[Federal Register Volume 88, Number 119 (Thursday, June 22, 2023)]
[Rules and Regulations]
[Pages 40685-40695]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2023-13319]



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 Rules and Regulations
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  Federal Register / Vol. 88, No. 119 / Thursday, June 22, 2023 / Rules 
and Regulations  

[[Page 40685]]



DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2023-0668; Project Identifier AD-2023-00199-R; 
Amendment 39-22453; AD 2023-11-07]
RIN 2120-AA64


Airworthiness Directives; Various Helicopters

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule; request for comments.

-----------------------------------------------------------------------

SUMMARY: The FAA is superseding Airworthiness Directive (AD) 2021-23-
13, which applied to all helicopters equipped with a radio (also known 
as radar) altimeter. AD 2021-23-13 required revising the limitations 
section of the existing rotorcraft flight manual (RFM) for your 
helicopter to incorporate limitations prohibiting certain operations 
requiring radio altimeter data when in the presence of 5G C-Band 
interference in areas as identified by Notices to Air Missions 
(NOTAMs). Since the FAA issued AD 2021-23-13, 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 (5G C-Band). This AD requires revising the limitations section 
of the existing RFM to incorporate limitations prohibiting certain 
operations requiring radio altimeter data, due to the presence of 5G C-
Band interference. The FAA is issuing this AD to address the unsafe 
condition on these products. In addition, the FAA is requesting comment 
on the change to the spurious emission level requirement.

DATES: This AD is effective June 22, 2023.
    The FAA must receive comments on this AD by August 7, 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 by 
searching for and locating Docket No. FAA-2023-0668; 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 final rule, any 
comments received, and other information. The street address for Docket 
Operations is listed above.

FOR FURTHER INFORMATION CONTACT: David Swartz, Continued Operational 
Safety Technical Advisor, COS Program Management Section, Operational 
Safety Branch, FAA; phone: 817-222-5390; email: 
[email protected].

SUPPLEMENTARY INFORMATION:

Background

    The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 
CFR part 39 to supersede AD 2021-23-13, Amendment 39 21811 (86 FR 
69992, December 9, 2021) (AD 2021-23-13). AD 2021-23-13 applied to all 
helicopters equipped with a radio (also known as radar) altimeter. The 
NPRM published in the Federal Register on April 12, 2023 (88 FR 21931). 
The NPRM was prompted by a determination that radio altimeters cannot 
be relied upon to perform their intended function if they experience 5G 
C-Band interference.
    In the NPRM, the FAA proposed minimum performance levels for radio 
altimeters that can be used across the affected fleet. The FAA proposed 
that a ``radio altimeter tolerant rotorcraft'' is one for which the 
radio altimeter, as installed, demonstrates: (1) tolerance to radio 
altimeter interference at or above a fundamental power spectral density 
(PSD) curve threshold, and (2) tolerance to an aggregate spurious 
emission level of -42 dBm/MHz in the 4200-4400 MHz radio altimeter 
band. For rotorcraft with radio altimeters that meet the proposed 
minimum performance levels, the FAA proposed to terminate the 
operational limitations imposed by AD 2021-23-13 with no further 
action. For rotorcraft with radio altimeters that do not meet the 
proposed minimum performance levels, the FAA proposed to maintain the 
requirements of AD 2021-23-13 by requiring revising the limitations 
section of the existing RFM to incorporate limitations prohibiting 
certain operations requiring radio altimeter data, due to the presence 
of 5G C-Band interference as identified by NOTAM until June 30, 2023. 
On or before June 30, 2023, the FAA proposed to also require, for non-
radio altimeter tolerant rotorcraft, revising the existing RFM to 
incorporate limitations prohibiting these same operations in the 
contiguous U.S. airspace.

Actions Since the NPRM Was Issued

    Since the FAA issued the NPRM, the FAA determined that the spurious 
emission level as proposed in the NPRM would not necessarily address 
the unsafe condition from both indoor and outdoor emitters. 
Additionally, commenters of the NPRM requested the FAA replace the 
proposed fixed aggregate spurious emission level with a spurious PSD 
tolerance curve.
    The FAA analyzed multiple scenarios to determine the worst case 
spurious emissions a rotorcraft could be exposed to from either indoor 
or outdoor 5G C-band base stations. The constraining scenario is a 
rotorcraft operation on a building or elevated platform with a nearby 
in-building 5G emitter. Figure 2 of paragraph (g)(1)(ii) of this AD is 
derived by assuming that the rotorcraft is at least 75 ft laterally 
from the in-building emitter with conducted spurious emissions of -30 
dBm/MHz. No building exit losses are accounted for in this curve.
    In this final rule, the FAA replaced the proposed fixed aggregate 
spurious emission level of -42 dBm/MHz (conducted) with a spurious PSD 
tolerance curve for 5G C-Band interference from a single base station. 
This form of the requirement is consistent with the transport and 
commuter category airplane

