulatory
alternatives and adopt the least costly, most cost-effective, or least
burdensome alternative that achieves the objectives of the rule. The
provisions of section 205 do not apply when they are inconsistent with
applicable law. Moreover, section 205 allows FAA to adopt an
alternative other than the least costly, most cost-effective, or least
burdensome alternative if the agency publishes with the final rule an
explanation why that alternative was not adopted.
[[Page 486]]
This proposed rule would not result in the expenditure by State,
local, or tribal governments, in the aggregate, of more than $187
million annually, but would result in the expenditure of that magnitude
by airlines and other private operators of RA-equipped aircraft. This
document seeks comments on the alternatives presented in section V.A.5
for achieving the purposes of FAA's safety mandate in support of the
spectrum auction mandate of Public Law 119-21 (One, Big, Beautiful Bill
Act).
E. Paperwork Reduction Act
The Paperwork Reduction Act of 1995 (44 U.S.C. 3507(d)) requires
FAA consider the impact of paperwork and other information collection
burdens imposed on the public. According to the 1995 amendments to the
Paperwork Reduction Act (5 CFR 1320.8(b)(2)(vi)), an agency may not
collect or sponsor the collection of information, nor may it impose an
information collection requirement unless it displays a currently valid
OMB control number.
FAA has determined there would be no new information collection
associated with the proposed requirement to operate aircraft with RA
systems that comply with the specified performance. This proposed
requirement will update the RA performance standard, but there will be
no new requirements beyond existing policy.
F. International Compatibility
In keeping with U.S. obligations under the Convention on
International Civil Aviation, it is FAA policy to conform to
International Civil Aviation Organization (ICAO) Standards and
Recommended Practices (SARPs) to the maximum extent practicable. FAA
has determined that there are currently no ICAO SARPs that correspond
to these proposed regulations. ICAO is planning updates to Annex 10
Volume V intended to help protect RAs from potentially harmful in-band
and adjacent band interference caused by non-aeronautical systems
operating in adjacent frequency bands. FAA will continue to work with
the international community to promote the spectrum compatibility
achieved by the proposed next generation RA system requirements.
Considering these SARPs have yet to be finalized, FAA seeks comment
on the interoperability of the proposed RA requirements across
international airspace and the feasibility of making such updates
within the proposed compliance timeline.
G. Environmental Analysis
The Department has analyzed the environmental impacts of this
proposed rule pursuant to the National Environmental Policy Act of 1969
(NEPA) (42 U.S.C. 4321 et seq.). FAA has determined that this rule is
categorically excluded pursuant to Paragraph B-2.6(d) of Appendix B to
FAA Order 1050.1G, FAA National Environmental Policy Act Implementing
Procedures (90 FR 29615, July 3, 2025). Categorical exclusions are
categories of actions that the agency has determined normally do not
significantly affect the quality of the human environment and therefore
do not require either an environmental assessment (EA) or an
environmental impact statement (EIS). See DOT Order 5610.1D Sec. 9. In
analyzing the applicability of a categorical exclusion, the agency must
also consider whether extraordinary circumstances are present that
would warrant the preparation of an EA or EIS. Id. Sec. 9(b). This
rulemaking, which requires all RAs to meet specific minimum performance
requirements to support resilience to interference from wireless
signals in neighboring spectrum bands, is categorically excluded
pursuant to Paragraph B-2.6(d) of FAA Order 1050.1G: ``Issuance of
regulatory documents (e.g., Notices of Proposed Rulemaking and issuance
of Final Rules) covering administrative or procedural requirements.
(Does not include air traffic procedures; specific air traffic
procedures that are categorically excluded are identified under
Appendix B, Paragraph B-2.5 of this Order).'' FAA does not anticipate
any environmental impacts, and there are no extraordinary circumstances
present in connection with this rulemaking.
H. Regulations Affecting Intrastate Aviation in Alaska
Section 1205 of the FAA Reauthorization Act of 1996 (110 Stat.
