[Federal Register Volume 88, Number 7 (Wednesday, January 11, 2023)]
[Proposed Rules]
[Pages 1520-1528]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2023-00420]


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 Proposed Rules
                                                 Federal Register
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 This section of the FEDERAL REGISTER contains notices to the public of 
 the proposed issuance of rules and regulations. The purpose of these 
 notices is to give interested persons an opportunity to participate in 
 the rule making prior to the adoption of the final rules.
 
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  Federal Register / Vol. 88, No. 7 / Wednesday, January 11, 2023 / 
Proposed Rules  

[[Page 1520]]



DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2022-1647; Project Identifier AD-2022-01379-T]
RIN 2120-AA64


Airworthiness Directives; Transport and Commuter Category 
Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: The FAA proposes to supersede Airworthiness Directive 2021-23-
12, which applies to all transport and commuter category airplanes 
equipped with a radio (also known as radar) altimeter. AD 2021-23-12 
requires revising the limitations section of the existing airplane/
aircraft flight manual to incorporate limitations prohibiting certain 
operations requiring radio altimeter data when in the presence of 5G C-
Band interference as identified by Notices to Air Missions. Since the 
FAA issued AD 2021-23-12, the FAA determined that additional 
limitations are needed due to the continued deployment of new 5G C-Band 
base stations whose signals are expected to cover most of the 
contiguous United States at transmission frequencies between 3.7-3.98 
GHz. This proposed AD would require revising the limitations section of 
the existing airplane/aircraft flight manual to incorporate limitations 
prohibiting certain operations requiring radio altimeter data, due to 
the presence of 5G C-Band interference. This proposed AD would also 
require modifying certain airplanes to allow safe operations in the 
United States 5G C-Band radio frequency environment. The FAA is issuing 
this AD to address the unsafe condition on these products.

DATES: The FAA must receive comments on this proposed AD by February 
10, 2023.

ADDRESSES: You may send comments, using the procedures found in 14 CFR 
11.43 and 11.45, by any of the following methods:
     Federal eRulemaking Portal: Go to regulations.gov. Follow 
the instructions for submitting comments.
     Fax: 202-493-2251.
     Mail: U.S. Department of Transportation, Docket 
Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New 
Jersey Avenue SE, Washington, DC 20590.
     Hand Delivery: Deliver to Mail address above between 9 
a.m. and 5 p.m., Monday through Friday, except Federal holidays.
    AD Docket: You may examine the AD docket at regulations.gov under 
Docket No. FAA-2022-1647; or in person at Docket Operations between 9 
a.m. and 5 p.m., Monday through Friday, except Federal holidays. The AD 
docket contains this NPRM, any comments received, and other 
information. The street address for Docket Operations is listed above.

FOR FURTHER INFORMATION CONTACT: Brett Portwood, Continued Operational 
Safety Technical Advisor, COS Program Management Section, Operational 
Safety Branch, FAA, 3960 Paramount Boulevard, Lakewood, CA 90712-4137; 
phone: 817-222-5390; email: [email protected].

SUPPLEMENTARY INFORMATION:

Comments Invited

    The FAA invites you to send any written relevant data, views, or 
arguments about this proposal. Send your comments to an address listed 
under ADDRESSES. Include ``Docket No. FAA-2022-1647; Project Identifier 
AD-2022-01379-T'' at the beginning of your comments. The most helpful 
comments reference a specific portion of the proposal, explain the 
reason for any recommended change, and include supporting data. The FAA 
will consider all comments received by the closing date and may amend 
the proposal because of those comments.
    Except for Confidential Business Information (CBI) as described in 
the following paragraph, and other information as described in 14 CFR 
11.35, the FAA will post all comments received, without change, to 
regulations.gov, including any personal information you provide. The 
agency will also post a report summarizing each substantive verbal 
contact received about this proposed AD.

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 that is relevant or responsive to 
this NPRM, it is important that you clearly designate the submitted 
comments as CBI. Please mark each page of your submission containing 
CBI as ``PROPIN.'' The 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 Brett 
Portwood, Continued Operational Safety Technical Advisor, COS Program 
Management Section, Operational Safety Branch, FAA, 3960 Paramount 
Boulevard, Lakewood, CA 90712-4137; phone: 817-222-5390; email: 
[email protected]. Any commentary that the FAA receives that is 
not specifically designated as CBI will be placed in the public docket 
for this rulemaking.

Background

    The FAA issued Airworthiness Directive (AD) 2021-23-12, Amendment 
39-21810 (86 FR 69984, December 9, 2021) (AD 2021-23-12), for all 
transport and commuter category airplanes equipped with a radio 
altimeter. AD 2021-23-12 was prompted by a determination that radio 
altimeters cannot be relied upon to perform their intended function if 
they experience interference from wireless broadband operations in the 
3.7-3.98 GHz frequency band (5G C-Band). AD 2021-23-12 requires 
revising the limitations section of the existing airplane/aircraft 
flight manual (AFM) to incorporate limitations prohibiting certain 
operations requiring radio altimeter data when in the presence of 5G C-
Band interference as identified by Notices to Air Missions (NOTAMs). 
The agency issued AD 2021-23-12 because radio altimeter anomalies that 
are undetected by the automation or pilot,

[[Page 1521]]

particularly close to the ground (e.g., landing flare), could lead to 
loss of continued safe flight and landing.