[[Page 40686]]

requirement. Similarly, it does not include an aggregation factor or 
any base station or aircraft antenna gain factors. The spurious PSD 
tolerance curve is based on a different spurious emission level than 
what the FAA proposed in the NPRM (-30 dBm/MHz conducted for a single 
indoor base station instead of -48 dBm/MHz conducted for a single 
outdoor base station and an aggregation factor of 6 dB which yielded an 
effective aggregate level of -42 dBm/MHz).
    As a result, the FAA is requesting comments on the spurious PSD 
tolerance curve in figure 2 to paragraph (g)(1)(ii) of this AD.

Comments on NPRM

    The FAA provided the public with an opportunity to comment on the 
NPRM and received submissions to Docket No. FAA-2023-0668 from Sikorsky 
Aircraft Corporation (Sikorsky), Bell Textron Inc. (Bell Textron), 
Thales Group (Thales), Air Medical Operators Association (AMOA), and 
Helicopter Association International (HAI) on behalf of 21 aviation 
coalition members.
    The following summarizes the comments received on the NPRM and 
provides the FAA's responses.

Request To Clarify AMOC Requirements for PSD Curve

    Comment summary: Bell Textron requested the FAA clarify whether 
alternative methods of compliance (AMOCs) will be required for filters 
or equipment that meet the fundamental PSD curve.
    FAA response: The AD specifies that radio altimeter tolerant 
aircraft must use a method approved by the FAA. The FAA developed a 
policy statement and requested public comments on this proposed policy 
on May 8, 2023 (88 FR 29554). The policy statement provides additional 
guidance on data submittals and approvals for this method. Therefore, 
operators will not need an AMOC provided their helicopter meets the 
fundamental and spurious emissions PSD curve thresholds specified in 
paragraphs (g)(1)(i) and (ii) of this AD.

Request To Revise Applicability

    Comment summary: Sikorsky requested that the FAA revise the 
proposed AD to exempt helicopters conducting ambulatory, health, 
emergency, and search and rescue (SAR) operations. Sikorsky stated that 
the inability to conduct these lifesaving operations due to the AD 
would have a negative humanitarian and social impact.
    FAA response: The FAA disagrees. The FAA has determined that 
regardless of the purpose of a flight, erroneous radio altimeter 
behavior creates an unacceptable risk during the operations prohibited 
by the AD. To remove those prohibitions from the RFM, operators may 
modify their helicopters to a radio altimeter tolerant helicopter as 
defined in paragraph (g)(1) of this AD.

Request To Clarify ``Critical Phase of Flight''

    Comment summary: In the NPRM, the FAA explained its determination 
that helicopters with a radio altimeter that demonstrates the 
tolerances in the AD would not experience interference during a 
critical phase of flight in the contiguous U.S. airspace. Sikorsky 
questioned whether by ``critical phase of flight'' the FAA means the 
definition in 14 CFR 135.100(c).
    FAA response: The commenter is correct that the term ``critical 
phase of flight'' in this AD has the same meaning as in 14 CFR 
135.100(c): all ground operations involving taxi, takeoff and landing, 
and all other flight operations conducted below 10,000 feet, except 
cruise flight.

Request To Clarify ``Prohibited Operations''

    Comment summary: In the NPRM, the FAA explained its proposed 
actions and stated that after July 1, 2023, helicopters without an 
upgraded radio altimeter would be ``subject to the prohibited 
operations.'' Sikorsky requested that the FAA clarify its use of the 
term ``prohibited operations,'' as avionic system architecture, 
redundancies, and associated functional hazard and system safety 
assessments could cause aircraft manufacturers, operators, and 
regulators to interpret the term differently.
    FAA response: The FAA's use of the term ``prohibited operations'' 
in the preamble of the NPRM refers to the four operations listed in 
figure 3 to paragraph (h) of this AD and figure 4 to paragraph (i) of 
this AD.