3213) requires the Administrator, when modifying 14 CFR regulations in
a manner affecting intrastate aviation in Alaska, to consider the
extent to which Alaska is not served by transportation modes other than
aviation, and to establish appropriate regulatory distinctions. FAA
expects reduced impact because this proposed rule would not apply to
aircraft equipped with RA that only conduct intrastate operations in
Alaska. However, this proposed rule could, if adopted, affect aviation
operations in Alaska because it applies to aircraft equipped with RA
based in Alaska that operate regularly to the 48 contiguous United
States, or aircraft based in the 48 contiguous United States that
operate regularly to and from Alaska. FCC is proposing to preserve the
status quo regarding its current licenses outside of the contiguous
United States, which would be permitted to continue in the entire 3.7-
4.2 GHz band. FCC notes that its proposal to only reallocate spectrum
within the contiguous U.S. would ensure the ongoing provision of
current C-band services necessary to protect life and property outside
the contiguous U.S.--including telehealth, E911, and education
services--for which C-band service may be the only option available,
such as in remote areas of Alaska. Therefore, FAA specifically requests
comments on the suitability of applying the proposed rule differently
for intrastate operations in Alaska.
VI. E.O. Determinations
A. E.O. 13132, Federalism
FAA has analyzed this proposed rule under the principles and
criteria of E.O. 13132, Federalism. FAA has determined this action
would not have a substantial direct effect on the States, or the
relationship between the Federal Government and the States, or on the
distribution of power and responsibilities among the various levels of
government, and, therefore, would not have federalism implications.
B. E.O. 13175, Consultation and Coordination With Indian Tribal
Governments
Consistent with E.O. 13175, Consultation and Coordination with
Indian Tribal Governments,\80\ and FAA Order 1210.20, American Indian
and Alaska Native Tribal Consultation Policy and Procedures,\81\ FAA
ensures Federally Recognized Tribes (Tribes) are given the opportunity
to provide meaningful and timely input regarding proposed Federal
actions that have the potential to affect uniquely or significantly
their respective Tribes. At this point, FAA has not identified any
unique or significant effects, environmental or otherwise, on Tribes
resulting from this proposed rule.
C. E.O. 13211, Regulations That Significantly Affect Energy Supply,
Distribution, or Use
FAA analyzed this proposed rule under E.O. 13211, Actions
Concerning Regulations that Significantly Affect Energy Supply,
Distribution, or Use (May 18, 2001). FAA has determined it would not be
a ``significant energy action'' under the E.O. and would not be likely
to have a significant adverse effect
[[Page 487]]
on the supply, distribution, or use of energy.
D. E.O. 13609, Promoting International Regulatory Cooperation
E.O. 13609, Promoting International Regulatory Cooperation,
promotes international regulatory cooperation to (1) meet shared
challenges involving health, safety, labor, security, environmental,
and other issues and reduce, eliminate, or (2) prevent unnecessary
differences in regulatory requirements. FAA has analyzed this action
under the policy and agency responsibilities of E.O. 13609. FAA has
determined this action would help prevent future differences between
U.S. aviation standards and those of other CAAs by being the first
nation to adopt and require these new RA system performance standards,
to set a standard for future harmonization with other CAAs, and inform
future wireless standards for the spectrum authorities of other nations
who are considering similar spectrum reallocation near the RA band.
E. Executive Order 14192, Unleashing Prosperity Through Deregulation
Executive Order 14192 (Unleashing Prosperity Through Deregulation)
requires that, for each new regulatory rule, an agency must identify 10
prior regulations for elimination. This proposed rule responds to
statutory requirements of section 40002 of the One Big Beautiful Bill
Act, which re-institutes FCC's general auction authority and
specifically directs the Commission to complete a system of competitive
bidding for not less than 100 megahertz in the Upper C-band. To ensure
safe, efficient, and reliable aviation operations in the presence of
wireless signals in the Upper C-band, FAA is proposing new regulations
that would require all RAs to meet specific minimum performance
requirements. This proposed rule, if finalized as proposed, is expected
to be an E.O. 14192 regulatory action.
VII. Additional Information
A. Comments Invited
FAA invites interested persons to participate in this rulemaking by
submitting written comments, data, or views. FAA also invites comments
relating to the economic, environmental, energy, or federalism impacts
that might result from adopting the proposals in this document. The
most helpful comments reference a specific portion of the proposal,
explain the reason for any recommended change, and include supporting
data. To ensure the docket does not contain duplicate comments,
commenters should submit only one time if comments are filed
electronically, or commenters should send only one copy of written
comments if comments are filed in writing.