Actions Since AD 2021-23-12

    Airplane Capability and Alterations: Since issuing AD 2021-23-12, 
the FAA has reviewed data from dozens of alternative method of 
compliance (AMOC) requests, demonstrating that these radio altimeters 
can be relied upon to perform their intended function when operating 
beyond a certain protection radius around 5G C-Band transmitters. The 
iterative AMOC process allowed the FAA to gain insight into 5G C-Band 
transmission impacts to runway safety zones \1\ in a progressively more 
sophisticated manner. At first, the FAA made conservative assumptions 
about the potential for impact on radio altimeters from 5G C-Band 
transmissions and applied them to all airport environments. During the 
FAA's initial analyses of AMOC requests, the FAA looked to protect 
against 5G C-Band interference during the most critical phases of 
flight (takeoffs and landings) by protecting a 2-nautical mile circle 
around the ends of runways. After some time and an improved 
understanding of the C-Band signals and their effects on specific radio 
altimeters, the FAA was able to reduce the protected area around the 
ends of runways and instead define a rectangular airspace area to 
protect around runways. The rectangular area was further refined into a 
trapezoidal area, which allowed for geographically expanded 5G C-Band 
transmissions that would not affect radio altimeter functions within 
the area. The FAA is now able to assess the 5G C-Band transmissions' 
impact to aviation operations in a specific area, taking into account 
the particularities of the signal and the airport environment. This 
assessment process is the Signal in Space (SiS) analysis. It includes a 
3-dimensional model for the runway safety zone and considers base 
station heights and terrain around the airport.
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    \1\ Runway safety zones are those areas around a runway where 
radio altimeters on transport and commuter category airplanes must 
function accurately and reliably during critical phases of flight 
where radio altimeter interference is most likely to result in a 
catastrophic accident.
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    The AMOC process also provided data about the varying levels of 
interference tolerance for a majority of radio altimeters on the 
market, allowing the FAA to understand the overall susceptibility to 
interference of the existing fleet of transport airplanes. In addition, 
the FAA learned about the aircraft alterations that can be accomplished 
quickly to improve a radio altimeter's tolerance to transmissions in 
adjacent or nearby spectrum bands. Now that the FAA better understands 
the performance of specific radio altimeters and the means to make them 
more tolerant of transmissions in adjacent or nearby spectrum bands, 
the FAA is proposing the updated corrective action presented in this 
proposed AD.
    5G Compatibility: AMOCs allowing operations otherwise prohibited by 
AD 2021-23-12 were based on voluntary operational mitigations 
undertaken by AT&T and Verizon, 5G C-Band licensees. The FAA, AT&T, and 
Verizon have collaborated extensively to ensure 5G C-Band radio 
frequency transmissions and aircraft operations can safely co-exist. In 
early January 2022, the FAA progressively tailored runway protection 
zones around airports to envelop only the airspace areas where critical 
phases of flight occur. The FAA has worked with AT&T and Verizon to 
improve the precision of the FAA's interference analyses used during 
the AMOC process. In turn, AT&T and Verizon coordinated their 
deployment around 5G C-Band mitigated airports (5G CMAs),\2\ including 
in some cases reducing emission power around airports and committing to 
antenna pointing angles in the vertical plane to limit the potential 
for interference within the tailored runway safety zones. This 
collaborative work has allowed safe transport and commuter airplane 
operations to continue in the short term.
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    \2\ For purposes of this proposed AD, a ``5G C-Band mitigated 
airport'' is an airport at which AT&T and Verizon have agreed to 
voluntarily limit their 5G deployment at the request of the FAA. The 
FAA will provide a list of these airports in the United States 
through the FAA Domestic Notice system. More information about 
Domestic Notices can be found on the FAA website at https://www.faa.gov/air_traffic/publications/domesticnotices/dom1_foreword.html.
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    Update to Safety Determination: The FAA's initial determination 
that radio altimeters cannot be relied upon to perform their intended 
function if they experience interference from wireless broadband 
operations in the 5G C-Band remains unchanged. Therefore, this proposed 
AD would continue prohibiting the use of the same operations identified 
in the original AD (AD 2021-23-12) except for the prohibition of 
Required Navigation Performance with Authorization Required (RNP AR) 
Instrument Approach Procedures (IAP). After further analysis, the FAA 
has determined that 5G C-Band interference does not create an unsafe 
condition for an airplane conducting RNP AR IAPs because RNP AR 
operations do not rely on direct radio altimeter inputs to determine 
arrival at altitude minimums or the flight path of the airplane. 
Therefore, this proposed AD would no longer prohibit RNP AR IAPs.
    The FAA has also gained a better understanding of 5G C-Band 
interference beyond its effect on the operations prohibited by AD 2021-
23-12. Since 5G C-band deployment began, the FAA has solicited reports 
of radio altimeter anomalies from aircraft pilots and operators.\3\ The 
FAA has received over 420 reports of radio altimeter anomalies 
occurring within a known location of a 5G C-Band deployment. 
Approximately 315 of these reports were determined to not be related to 
5G C-Band interference and were resolved through normal continued 
operational safety procedures. But for roughly 100 of the anomaly 
reports occurring within NOTAM areas, the FAA has excluded other 
potential causes for the anomaly, but could not rule out 5G C-Band 
interference as the potential source of the radio altimeter anomalies. 
These approximately 100 incidents included various flight deck effects 
such as erroneous Terrain Awareness and Warning System (TAWS) warnings, 
erroneous Traffic Collision Avoidance System (TCAS) warnings, erroneous 
landing gear warnings, and the erroneous display of radio altimeter 
data. Although these flight deck effects are less severe than the 
hazards associated with low-visibility landings, the FAA is concerned 
that to the extent 5G C-Band operations contributed to such events, the 
effects will occur more frequently as telecommunication companies 
continue to deploy 5G C-Band services throughout the country. The FAA 
has assessed the cumulative effects of increasing numbers of erroneous 
warnings across the fleet of transport and commuter airplanes. Although 
they may seem minor in isolation such that some may consider them a 
mere nuisance, these warnings have safety implications over time. The 
erroneous warnings increase flightcrew workload as they try to 
ascertain the validity of the warning. Repeated determinations that the 
warning occurred in error will lead to flightcrew desensitization to 
warnings from these safety systems.\4\ In other words, as the