Request To Extend Compliance Time

    Comment summary: Sikorsky and HAI requested additional time to 
comply with the AD. The commenters were concerned that operators will 
not have sufficient time to equip with an upgraded radio altimeter 
before June 30, 2023, in order to avoid the flight restrictions. 
According to HAI, no currently available radio altimeters meet the 
proposed performance levels, and about one-third of the fleet will need 
to purchase a different make/model radio altimeter and the appropriate 
filter solution. The commenters stated that an extension would avoid 
operational and economic impacts to U.S. transportation, as well as 
disturbances to aerospace production.
    FAA response: The FAA carefully considered the impact of the flight 
restrictions on the unmodified fleet after June 30, 2023, and did not 
take the decision to prohibit these operations lightly. The June 30, 
2023, date was driven by the unsafe condition over which the FAA has no 
control. After refraining from operating at the levels authorized by 
the Federal Communications Commission (FCC) for a year and a half, 
wireless companies are now able to operate at higher levels, yet still 
not at the levels authorized. Additionally, the FAA anticipates 19 
additional telecommunications companies will begin transmitting in the 
C-Band after June 30, 2023. Although the FAA continues to work with the 
companies that intend to transmit in the 3.7-3.98-GHz band near 5G C-
Band mitigated airports (5G CMAs), the FAA has no agreement with those 
companies to provide the FAA with tower locations and other information 
necessary to support the current NOTAM/AMOC process. Therefore, the FAA 
will not be able to extend the June 30, 2023, date.

Request To Change Number of Affected Helicopters

    Comment summary: Sikorsky requested that the FAA revise the number 
of affected helicopters in the estimated costs section of the preamble. 
Sikorsky stated that the number in the NPRM (1,128 helicopters of U.S. 
registry) is incorrect as the Sikorsky civil fleet of Model S-76A, S-
76A+, S-76/B, S-76C, S-76C++, S-76D, and S-92A helicopters, which 
utilize single and/or dual radio altimeters, total 1,153. AMOA also 
stated the proposed AD does not contain an accurate estimate of 
affected helicopters.
    FAA response: The FAA has reevaluated and revised the affected 
fleet size for U.S registered helicopters. The FAA estimates that 
approximately 5,500 helicopters of U.S. registry are likely to have a 
radio altimeter installed and are therefore required to revise their 
existing RFM or modify to become a radio altimeter tolerant rotorcraft. 
The FAA also estimates that 1,128 of the 5,500 helicopters of U.S. 
registry are equipped to be able to perform the operations prohibited 
by this AD for non-radio altimeter rotorcraft. The FAA has revised the 
estimated costs of this final rule accordingly.

Allocation of Federal Funding

    Comment summary: HAI recommended that federal funding be allocated 
to helicopter operators to help cover the costs associated with

[[Page 40687]]

equipage. This financial support would alleviate the burden on 
operators and facilitate the implementation of necessary retrofits.
    FAA response: The FAA appreciates the cost to retrofit helicopters 
with tolerant radio altimeters. However, the FAA's budget does not 
include allocations for AD costs incurred in modifying privately owned 
equipment.

Request To Revise Cost Estimates for Modification and RFM Updates

    Comment summary: Sikorsky, Bell Textron, HAI, and AMOA commented 
that the NPRM estimated only the cost to revise the RFM update and did 
not include costs to replace or modify the radio altimeters. AMOA also 
stated the NPRM does not realistically address the capacity and likely 
backlog associated with the modifications.
    FAA response: Based on these comments, the FAA has revised the 
estimated costs to include the costs associated with modifying the 
helicopter to be a radio altimeter tolerant rotorcraft. The cost 
analysis in FAA AD rulemaking actions typically considers only the 
direct costs associated with the specific actions required by the AD. 
The FAA does not include secondary or indirect costs, such as those 
resulting from delays or supply issues. The FAA lacks the data 
necessary to quantify those costs, which might vary significantly among 
operators; the commenters did not provide such data either.
    Comment summary: Sikorsky stated that the FAA's estimate of the 
hourly labor rate and number of hours associated with updating the RFM 
is too low. Sikorsky suggested that typical RFM updates/releases 
include technical writing, peer reviews, airworthiness approvals, and 
release.
    FAA response: The FAA disagrees. The FAA uses 1 work-hour as a 
standard estimate in ADs that require an administrative function such 
as a revision to a flight manual.