FAA will file in the docket all comments it receives, as well as a
report summarizing each substantive public contact with FAA personnel
concerning this proposed rule. Before acting on this proposal, FAA will
consider all comments it receives on or before the closing date for
comments. FAA will consider comments filed after the comment period has
closed if it is possible to do so without incurring expense or delay.
FAA may change this proposal in light of the comments it receives.
FCC has initiated a rulemaking on repurposing the 3.98-4.2 GHz band
for advanced wireless services consistent with the One Big Beautiful
Bill Act. As part of that rulemaking, FCC seeks comments on issues
related to adjacent band coexistence.\82\ Interested parties should
also submit comments in FCC's proceeding.
B. Confidential Business Information
Confidential Business Information (CBI) is commercial or financial
information that is both customarily and actually treated as private by
its owner. Under the Freedom of Information Act (FOIA) (5 U.S.C. 552),
CBI is exempt from public disclosure. If your comments responsive to
this NPRM contain commercial or financial information that is
customarily treated as private, that you actually treat as private, and
is relevant or responsive to this NPRM, it is important you clearly
designate the submitted comments as CBI. Please mark each page of your
submission containing CBI as ``PROPIN.'' FAA will treat such marked
submissions as confidential under the FOIA, and they will not be placed
in the public docket of this NPRM. Submissions containing CBI should be
sent to the person in the FOR FURTHER INFORMATION CONTACT section of
this document. Any commentary FAA receives that is not specifically
designated as CBI will be placed in the public docket for this
rulemaking.
C. Electronic Access and Filing
A copy of this NPRM, all comments received, any final rule, and all
background material may be viewed online at www.regulations.gov using
the docket number listed above. Electronic retrieval help and
guidelines are available on the website. It is available 24 hours each
day, 365 days each year. An electronic copy of this document may also
be downloaded from the Office of the Federal Register's website at
www.federalregister.gov and the Government Publishing Office's website
at www.govinfo.gov. A copy may also be found at FAA's Regulations and
Policies website at www.faa.gov/regulations_policies.
Copies may also be obtained by sending a request to the Federal
Aviation Administration, Office of Rulemaking, ARM-1, 800 Independence
Avenue SW, Washington, DC 20591, or by calling (202) 267-9677.
Commenters must identify the docket or notice number of this
rulemaking.
All documents FAA considered in developing this proposed rule,
including economic analyses and technical reports, may be accessed in
the electronic docket for this rulemaking.
D. Small Business Regulatory Enforcement Fairness Act
The Small Business Regulatory Enforcement Fairness Act of 1996
(Pub. L. 104 121, 110 Stat. 857, Mar. 29, 1996) requires FAA to comply
with small entity requests for information or advice about compliance
with statutes and regulations within its jurisdiction. A small entity
with questions regarding this document may contact its local FAA
official, or the person listed under the FOR FURTHER INFORMATION
CONTACT heading at the beginning of the preamble. To find out more
about SBREFA on the internet, visit www.faa.gov/regulations_policies/rulemaking/sbre_act/.
Endnotes
\1\ RA systems are generally comprised of a transceiver,
cabling, and antennas. When necessary, different terms are used
throughout the NPRM to discuss the RA system as a whole or specific
parts of it.
\2\ Available at https://www.fcc.gov/ecfs/search/search-filings/filing/1022884849315.
\3\ Public Law 119-21 (July 4, 2025), available at https://www.congress.gov/bill/119th-congress/house-bill/1.
\4\ In the Matter of Upper C-band (3.98-4.2 GHz), 90 FR 56076
(proposed December 5, 2025) available at https://www.federalregister.gov/documents/2025/12/05/2025-22020/in-the-matter-of-upper-c-band-398-42-ghz.
\5\ Letter from Henry G. Hultquist, Vice President-Federal
Regulatory, AT&T Services, Inc., et al., to Marlene H. Dortch,
Secretary, FCC, GN Docket No. 18-122 (filed Mar. 31, 2023)
(discussing voluntary commitments), available at https://www.fcc.gov/ecfs/search/search-filings/filing/1033142661477.
\6\ https://www.rtca.org/sc-239/.
\7\ https://www.eurocae.net/working-group/wg-119/.
[[Page 488]]
\8\ See Letter from Dorothy B. Reimold, Vice President Civil
Aviation, Aerospace Industries Assoc., et al., to Marlene H. Dortch,
Secretary, FCC, GN Docket No. 25-59, at 1-2 (filed Aug. 21, 2025).