[[Page 1522]]

flightcrew becomes more desensitized to erroneous warnings, they are 
less likely to react to an accurate warning, negating the safety 
benefits of the warning altogether and likely leading to a catastrophic 
incident.
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    \3\ Special Airworthiness Information Bulletin AIR-21-18R1 and 
subsequent revisions encouraged pilots to submit detailed reports of 
radio altimeter anomalies using the Radio Altimeter Anomaly 
Reporting Form available on the FAA website at www.faa.gov/air_traffic/nas/RADALT_reports/.
    \4\ FAA research on nuisance alerts in the air traffic control 
(ATC) environment has shown that nuisance alerts can desensitize 
people toward the alert and lead to slower responses to real alerts. 
When people experience frequent false or low-urgency alerts, they 
tend to respond less quickly and less accurately to real and high-
urgency alerts. Further, when there is a high incidence of nuisance 
alerts, people may suppress the alert before determining its actual 
status or may no longer treat the alert as mandatory. In both cases, 
overall alarm compliance decreases and they may stop responding to 
every alert. See Nuisance Alerts in Operational ATC Environments: 
Classification and Frequencies, Friedman-Berg, Allendoerfer, and Pai 
(2008). A copy of this paper can be found on the FAA website at 
https://hf.tc.faa.gov/publications/2008-nuisance-alerts-in-operational-atc-environments/full_text.pdf.
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    AD 2021-23-12 does not mitigate the hazards associated with 
erroneous system warnings, focusing instead on the potentially more 
severe hazards associated with certain low visibility operations. 
Additionally, AD 2021-23-12 does not address other operations near 
airports, such as Category I instrument landing system (ILS) or visual 
flight rule (VFR) approaches. Therefore, the FAA has determined that 
additional corrective action is required to address this unsafe 
condition and proposes to supersede AD 2021-23-12.
    Why New Corrective Action is Needed: In addition to the hazards due 
to the cumulative effects of nuisance warnings described earlier, the 
FAA expects an increase in the number of 5G C-Band base stations around 
airports in the national airspace system (NAS) and expects these 
stations to transmit in the entire 5G C-Band frequency band (from 3.7 
to 3.98 GHz). Since the FAA issued AD 2021-23-12, which focused solely 
on the airport environment, 5G C-Band base stations have increasingly 
begun transmission in other areas of the country. Whereas 5G 
transmissions were initially limited to 3.7 to 3.8 GHz, these 
transmissions have also begun to expand to 3.8 to 3.98 GHz, and the FAA 
expects deployment at the higher end of the frequency range to expand 
after July 1, 2023.\5\ These higher frequencies are nearer to the 
spectrum allocation where radio altimeters operate (4.2 to 4.4 GHz), 
which means that the potential for interference to radio altimeters 
from in-band and spurious \6\ emissions may be more likely. In 
addition, the FAA expects approximately 19 additional telecommunication 
companies in addition to AT&T and Verizon will begin transmitting in 
the C-Band at some point after June 2023.\7\ As the 21 
telecommunication companies authorized to transmit 5G C-Band continue 
to expand transmissions throughout the country, using NOTAMs to 
identify affected areas and assessing proposed AMOCs will become 
untenable. NOTAMs are temporary means of disseminating information 
until the information can be publicized by other means. Given 5G C-Band 
signals are not expected to be temporary and that 5G signals will cover 
the contiguous U.S., NOTAMs are no longer the best means of 
communicating the location of the 5G C-Band environment. In addition, 
given the information gleaned over the past year, the FAA is now able 
to identify the conditions under which radio altimeters can be relied 
on to perform their intended function in the presence of a 5G C-Band 
environment. Therefore, case-by-case AMOC approvals that allow 
performing certain operations otherwise prohibited by an AD are no 
longer the most efficient way for airplane operators to show that their 
radio altimeters perform their intended function in the 5G C-Band 
environment.
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    \5\ FCC licenses authorized 5G transmissions from 3.7 to 3.98 
GHz.
    \6\ The tolerance to 5G spurious emissions is the level of 
aggregate interference in the radio altimeter band below which the 
installed radio altimeter system will meet its performance standards 
and perform its intended function.
    \7\ The additional 19 telecommunications companies will have 
access to the FCC-licensed spectrum after current users vacate use 
of the frequencies.
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    Determination of Airplane Radio Altimeter Tolerance Requirements: 
The FAA is proposing interference tolerance requirements for radio 
altimeters that can be used across the affected fleet. Airplanes 
meeting these proposed minimum performance levels would be allowed to 
use the prohibited operations at the airports identified by an FAA 
Domestic Notice \8\ after July 1, 2023. Airplanes operating under 14 
CFR part 121 would also be required to have a radio altimeter that 
meets the proposed minimum performance standards (i.e., tolerance 
requirements) on or before February 1, 2024.
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    \8\ Domestic Notices publish special notices or notices 
containing graphics pertaining to almost every aspect of aviation, 
such as military training areas, large scale sporting events, air 
show information, Special Management Programs (STMPs), and airport-
specific information.
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    The FAA determined the proposed interference tolerance requirements 
by using the fuller understanding of specific radio altimeter 
capabilities the FAA gained during the AMOC process. This process 
revealed the radio altimeter modifications that would not require a 
substantial system redesign, allowing aircraft operators to readily 
replace radio altimeters or install filters that allowed the aircraft 
to operate safely in a mitigated 5G environment.
    The interference tolerance requirements are represented by a power 
spectral density (PSD) curve. The PSD curve, as depicted in figure 1 to 
paragraph (g)(2) of this proposed AD, represents the height over the 
ground and received power from a 5G C-Band emitter, at or below which 
the radio altimeter is expected to function reliably, measured in 
decibels per megahertz. These measurements are limited to the 5G CMAs 
that will be listed in an FAA Domestic Notice. For purposes of this 
proposed AD, a ``radio altimeter tolerant airplane'' (also known within 
industry as a Group 4 airplane) is one for which the radio altimeter, 
as installed, demonstrates tolerance to radio altimeter interference at 
or above PSD curve threshold specified in figure 1 to paragraph (g)(2) 
of this proposed AD. A radio altimeter tolerant airplane also 
demonstrates tolerance to a spurious emission level of -48 dBm/MHz in 
the 4200-4400 MHz radio altimeter band. For purposes of this proposed 
AD, a ``non-radio altimeter tolerant airplane'' (also known in industry 
as a Group 1, 2, or 3 airplane) is one for which the radio altimeter, 
as installed, does not demonstrate those tolerances. Some radio 
altimeters may already demonstrate tolerance to the 5G C-Band emissions 
without modification. Some may need to install filters between the 
radio altimeter and antenna to increase a radio altimeter's tolerance. 
For others, the addition of a filter will not be sufficient to address 
interference susceptibility; therefore, the radio altimeter will need 
to be replaced with an upgraded radio altimeter. The FAA has determined 
that radio altimeter tolerant airplanes will not experience the unsafe 
condition at any airport identified by the FAA as a 5G CMA in an FAA 
Domestic Notice.
    Areas of Operation: Over the past year, the FAA and the aviation 
industry, using data voluntarily provided by AT&T and Verizon, have 
identified maximum power levels for 5G C-Band transmissions that would 
permit safe aircraft operations. These power levels were identified 
using a SiS analysis that considers factors specific to an airport. 
That is, the SiS analysis considers specific 5G C-Band base station 
data to predict the 5G signal characteristics in the runway safety 
zone. The base station data includes 5G C-Band tower or antenna 
locations, fundamental transmission power levels, and antenna height. 
Using this analysis, the FAA has found that airplanes meeting the 
proposed standards as represented by the PSD curve can safely perform 
the