Concern for Agreed-On Mitigations

    Comment summary: Bell Textron, Thales, and Sikorsky expressed 
concern that the FAA does not have authority to enforce the voluntary 
agreements between the FAA and the telecommunications companies. Bell 
Textron requested that the FCC mandate the voluntary mitigations so 
that already designed filters will be a lasting solution. Thales and 
Sikorsky also requested that the proposed AD include necessary spurious 
data that 5G network operators should disclose to the FAA, to determine 
what action may be necessary to ensure safe aviation operations in the 
U.S.
    FAA response: Thales, Sikorsky, and Bell Textron are correct that 
the agreements between the FAA and the telecommunications companies 
have been voluntary because the FAA does not have enforcement authority 
over the companies' use of licenses they receive from the FCC. However, 
the FAA, National Telecommunications and Information Administration 
(NTIA), and FCC have worked extensively and collaboratively with the 
licensees to ensure that the agreements confirm necessary notification 
and coordination, that mitigations are in place with network 
deployments, and that the agreements are enforceable by the FCC. These 
March 31, 2023, voluntary agreements \1\ allow the FAA to continue to 
address aviation safety when analysis indicates that a proposed base 
station will exceed the permitted PSD values, which ensures the FAA can 
protect offshore radio altimeter coupled approaches, hover autopilot 
modes that use radio altimeters, search and rescue autopilot modes that 
use radio altimeters, and Category A, B, and Performance takeoff and 
landing operations without limitations. The FAA disagrees with the 
request that the AD include necessary spurious data that 5G network 
operators should disclose to the FAA, because the spurious PSD curve in 
this AD is based on the spurious emission limits documented in the 
March 31, 2023, agreement.
---------------------------------------------------------------------------

    \1\ A copy of the letter from AT&T, Verizon, T-Mobile, and 
UScellular dated March 31, 2023, documenting their voluntary 
commitments to transmit within mitigated parameters is in Docket No. 
FAA-2023-0668 and can be found on the FCC's website at: https://www.fcc.gov/ecfs/search/search-filings/filing/1033142661477.
---------------------------------------------------------------------------

    The FAA will continue to work with the FCC and NTIA in this regard 
to ensure continuing aviation safety. As stated in the voluntary 
agreement letter dated March 31, 2023, the commitments of AT&T, T-
Mobile, UScellular, and Verizon will last until January 1, 2028, at 
which point they will sunset unless extended or reduced by mutual 
agreement. A mid-term check-in involving the FAA, the FCC, and 
telecommunications companies will occur in July 2026 to assess the 
status of aviation's long-term migration to next-generation radio 
altimeters and the need for the sustainment of these commitments.

Request To Clarify Compliance Methods

    Comment summary: Bell Textron requested the FAA clarify the process 
for determining how a radio altimeter will meet the tolerance 
requirements in the AD. Bell Textron also requested that the FAA 
clarify the requirement in the proposed AD of ``using a method approved 
by the FAA.'' HAI requested the FAA revise the proposed AD to include a 
reference to an issue paper, advisory circular, or other means of 
compliance document that has gone through the public comment process.
    FAA response: The FAA developed a policy statement and requested 
public comments on this proposed policy on May 8, 2023 (88 FR 29554). 
The proposed policy describes an acceptable framework and method for 
demonstrating that an airplane or helicopter is radio altimeter 
tolerant. The policy discusses compliance methods that should be 
applied to programs for type certificates, amended type certificates, 
supplemental type certificates (STCs), and amended STCs. The policy 
proposes methods that may be used for compliance only if approved by 
the FAA in writing.

Request To Revise PSD Curve

    Comment summary: HAI, Sikorsky, and Thales requested that the FAA 
revise the proposed AD to include the frequency range to which the PSD 
tolerance curve thresholds apply, a new figure indicating the spurious 
tolerance similar to the figure with the PSD tolerance curve, and a 
specification of the altitude dependence for spurious tolerance.
    FAA response: The FAA agrees and has added the frequency range to 
the PSD tolerance curve. Additionally, as explained previously under 
``Actions Since the NPRM Was Issued,'' the FAA has replaced the 
proposed fixed emission level with a spurious PSD tolerance curve.

Request To Allow Modification as Minor Change

    Comment summary: HAI requested that the FAA revise the proposed AD 
to allow modification of the helicopter as a minor change to type 
design, to help expedite approvals and make the best use of resources.
    FAA response: The FAA disagrees. Under 14 CFR 21.95, minor design 
changes may be approved before an applicant submits to the FAA any 
substantiating data. Radio altimeters are critical sensors that must be 
shown to perform their intended function, and the modified hardware or 
software must be shown to still meet the aircraft-level system safety 
requirements. For example, a filter may alter the radio altimeter 
performance, which may have an appreciable effect on reliability, 
operational characteristics, or other

[[Page 40688]]

characteristics affecting airworthiness. For this reason, the FAA 
determined that FAA approval of the method used for the modification is 
necessary before operators could show compliance with this AD.