\9\ National Telecommunications and Information Administration
(NTIA) Comments re Upper C-band NOI, available at: https://www.fcc.gov/ecfs/search/search-filings/filing/1080426626787.
\10\ Speech: ``Downward Pressure on the Accident Rate''.
Nicholas A. Sabatini. International Society of Air Safety
Investigators. May 12, 2006. Retrieved September 2025 from China
Aviation Daily, http://www.chinaaviationdaily.com/news/0/456.html.
\11\ The aircraft used in this analysis average 14 to 21 years
in age depending on CFR operation. While aircraft retirement age can
vary significantly depending on multiple factors, FAA assumes
impacted aircraft will average 20 more years of service with an
updated RA system installed.
\12\ In the Matter of Expanding Flexible Use in the 3.7-4.2 GHz
Band, GN Docket No. 18-122, Federal Communications Commission (March
3, 2020), available at https://www.fcc.gov/document/fcc-expands-flexible-use-c-band-5g-0.
\13\ RTCA Paper No. 274-20/PMC-2073, Assessment of C-Band Mobile
Telecommunications Interference Impact on Low Range Radar Altimeter
Options, dated October 7, 2020, page i. This document is available
on www.regulations.gov in Docket No. FAA-2021-0953, and at https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-Interference-Assessment-Report_274-20-PMC-2073_accepted_changes.pdf.
\14\ Airworthiness Directive; Transport and Commuter Category
Airplanes, 86 FR 69984 (12/09/21) available at https://www.federalregister.gov/documents/2021/12/09/2021-26777/airworthiness-directives-transport-and-commuter-category-airplanes.
\15\ Airworthiness Directives; Various Helicopters 86 FR 6992
(12/09/21) available at https://www.federalregister.gov/documents/2021/12/09/2021-26779/airworthiness-directives-various-helicopters.
\16\ AD 2022-03-05 for Boeing 747-8 and 777 models, AD 2022-02-
16 for Boeing 787 models, AD 2022-03-20 for newer Boeing 737 models,
AD 2022-04-05 for Boeing 757 and 767 models, AD 2022-05-04 for older
Boeing 737 models, AD 2022-06-16 for older Boeing 747 models, and AD
2022-09-18 for legacy McDonnell Douglas models.
\17\ https://www.fcc.gov/document/fcc-announces-winning-bidders-37-ghz-service-auction.
\18\ For example, one voluntary agreement is available at
https://www.fcc.gov/ecfs/search/search-filings/filing/1033142661477.
\19\ Airworthiness Directives; Transport and Commuter Category
Airplanes, 88 FR 34065 (May 26, 2023) available at https://www.federalregister.gov/documents/2023/05/26/2023-11371/airworthiness-directives-transport-and-commuter-category-airplanes.
\20\ Airworthiness Directives; Various Helicopters, 88 FR 40685
(June 22, 2023) available at https://www.federalregister.gov/documents/2023/06/22/2023-13319/airworthiness-directives-various-helicopters.
\21\ AD 2023-12-05 for Boeing 747-8 and 777 models, AD 2023-12-
10 for Boeing 787 models, AD 2023-12-11 for newer Boeing 737 models,
AD 2023-12-12 for Boeing 757 and 767 models, AD 2023-12-13 for older
Boeing 737 models, AD 2023-12-14 for older Boeing 747 models, and AD
2023-12-15 for legacy McDonnell Douglas models.
\22\ AD 2025-04-08 for MHI RJ regional jet models; AD 2023-06-13
for Bombardier Model BD-700-2A12 airplanes, which was subsequently
replaced by AD 2023-14-01; AD 2023-03-06 for Bombardier Model BD-
700-1A10 and -1A11 airplanes, which was subsequently replaced by AD
2023-13-15; AD 2023-06-13 for Bombardier Model BD-700-2A12
airplanes; and AD 2023-14-02 for Airbus Model BD-500-1A10 and -1A11
airplanes.
\23\ https://www.rtca.org/sc-239/.
\24\ https://www.eurocae.net/working-group/wg-119/.
\25\ See Letter from Dorothy B. Reimold, Vice President Civil
Aviation, Aerospace Industries Assoc., et al., to Marlene H. Dortch,
Secretary, FCC, GN Docket No. 25-59, at 1-2 (filed Aug. 21, 2025).