[[Page 1523]]

prohibited operations specified in this proposed AD at 5G CMAs. These 
operations are safe for radio altimeter tolerant airplanes to perform 
at these airports as long as telecommunication companies transmit at 
parameters under the current voluntary agreements with the FAA and FCC.
    Compatibility with 5G C-Band Providers: The FAA has determined that 
any 5G C-Band provider that maintains the mitigated actions will not 
have an effect on the safety of transport and commuter airplanes with 
radio altimeters that meet the interference tolerance requirements at 
5G CMAs. The FAA will assess the effects of any changes to transmission 
parameters at 5G CMAs to determine whether they would result in a 
hazard to air navigation. If the transmission changes negatively affect 
the safe operation of a radio altimeter tolerant airplane at that 
airport, the FAA will remove that airport from the 5G CMAs list.
    Therefore, the FAA has determined that an unsafe condition exists 
when performing certain operations in the presence of 5G C-Band 
transmissions affecting the proper function of radio altimeters. For 
that reason, operators would be required to revise their existing AFM 
to prohibit these operations unless operating a radio altimeter 
tolerant airplane at a 5G CMA. This proposed requirement would take 
effect on July 1, 2023.
    In addition, the FAA proposes to prohibit operations under part 121 
in the U.S. after February 1, 2024, unless such operations are 
conducted with a radio altimeter tolerant airplane. As explained 
earlier, the FAA expects erroneous system warnings due to a 
malfunctioning radio altimeter to lead to flightcrew becoming 
desensitized to system warnings. Such desensitization negates the 
safety benefits of the warning itself and can lead to a catastrophic 
event. To minimize the number of erroneous system messages and the 
unsafe condition they produce, the FAA is proposing to require all 
airplanes operating under part 121 meet the PSD performance curve to 
operate in the contiguous U.S. after February 1, 2024. This is the date 
the FAA has determined to be as soon as reasonably practical, 
consistent with FAA policy.\9\ Non-radio altimeter tolerant airplanes 
can operate under part 121 subject to the revised AFM limitations until 
February 1, 2024, without meeting the radio altimeter performance 
requirements proposed in this AD. If this AD is finalized as proposed, 
after February 1, 2024, airplanes operating under part 121 must meet 
the radio altimeter tolerant requirements specified in figure 1 to 
paragraph (g)(2) of this proposed AD.
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    \9\ PS-ANM-25-05, Risk Assessment Methodology for Transport 
Category Airplanes, available at drs.faa.gov/browse/excelExternalWindow/4E5AE8707164674A862579510061F96B.0001.
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FAA's Determination