Request To Clarify Need for Substantial Redesign of Radio Altimeters

    Comment summary: In the NPRM, the FAA stated that the radio 
altimeter modifications would not require a substantial system 
redesign, because operators could readily replace radio altimeters or 
install filters. Bell Textron commented that anything other than a 
simple filter is a substantial system redesign and requested the FAA 
provide clarification that some radio altimeters will require 
substantial system redesign.
    FAA response: The FAA acknowledges that certain radio altimeter 
changes may require a substantial system design at the radio altimeter 
or radio altimeter integration level. For some radio altimeters, 
hardware or software design changes may be needed to address 
attenuation effects of additional filtering. However, some of these 
changes may not be considered a substantial system design. For example, 
the FAA has allowed many of the radio altimeter filtering that was 
implemented as AMOCs to AD 2021-23-12 to be certified as a minor 
technical standard order (TSO) change as the change was not extensive 
enough to require a substantially complete investigation.

Request for FAA To Retain NVG Exemptions

    Comment summary: AMOA expressed concern about the proposed AD's 
effect on Exemption No. 18973, which provides relief to helicopter air 
ambulance operators conducting operations under part 135 with night 
vision goggles (NVGs). The commenter stated that the exemption is tied 
to NOTAMs for identifying areas where the radio altimeter is 
unreliable, and in the NPRM, the FAA advised that NOTAMs would no 
longer be used to communicate the location of the 5G C-Band 
environment. Sikorsky also noted that the NPRM does not address this 
exemption.
    FAA response: The FAA acknowledges that Exemption No. 18973 and 
related exemptions providing relief for helicopter operations under 
parts 91, 135, and 141 with NVGs remain in the public interest. The FAA 
is aware of the issue raised by the commenters and will address with 
follow-on actions for those exemption holders.

Request for AD Applicability for Offshore Operations

    Comment summary: In the NPRM, the FAA proposed to retain the 
prohibition on offshore operations from AD 2021-23-13. HAI and Sikorsky 
noted that the NPRM does not specify where or how far off shore the AD 
would apply. HAI explained that typically, offshore oil rigs are 
situated 3 to 200 miles from the shore, making it crucial to determine 
the range of the restrictions. Sikorsky commented the NPRM has no 
guidance with respect to offshore operations and asked for 
clarification of the proposed FAA offshore applicability limit from the 
U.S. coastline.
    FAA response: The flight restrictions required by this AD are 
limited to the airspace of the contiguous U.S., which means the 
prohibitions are applicable everywhere in the contiguous U.S. to 
include the airspace overlying the waters up to 12 nautical miles from 
the coast, including from the islands of those states. The FAA notes 
that based on the most susceptible radio altimeters, 5G C-Band 
interference could extend up to 20 nautical miles beyond the boundaries 
of the airspace of the contiguous U.S. on non-radio altimeter tolerant 
rotorcraft, so rotorcraft operators should also exercise caution in 
operating in those areas.

Request for Continued Collaboration

    Comment summary: HAI stated that, given that FAA will no longer 
have access to 5G C-Band emitter locations after June 2023, enhanced 
communication and collaboration between FAA and industry stakeholders 
will be paramount. HAI offered to assist FAA in tailoring where 
barometric minimums should be required under a formal information-
sharing agreement.
    FAA response: FAA appreciates the effort HAI and the rotorcraft 
offshore industry have gone through to protect the safety of those 
operations and looks forward to continued collaboration with all 
stakeholders on sharing data on offshore 5G C-Band mitigation efforts.