\26\ Minimum Performance Standards--Airborne Ground Proximity
Warning Equipment, RTCA/DO-161A, RTCA Incorporated.
\27\ Ground Proximity Warning Systems, 39 FR 44439 (Dec. 18,
1974).
\28\ Special Federal Aviation Regulation No. 30; Ground
Proximity Warning System, 43 FR 28176 (June 29, 1978).
\29\ Ground Proximity Warning Systems, 57 FR 9944 (March 20,
1992).
\30\ Terrain Awareness and Warning System, 65 FR 16736 (March
29, 2001).
\31\ Public Law 100-223, 101 Stat. 1486 (1987), available at
https://www.govinfo.gov/app/details/STATUTE-101/STATUTE-101-Pg1486.
\32\ Traffic Alert and Collision Avoidance System, 54 FR 940
(Jan. 10, 1989).
\33\ Collision Avoidance Systems, 68 FR 15884 (Apr. 1, 2003).
\34\ Regulation of Fractional Aircraft Ownership Programs and
On-Demand Operations, 68 FR 54520 (Sept. 17, 2003).
\35\ See Airborne Low-Altitude Windshear Equipment Requirements,
55 FR 13236 (Apr. 9, 1990).
\36\ Public Law 112-95, 126 Stat. 11 (Feb. 14, 2012) available
at https://www.congress.gov/bill/112th-congress/house-bill/658/text.
\37\ Helicopter Air Ambulance, Commercial Helicopter, and Part
91 Helicopter Operations, 79 FR 9932 (Feb. 21, 2014), available at
https://www.federalregister.gov/documents/2014/02/21/2014-03689/helicopter-air-ambulance-commercial-helicopter-and-part-91-helicopter-operations.
\38\ Integration of Powered-Lift: Pilot Certification and
Operations; Miscellaneous Amendments Related to Rotorcraft and
Airplanes, 89 FR 92296 (Nov. 21, 2024), available at https://www.federalregister.gov/documents/2024/11/21/2024-24886/integration-of-powered-lift-pilot-certification-and-operations-miscellaneous-amendments-related-to.
\39\ Pilot, Flight Instructor, and Pilot School Certification,
74 FR 42500 (Aug. 21, 2009).
\40\ https://my.rtca.org/productdetails?id=a1B36000001IcnqEAC.
\41\ https://www.eurocae.net/product/ed-30-mps-for-airborne-low-range-radio-radar-altimeter-equipment/.
\42\ https://drs.faa.gov/browse/excelExternalWindow/3E13DA064E29A5F586257A1B005889A8.0001.
\43\ Airworthiness Directives; Transport and Commuter Category
Airplanes, 88 FR 34065 (May 26, 2023) available at https://www.federalregister.gov/documents/2023/05/26/2023-11371/airworthiness-directives-transport-and-commuter-category-airplanes.
\44\ See Letter from Dorothy B. Reimold, Vice President Civil
Aviation, Aerospace Industries Assoc., et al., to Marlene H. Dortch,
Secretary, FCC, GN Docket No. 25-59, at 1-2 (filed Aug. 21, 2025).
\45\ FAA Advisory Circular (AC) 20-199, Installation of an
Airborne Low-Range Radio Altimeter System, will be available for
review and comment at https://www.faa.gov/aircraft/draft_docs, under
``Aircraft Certification Service (AIR) Draft Documents''
\46\ NPRM published in 90 FR 38212.
\47\ https://www.govinfo.gov/content/pkg/USCODE-2023-title49/pdf/USCODE-2023-title49-subtitleVII-partA-subpartiii-chap448-sec44809.pdf.
\48\ In the Matter of Upper C-band (3.98-4.2 GHz), 90 FR 56076
(proposed December 5, 2025), paragraph 20, available at https://www.federalregister.gov/documents/2025/12/05/2025-22020/in-the-matter-of-upper-c-band-398-42-ghz.
\49\ In the Matter of Upper C-band (3.98-4.2 GHz), 90 FR 56076
(proposed December 5, 2025), paragraphs 119-122, available at
https://www.federalregister.gov/documents/2025/12/05/2025-22020/in-the-matter-of-upper-c-band-398-42-ghz.
\50\ Defined as conducting at least one U.S. operation between
04/01/2024 and 09/01/2025.