    The FAA is issuing this NPRM after determining that the unsafe 
condition described previously is likely to exist or develop on other 
products of the same type design.

Proposed AD Requirements in This NPRM

    This proposed AD would maintain the requirements of AD 2021-23-12, 
except for the limitation pertaining to RNP AR IAPs by requiring, 
before further flight, revising the existing AFM to incorporate 
limitations prohibiting the following operations in the presence of 5G 
C-Band wireless broadband interference as identified by NOTAM (NOTAMs 
will be issued to state the specific airports where the radio altimeter 
is unreliable due to the presence of 5G C-Band wireless broadband 
interference). Alternatively, operators may incorporate the AFM 
revision required by paragraph (g) of AD 2021-23-12.

 Instrument Landing System (ILS) Instrument Approach Procedures 
(IAP) SA CAT I, SA CAT II, CAT II, and CAT III
 Automatic Landing operations
 Manual Flight Control Guidance System operations to landing/
head-up display (HUD) to touchdown operation
 Use of Enhanced Flight Vision System (EFVS) to touchdown under 
14 CFR 91.176(a).

    On or before June 30, 2023, this proposed AD would also require 
revising the existing AFM to incorporate limitations prohibiting these 
same operations at all airports for non-radio altimeter tolerant 
airplanes. For radio altimeter tolerant airplanes, the prohibited 
operations would be allowed at 5G CMAs as identified in an FAA Domestic 
Notice.
    On or before February 1, 2024, this proposed AD would require that 
airplanes operating under Part 121 be modified from a non-radio 
altimeter tolerant airplane to a radio altimeter tolerant airplane.

Interim Action

    The FAA considers that this AD, if adopted as proposed, would be an 
interim action. Once the Technical Standard Order (TSO) standard for 
radio altimeters is established, which will follow the existing 
international technical consensus on the establishment of the minimum 
operational performance standards (MOPS), the FAA anticipates that the 
MOPS will be incorporated into the TSO. The FAA also anticipates that 
aircraft incorporating equipment approved under the new Radio Altimeter 
TSO will be able to operate in both 5G CMAs and non-5G CMAs with no 5G 
C-Band-related AFM limitations. Once a new radio altimeter TSO is 
developed, approved, and available, the FAA might consider additional 
rulemaking.

Costs of Compliance

    The FAA is requesting comments on this evaluation of costs and 
benefits for the proposed airworthiness directive. If adopted as 
proposed, this AD would affect approximately 7,993 airplanes of U.S. 
registry, all of which would require two AFM revisions. In Special 
Airworthiness Information Bulletin AIR-21-18R2, the FAA requested radio 
altimeter retrofit plans, timelines, and completion information from 
the aviation industry. The FAA did not receive comprehensive data, but 
based on the limited information the agency did receive, the FAA 
extrapolated impacts across industry. Based on that information, the 
FAA roughly estimates that almost 7,000 airplanes on the U.S. registry 
are already equipped or are being retrofitted to address radio 
altimeter interference tolerance before publication of this AD, or are 
not operated under 14 CFR part 121, and thus would only require AFM 
revisions to comply with this AD as proposed. Based on information 
received, some operators will comply with the proposed modification by 
replacing the radio altimeter and others by installing an externally 
mounted filter. The FAA estimates that approximately 180 airplanes 
would require radio altimeter replacement and 820 airplanes would 
require addition of radio altimeter filters to comply with the proposed 
modification requirement. As such, the FAA estimates the following 
costs to comply with this proposed AD, for a total cost of compliance 
of up to $26,049,810:

[[Page 1524]]



                                                 Estimated Costs
----------------------------------------------------------------------------------------------------------------
                                                                                                 Cost on U.S.
             Action                  Labor cost          Parts cost       Cost per product        operators
----------------------------------------------------------------------------------------------------------------
AFM revision for all airplanes.  1 work-hour x $85   $0................  $85...............  $679,405 for 7,993
                                  per hour = $85.                                             affected
                                                                                              airplanes.
AFM revision (radio altimeter    1 work-hour x $85   $0................  $85...............  $679,405 for 7,993
 tolerant specific limitations).  per hour = $85.                                             affected
                                                                                              airplanes.
Modification (radio altimeter    ..................  ..................  Up to $80,000       Up to $14,400,000
 replacement option).                                                     (includes parts     for 180 affected
                                                                          and labor).         airplanes.
Modification (filter addition    12 work-hours x     $4,000 per filter.  $5,020 per filter.  Up to $10,291,000
 option).                         $85 per hour =                                              for 820 affected
                                  $1,020 per filter.                                          airplanes with 2
                                                                                              or 3 filters per
                                                                                              airplane.
----------------------------------------------------------------------------------------------------------------