\51\ AC 150/5390-2D, Heliport Design, available at https://www.faa.gov/airports/resources/advisory_circulars/index.cfm/go/document.current/documentnumber/150_5390-2.
\52\ Letter from Henry G. Hultquist, Vice President-Federal
Regulatory, AT&T Services, Inc., et al., to Marlene H. Dortch,
Secretary, FCC, GN Docket No. 18-122 (filed Mar. 31, 2023)
(discussing voluntary commitments), available at https://www.fcc.gov/ecfs/search/search-filings/filing/1033142661477.https://www.fcc.gov/ecfs/search/search-filings/filing/1033142661477.
\53\ Demonstration of Radio Altimeter Tolerant Aircraft, 88 FR
46055 (July 19, 2023), available at https://www.federalregister.gov/documents/2023/07/19/2023-14927/demonstration-of-radio-altimeter-tolerant-aircraft.
\54\ Airworthiness Directives; Transport and Commuter Category
Airplanes, 88 FR 34065 (05/26/2023) available at https://www.federalregister.gov/documents/2023/05/26/2023-11371/airworthiness-directives-transport-and-commuter-category-airplanes.
\55\ Airworthiness Directives; Various Helicopters, 88 FR 40685
(06/22/2023) available at https://www.federalregister.gov/documents/2023/06/22/2023-13319/airworthiness-directives-various-helicopters.
\56\ Risk of Potential Adverse Effects on Radio Altimeters (RA)
when Operating in the
[[Page 489]]
Presence of 5G C-Band Wireless Broadband Signals, SAFO 21007 (Dec.
23, 2021), available at https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/SAFO21007R1.pdf.
\57\ Clarification to FAA Order 8400.13, Procedures for the
Evaluation and Approval of Facilities for Special Authorization
Category I Operations and All Category II and III Operations,
available at https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afx/afs/afs400/afs420/order_ac_memo/Clarification_Order_8400.13_5G-C-Band.pdf.
\58\ Colo Void Clause Coalition; Antenna Systems Co-Location;
Voluntary Best Practices, 87 FR 39746 (July 5, 2022), available at
https://www.federalregister.gov/documents/2022/07/05/2022-14306/colo-void-clause-coalition-antenna-systems-co-location-voluntary-best-practices.
\59\ Airworthiness Directives; Transport and Commuter Category
Airplanes, 88 FR 34065 (May 26, 2023) available at https://www.federalregister.gov/documents/2023/05/26/2023-11371/airworthiness-directives-transport-and-commuter-category-airplanes;
Airworthiness Directives; Various Helicopters, 88 FR 40685 (June 22,
2023) available at https://www.federalregister.gov/documents/2023/06/22/2023-13319/airworthiness-directives-various-helicopters; and
AD 2023-12-05 for Boeing 747-8 and 777 models, AD 2023-12-10 for
Boeing 787 models, AD 2023-12-11 for newer Boeing 737 models, AD
2023-12-12 for Boeing 757 and 767 models, AD 2023-12-13 for older
Boeing 737 models, AD 2023-12-14 for older Boeing 747 models, and AD
2023-12-15 for legacy McDonnell Douglas models.
\60\ OMB Circular A-4 can be found at https://www.whitehouse.gov/wp-content/uploads/2025/08/CircularA-4.pdf.
\61\ The MITRE Corporation (MITRE) is a private, not-for-profit
company that provides research and development services, primarily
to the federal government. The data provided by MITRE consists of
FAA fleet data combined with RA equipage specifications and number
of aircraft operations.
\62\ The core 30 airports are the busiest 30 U.S. commercial
airports by passenger emplacements, the list of which can be found
at https://www.aspm.faa.gov/aspmhelp/index/Core_30.html.
\63\ The Aerology analysis of 2019 METAR data from the core 30
airports can be found at https://aerology.substack.com/p/what-does-low-visibility-mean.
\64\ Data on operations is sourced from the FAA Operations
Network (OPSNET), with the 2024 data provided in the most recent FAA
Air Traffic by the Numbers found at https://www.faa.gov/air_traffic/by_the_numbers.
\65\ The airspace limitation order can be found at https://www.federalregister.gov/documents/2025/11/12/2025-19850/emergency-order-establishing-operating-limitations-on-the-use-of-navigable-airspace.