    The benefits of the proposed AD would include the value of reducing 
aviation accident risks that are mitigated by TAWS, TCAS, and airborne 
windshear warning and flight guidance systems (windshear systems), all 
of which rely on proper performance of radio altimeters to perform 
their intended function. TAWS, TCAS, and windshear systems are examples 
of safety-enhancing systems required for operation under 14 CFR part 
121. The FAA required these systems to address hazards which have 
caused accidents and fatalities during commercial air transportation in 
the United States. This proposed AD would maintain the same level of 
safety afforded by these and other safety systems before the use of the 
C-Band by 5G broadband networks. This proposed AD would also minimize 
erroneous system messages and the unsafe condition they produce.

Authority for This Rulemaking

    Title 49 of the United States Code specifies the FAA's authority to 
issue rules on aviation safety. Subtitle I, section 106, describes the 
authority of the FAA Administrator. Subtitle VII, Aviation Programs, 
describes in more detail the scope of the Agency's authority.
    The FAA is issuing this rulemaking under the authority described in 
Subtitle VII, Part A, Subpart III, Section 44701: General requirements. 
Under that section, Congress charges the FAA with 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 that 
authority because it addresses an unsafe condition that is likely to 
exist or develop on products identified in this rulemaking action.

Regulatory Findings

    The FAA has determined that this proposed AD would not have 
federalism implications under Executive Order 13132. This proposed AD 
would not have a substantial direct effect on the States, on the 
relationship between the national Government and the States, or on the 
distribution of power and responsibilities among the various levels of 
government.
    For the reasons discussed above, I certify that the proposed 
regulation:
    (1) Is not a ``significant regulatory action'' under Executive 
Order 12866, and
    (2) Would not affect intrastate aviation in Alaska.

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.
    The 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. The FAA invites interested parties 
to submit data and information regarding the potential economic impact 
that would result from the proposal. The FAA will consider comments 
when making a determination or when completing a Final Regulatory 
Flexibility Assessment. An IRFA contains 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.

Reasons the Action Is Being Considered

    AD 2021-23-12 requires revising the limitations section of the 
existing AFM to incorporate limitations prohibiting certain operations 
requiring radio altimeter data when in the presence of 5G C-Band 
interference as identified by NOTAMs. Since the FAA issued AD 2021-23-
12, the FAA determined that more restrictive limitations are needed due 
to the continued deployment of new 5G C-Band base stations whose 
signals are expected to cover most of the contiguous United States at 
transmission frequencies between 3.7-3.98 GHz. This proposed AD would 
address the unsafe condition resulting from the continued deployment of 
5G C-Band transmissions and their interference to radio altimeters.

Objectives of the Proposed Rule

    This proposed AD would require revising the limitations section of 
the existing AFM to incorporate limitations prohibiting certain 
operations requiring radio altimeter data, due to the presence of 5G C-
Band interference. This proposed AD would also require modifying 
certain airplanes to allow safe operations in the United States 5G C-
Band-radio frequency environment.

[[Page 1525]]

Description and Estimate of the Number of Small Entities

    The 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.
    The SBA established size standards for various types of economic 
activities, or industries, under the North American Industry 
Classification System (NAICS).\10\ These size standards generally 
define small businesses based on the number of employees or annual 
receipts. The following table shows the SBA size standards for 
certificate holders as an example. Note that the SBA definition of a 
small business applies to the parent company and all affiliates as a 
single entity.
---------------------------------------------------------------------------

    \10\ Small Business Administration (SBA) Table of Size 
Standards. Effective December 19, 2022. https://www.sba.gov/document/support--table-size-standards.

                                Small Business Size Standards: Air Transportation
----------------------------------------------------------------------------------------------------------------
              NAICS code                          Description                        SBA size standard
----------------------------------------------------------------------------------------------------------------
481111...............................  Scheduled Passenger Air            1,500 employees.
                                        Transportation.
481112...............................  Scheduled Freight Air              1,500 employees.
                                        Transportation.
481211...............................  Nonscheduled Chartered Passenger   1,500 employees.
                                        Air Transportation.
481212...............................  Nonscheduled Chartered Freight     1,500 employees.
                                        Air Transportation.
481219...............................  Other Nonscheduled Air             $25 million.
                                        Transportation.
----------------------------------------------------------------------------------------------------------------

    Certificate holders affected by the proposed AD are those 
authorized to conduct operations under 14 CFR part 121. To identify 
small entities, the FAA reviewed readily available data sources (e.g., 
company websites) and data available to the FAA through its certificate 
oversight functions to determine whether the certificate holder meets 
the applicable size standard. The following table provides a summary of 
the estimated number of small entities to which this proposed AD would 
apply.