\66\ The A4A cost estimate can be found in their November 10,
2025 press release at https://www.airlines.org/news-update/new-data-shows-huge-impact-of-the-government-shutdown-on-airlines-and-our-customers/.
\67\ The part 129 foreign carrier totals for this fleet only
include operators and aircraft that have had at least one U.S.
operation in the 17-month period from April 1, 2024, to September 1,
2025.
\68\ Speech: ``Downward Pressure on the Accident Rate''.
Nicholas A. Sabatini. International Society of Air Safety
Investigators. May 12, 2006. Retrieved September 2025 from China
Aviation Daily. http://www.chinaaviationdaily.com/news/0/456.html.
\69\ Transport and commuter category airplane costs are found in
the associated final rule for Airworthiness Directive; Transport and
Commuter Category Airplanes (05/26/2023), available at https://www.federalregister.gov/documents/2023/05/26/2023-11371/airworthiness-directives-transport-and-commuter-category-airplanes.
\70\ Rotorcraft costs are found in the associated final rule for
Airworthiness Directives; Various Helicopters (12/09/21) available
at https://www.federalregister.gov/documents/2021/12/09/2021-26779/airworthiness-directives-various-helicopters.
\71\ The 10,341 RA units for aircraft operating under part 129
do not include units that are covered under the second compliance
deadline (786 airplane and 27 rotorcraft RAs).
\72\ This group consists of the 16,657 aircraft under part 91,
8,314 aircraft under part 135, and the 18 rotorcraft and 531
airplanes with less than 30 passenger seats or 7,500 lbs. cargo
capacity under part 129.
\73\ The IBA estimate is found at https://www.iba.aero/resources/articles/the-direct-cost-of-grounding-the-boeing-737-max-8-fleet/.
\74\ The 2010 Total Delay Impact Study can be found at https://rosap.ntl.bts.gov/view/dot/6234.
\75\ The BLS CPI-U values can be found at https://data.bls.gov/timeseries/CUUR0000SA0.
\76\ New ADs to address Upper C-band RA interference would
likely align with current ADs addressing Lower C-band interference
and would prohibit operating Boeing 777s and 747-8s, making up 295
of the 8,014 airplanes operating under part 121 and 1,100 out of
5,050 operating under part 129, with the potential for issuance of
further ADs covering additional models.
\77\ Information on NAICS can be found at https://www.census.gov/naics/.
\78\ Transport and commuter category airplane costs are found in
the associated final rule for Airworthiness Directive; Transport and
Commuter Category Airplanes (05/26/2023), available at https://www.federalregister.gov/documents/2023/05/26/2023-11371/airworthiness-directives-transport-and-commuter-category-airplane
and rotorcraft costs are found in the associated final rule for
Airworthiness Directives; Various Helicopters (12/09/21) available
at https://www.federalregister.gov/documents/2021/12/09/2021-26779/airworthiness-directives-various-helicopters.
\79\ The 2022 U.S. SUSB files can be found at https://www.census.gov/data/tables/2022/econ/susb/2022-susb-annual.html.
\80\ 65 FR 67249 (Nov. 6, 2000).
\81\ FAA Order No. 1210.20 (Jan.28, 2004), available at
www.faa.gov/documentLibrary/media/1210.pdf.
\82\ In the Matter of Upper C-band (3.98-4.2 GHz), 90 FR 56076
(proposed December 5, 2025) available at https://www.federalregister.gov/documents/2025/12/05/2025-22020/in-the-matter-of-upper-c-band-398-42-ghz.
List of Subjects
14 CFR Part 91
Air carriers, Air taxis, Aircraft, Aviation safety.
14 CFR Part 121
Air carriers, Aircraft, Aviation safety, Safety.
14 CFR Part 129
Air carriers, Aircraft, Aviation safety.
The Proposed Amendment
For the reasons discussed in the preamble, the Federal Aviation
Administration proposes to amend chapter I of title 14, Code of Federal
Regulations as follows:
PART 91--GENERAL OPERATING AND FLIGHT RULES
0
1. The authority citation for part 91 continues to read as follows:
Authority: 49 U.S.C. 106(f), 40101, 40103, 40105, 40113, 40120,
44101, 44111, 44701, 44704, 44709, 44711, 44712, 44715, 44716,
44717, 44722, 46306, 46315, 46316, 46504, 46506-46507, 47122, 47508,
47528-47531, 47534; Pub. L. 114-190, 130 Stat. 615 (49 U.S.C. 44703
note); Sec. 828 of Pub. L. 118-63, 138 Stat. 1330 (49 U.S.C. 44703
note); articles 12 and 29 of the Convention on International Civil
Aviation (61 Stat. 1180), (126 Stat. 11).