                                       Estimated Number of Small Entities
----------------------------------------------------------------------------------------------------------------
                                                                   Number of      Number small    Percent small
                           Category                                entities         entities         entities
----------------------------------------------------------------------------------------------------------------
Major.........................................................               6                0                0
National......................................................              15                7               47
Passenger and Cargo Charter...................................              12                8               67
Regional......................................................              15                7               47
Specialty Cargo...............................................              14                9               64
                                                               -------------------------------------------------
    Total.....................................................              62               31               50
----------------------------------------------------------------------------------------------------------------

Projected Reporting, Recordkeeping, and Other Compliance Requirements

    No new recordkeeping or reporting requirements are associated with 
the proposed AD. Small entity compliance with the proposed AD would 
entail incorporation of AFM revisions at an approximate cost of $170 
per airplane. As discussed previously, the FAA estimates that the 
majority of airplanes operated by small entities will already be 
equipped in a manner that complies with the proposed requirements of 
this AD. Given the relatively small aircraft fleet sizes for small 
entity airlines, the FAA anticipates that a small number of airplanes 
would need to have radio altimeter filters installed (at an approximate 
cost of $5,020 per filter), and a smaller number of airplanes will 
require a radio altimeter replacement (at an approximate cost of up to 
$80,000 per airplane). These costs represent a small percentage of the 
overall cost of owning and operating a transport category airplane, and 
to the extent that small entities provide more unique services or serve 
markets with less competition, these entities might be able to pass on 
these small incremental costs of AD compliance in the form of price 
increases.

All Federal Rules That May Duplicate, Overlap, or Conflict

    There are no relevant Federal rules that may duplicate, overlap, or 
conflict with the proposed AD.

Significant Alternatives Considered

    This AD specifies the only feasible alternatives identified for 
mitigating the unsafe condition. If a less burdensome method for 
mitigating the unsafe condition is identified, the FAA will consider 
proposed alternative methods of compliance, if requested, using the 
procedures found in 14 CFR 39.19.

List of Subjects in 14 CFR Part 39

    Air transportation, Aircraft, Aviation safety, Incorporation by 
reference, Safety.

The Proposed Amendment

    Accordingly, under the authority delegated to me by the 
Administrator, the FAA proposes to amend 14 CFR part 39 as follows:

PART 39--AIRWORTHINESS DIRECTIVES

0
1. The authority citation for part 39 continues to read as follows:

    Authority:  49 U.S.C. 106(g), 40113, 44701.


Sec.  39.13  [Amended]

0
2. The FAA amends Sec.  39.13 by:
0
a. Removing Airworthiness Directive (AD) 2021-23-12, Amendment 39-21810 
(86 FR 69984, December 9, 2021), and
0
b. Adding the following new AD:


[[Page 1526]]


Transport and Commuter Category Airplanes: Docket No. FAA-2022-1647; 
Project Identifier AD-2022-01379-T.

(a) Comments Due Date

    The FAA must receive comments on this airworthiness directive 
(AD) action by February 10, 2023.

(b) Affected ADs

    This AD replaces AD 2021-23-12, Amendment 39-21810 (86 FR 69984, 
December 9, 2021) (AD 2021-23-12).

(c) Applicability

    This AD applies to all transport and commuter category airplanes 
equipped with a radio (also known as radar) altimeter. These radio 
altimeters are installed on various transport and commuter category 
airplanes including, but not limited to, the airplanes for which the 
design approval holder is identified in paragraphs (c)(1) through 
(19) of this AD.

(1) The Boeing Company
(2) Airbus SAS
(3) Bombardier Inc.
(4) Embraer S.A.
(5) Gulfstream Aerospace Corporation
(6) Gulfstream Aerospace LP
(7) Textron Aviation Inc.
(8) Pilatus Aircraft Limited
(9) Fokker Services B.V.
(10) Saab AB, Support and Services
(11) DeHavilland Aircraft of Canada Limited
(12) Airbus Canada Limited Partnership
(13) ATR-GIE Avions de Transport R[eacute]gional
(14) Yabor[atilde] Ind[uacute]stria Aeron[aacute]utica S.A.
(15) MHI RJ Aviation ULC
(16) BAE Systems (Operations) Limited
(17) Lockheed Martin Corporation/Lockheed Martin Aeronautics Company
(18) Viking Air Limited
(19) Dassault Aviation

(d) Subject

    Air Transport Association (ATA) of America Code 31, Indicating/
Recording System; 34, Navigation.

(e) Unsafe Condition

    This AD was prompted by determination that radio altimeters 
cannot be relied upon to perform their intended function if they 
experience interference from wireless broadband operations in the 
3.7-3.98 GHz frequency band (5G C-Band). The FAA is issuing this AD 
because radio altimeter anomalies that are undetected by the 
automation or pilot, particularly close to the ground (e.g., landing 
flare), could lead to loss of continued safe flight and landing. 
Additionally, radio altimeter anomalies could lead to increased 
flightcrew workload and flightcrew desensitization to warnings.

(f) Compliance

    Comply with this AD within the compliance times specified, 
unless already done.