0
2. Add Sec. 91.220 to read as follows:
Sec. 91.220 Radio Altimeter Systems
(a) After [two years after the initial RA performance deadline], no
person may operate an aircraft in the airspace of the 48 contiguous
United States and the District of Columbia with a radio altimeter
unless the radio altimeter system meets the performance requirements of
paragraph (b).
(b) The radio altimeter system must operate at an altitude of 0-500
feet above ground level in the interference environment defined in
table 1:
[[Page 490]]
Table 1
------------------------------------------------------------------------
Power flux-density, single
Frequency range (MHz) polarization, RMS (dBW/m\2\/
MHz)
------------------------------------------------------------------------
3000 <= f < 4000.......................... 9.5
4000 <= f < 4100.......................... 9.5
4100 <= f < 4150.......................... 9.5
4150 <= f < 4160.......................... 6.5
4160 <= f < 4180.......................... -1
4180 <= f < 4190.......................... -17
4190 <= f < 4200.......................... -34
------------------------------------------------------------------------
4200 <= f <= 4400......................... -82
------------------------------------------------------------------------
4400 < f <= 4410.......................... -33
4410 < f <= 4430.......................... -21
4430 < f <= 4440.......................... -8
4440 < f <= 4450.......................... -1
4450 < f <= 4460.......................... 6.5
4460 < f <= 4500.......................... 9.5
4500 < f <= 4600.......................... 9.5
4600 < f <= 5600.......................... 9.5
------------------------------------------------------------------------
PART 121--GENERAL OPERATING AND FLIGHT RULES
0
3. The authority citation for part 121 continues to read as follows:
Authority: 49 U.S.C. 106(f), 40103, 40113, 40119, 41706, 42301
preceding note added by Pub. L. 112-95, sec. 412, 126 Stat. 89,
44101, 44701-44702, 44705, 44709-44711, 44713, 44716-44717, 44722,
44729, 44732; 46105; Pub. L. 111-216, 124 Stat. 2348 (49 U.S.C.
44701 note); Pub. L. 112-95, 126 Stat. 62 (49 U.S.C. 44732 note);
Pub. L. 115-254, 132 Stat. 3186 (49 U.S.C. 44701 note).
0
4. Add Sec. 121.326 to read as follows:
Sec. 121.326 Radio Altimeter Systems
After [the initial RA performance deadline], no person may operate
an aircraft under this part in the airspace of the 48 contiguous United
States and the District of Columbia with a radio altimeter unless the
radio altimeter system meets the performance requirements of Sec.
91.220(b) of this chapter.
PART 129--GENERAL OPERATING AND FLIGHT RULES
0
5. The authority citation for part 129 continues to read as follows:
Authority: 49 U.S.C. 1372, 40113, 40119, 44101, 44701-44702,
44705, 44709-44711, 44713, 44716-44717, 44722, 44901-44904, 44906,
44912, 46105, Pub. L. 107-71 sec. 104.
0
6. Add Sec. 129.16 to read as follows:
Sec. 129.16 Radio Altimeter Systems
(a) After [the initial RA performance deadline], no person may
operate an aircraft with 30 or more passenger seats or a payload
capacity of more than 7,500 pounds under this part in the airspace of
the 48 contiguous United States and the District of Columbia with a
radio altimeter unless the radio altimeter system meets the performance
requirements of Sec. 91.220(b) of this chapter.
(b) After [two years after the initial RA performance deadline], no
person may operate an aircraft under this part in the airspace of the
48 contiguous United States and the District of Columbia with a radio
altimeter unless the radio altimeter system meets the performance
requirements of Sec. 91.220(b) of this chapter.
Issued under authority provided by 49 U.S.C. 106(f) and
44701(a), in Washington, DC.
Hugh J. Thomas,
Acting Executive Director, Flight Standards Service.
[FR Doc. 2026-00051 Filed 1-5-26; 4:15 pm]
BILLING CODE 4910-13-P