(g) Definitions

    (1) For purposes of this AD, a ``5G C-Band mitigated airport'' 
(5G CMA) is an airport at which the telecommunications companies 
have agreed to voluntarily limit their 5G deployment at the request 
of the FAA, as identified by an FAA Domestic Notice.
    (2) For purposes of this AD, a ``radio altimeter tolerant 
airplane'' is one for which the radio altimeter, as installed, 
demonstrates the tolerances specified in paragraphs (g)(2)(i) and 
(ii) of this AD, using a method approved by the FAA.
    (i) Tolerance to radio altimeter interference at or above the 
power spectral density (PSD) curve threshold specified in figure 1 
to paragraph (g)(2) of this AD.
    (ii) Tolerance to an aggregate base station conducted spurious 
emission level of -48 dBm/MHz in the 4200-4400 MHz radio altimeter 
band.
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[GRAPHIC] [TIFF OMITTED] TP11JA23.013

    (3) For purposes of this AD, a ``non-radio altimeter tolerant 
airplane'' is one for which the radio altimeter, as installed, does 
not demonstrate the tolerances specified in paragraphs (g)(2)(i) and 
(ii) of this AD.

(h) Airplane/Aircraft Flight Manual (AFM) Revision for All Airplanes

    Before further flight: Revise the Limitations Section of the 
existing AFM by incorporating the limitations specified in figure 2 
to paragraph (h) of this AD. This may be done by inserting a copy of 
figure 2 to paragraph (h) of this AD into the existing AFM. If an 
operator has complied with paragraph (g) of

[[Page 1527]]

AD 2021-23-12, that action satisfies the requirements of this 
paragraph.
[GRAPHIC] [TIFF OMITTED] TP11JA23.014

(i) AFM Revision for Non-Radio Altimeter Tolerant Airplanes

    For non-radio altimeter tolerant airplanes, do the actions 
specified in paragraphs (i)(1) and (2) of this AD.
    (1) On or before June 30, 2023, revise the Limitations Section 
of the existing AFM by incorporating the limitations specified in 
figure 3 to paragraph (i) of this AD. This may be done by inserting 
a copy of figure 3 to paragraph (i) of this AD into the existing 
AFM. Incorporating the AFM revision required by this paragraph 
terminates the AFM revision required by paragraph (h) of this AD.
    (2) Before further flight after incorporating the limitations 
specified in figure 3 to paragraph (i) of this AD, remove the AFM 
revision required by paragraph (h) of this AD.
[GRAPHIC] [TIFF OMITTED] TP11JA23.015

(j) AFM Revision for Radio Altimeter Tolerant Airplanes

    For radio altimeter tolerant airplanes, do the actions specified 
in paragraphs (j)(1) and (2) of this AD.
    (1) On or before June 30, 2023, revise the Limitations Section 
of the existing AFM by incorporating the limitations specified in 
figure 4 to paragraph (j) of this AD. This may be done by inserting 
a copy of figure 4 to paragraph (j) of this AD into the existing 
AFM. Incorporating the AFM revision required by this paragraph 
terminates the AFM revision required by paragraph (h) of this AD.
    (2) Before further flight after incorporating the limitations 
specified in figure 4 to paragraph (j) of this AD, remove the AFM 
revision required by paragraph (h) of this AD.

[[Page 1528]]

[GRAPHIC] [TIFF OMITTED] TP11JA23.016

(k) Modification

    (1) For non-radio altimeter tolerant airplanes operating under 
14 CFR part 121: On or before February 1, 2024, modify each airplane 
to a radio altimeter tolerant airplane and accomplish the actions 
specified in paragraphs (k)(i) and (ii) of this AD.
    (i) Revise the Limitations Section of the existing AFM by 
incorporating the limitations specified in figure 4 to paragraph (j) 
of this AD. This may be done by inserting a copy of figure 4 to 
paragraph (j) of this AD into the existing AFM.
    (ii) Remove the AFM revision required by paragraph (i) of this 
AD.
    (2) For non-radio altimeter tolerant airplanes not operating 
under part 121, accomplishing the modification and AFM revision 
specified in paragraph (k)(1) of this AD is optional.

(l) Alternative Methods of Compliance (AMOCs)

    (1) The Manager, Operational Safety Branch, FAA, has the 
authority to approve AMOCs for this AD, if requested using the 
procedures found in 14 CFR 39.19. In accordance with 14 CFR 39.19, 
send your request to your principal inspector or responsible Flight 
Standards Office, as appropriate. If sending information directly to 
the manager of the Operational Safety Branch, send it to the 
attention of the person identified in paragraph (m) of this AD. 
Information may be emailed to: [email protected].
    (2) Before using any approved AMOC, notify your appropriate 
principal inspector, or lacking a principal inspector, the manager 
of the responsible Flight Standards Office.
    (3) AMOCs approved for AD 2021-23-12 are approved as AMOCs for 
the requirements specified in paragraph (h) of this AD.

(m) Related Information

    For more information about this AD, contact Brett Portwood, 
Continued Operational Safety Technical Advisor, COS Program 
Management Section, Operational Safety Branch, FAA, 3960 Paramount 
Boulevard, Lakewood, CA 90712-4137; phone: 817-222-5390; email: 
[email protected].

(n) Material Incorporated by Reference

    None.

    Issued on January 6, 2023.
Gaetano A. Sciortino,
Acting Director, Compliance & Airworthiness Division, Aircraft 
Certification Service.
[FR Doc. 2023-00420 Filed 1-9-23; 8:45 am]
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