Request To Change Limitations

    Comment summary: Bell Textron commented there are hover modes that 
provide workload relief under VFR (visual flight rules) conditions and, 
because the operation is under VFR, the pilot would be attentive and 
mitigate any hazards. Bell Textron requested that the FAA change the 
proposed prohibition of ``Engaging hover autopilot modes that require 
radio altimeter data'' to ``Engaging hover autopilot modes that require 
radio altimeter data for safe flight (e.g., those approved for IMC 
[instrument meteorological conditions] or IFR [instrument flight rules] 
conditions where visual reference is not adequate for detection and 
response).''
    FAA response: The FAA disagrees. The FAA considered both VFR and 
IFR operational scenarios and could not rule out an unsafe condition 
when operating under VFR conditions. Additionally, the proposed 
language is vague and could lead to operator confusion and operations 
with unsafe conditions.
    Comment Summary: Bell Textron commented that there are approach 
modes other than SAR modes that could result in an unsafe condition if 
there is erroneous radio altimeter data. The commenter further stated 
that if there are visual references, the pilot can safely mitigate 
hazards from erroneous radio altimeter data. Bell Textron requested the 
FAA change the proposed prohibition of ``Engaging Search and Rescue 
(SAR) autopilot modes that require radio altimeter data'' to ``Engaging 
Search and Rescue (SAR) approach or similar low altitude/low speed 
autopilot approach modes that require radio altimeter data where visual 
reference is not adequate for detection and response.''
    FAA response: The FAA disagrees. If there are autopilot modes 
similar to SAR modes, where the radio altimeter data is used to hold 
altitude, the hover mode prohibition will cover those modes. The FAA 
disagrees that if there are visual references, the pilot can always 
safely mitigate hazards from erroneous radio altimeter data while in 
SAR modes.
    No change to the proposed hover mode prohibition is necessary as a 
result of this comment. Where both barometric minimums and radar 
altimeter minimums are available as options in an approach, a non-radio 
altimeter tolerant rotorcraft may still conduct those approaches using 
the barometric minimums.
    Comment Summary: Bell Textron stated there can be Category A or B 
procedures that provide an alternative means to determine the height 
above the ground. The commenter requested the FAA change the proposed 
prohibition of ``Performing takeoffs and landings in accordance with 
any procedure (Category A, Category B, or by Performance Class in the 
Rotorcraft Flight Manual or Operations Specification) that requires the 
use of radio altimeter data'' to ``Performing takeoffs and landings in 
accordance with any procedure (Category A, Category B, or by 
Performance Class in the Rotorcraft Flight Manual or Operations 
Specification) that requires

[[Page 40689]]

the use of radio altimeter data without the option to alternatively use 
barometric data or visual references.''
    FAA response: The phrase ``that requires the use of radio altimeter 
data'' in the prohibition raised by the commenter is intended to limit 
the applicability of that prohibition to procedures that rely on the 
data provided by the radio altimeter. If the RFM contains an 
alternative procedure for a Category A, B, or performance class takeoff 
or landing using a means that does not rely on radio altimeter data, 
then that procedure is not prohibited by this AD.

Request for PSD Curve Changes

    Comment summary: Thales and Sikorsky requested the FAA clarify 
several aspects of the PSD curve in figure 1 to paragraph (g)(1) of the 
proposed AD: whether no elevation mask is taken into account at and 
below 350 feet above ground level (AGL); at which base station emission 
power the PSD curve corresponds (62 dBm/MHz or 65 dBm/MHz); and whether 
the radio altimeter performance criteria in the curve explicitly covers 
any 5G emitter station up to 65dBm/MHz in the applicable 3.7-3.98 GHz 
band, for the foreseeable future. Sikorsky requested that the FAA 
revise the proposed AD to include a PSD curve that reflects the reduced 
EIRP levels for helicopters operating at the 188 priority airports (5G 
CMAs) with reduced power levels.
    FAA response: The notch in the PSD curve does not mean that no 
elevation mask is taken into account below 350 feet AGL. The elevation 
mask is taken into consideration below 350 feet AGL and is a 
fundamental consideration in how the PSD curve was established. The PSD 
curve was calibrated to cover 5G C-Band emitter stations up to 65 dBm/
MHz in the applicable 3.7-3.98 GHz band.
    The FAA disagrees with revising the AD to include a PSD curve for 
helicopters operating at airports with reduced power levels. The radio 
altimeter tolerant rotorcraft PSD curve was established to provide safe 
operation anywhere in the contiguous U.S. regardless of whether a 
helicopter is operating at a 5G CMA.
    Comment summary: Sikorsky requested the FAA revise the PSD curve in 
figure 1 to paragraph (g)(1) of the proposed AD to change the height 
above ground at -6 dBm/MHz from 270 feet to 370 feet, and at -16 dBm/
MHz from 300 feet to 400 feet. Sikorsky stated its request was based on 
several factors, including the minimum safe obstacle avoidance distance 
and the typical decision height used for radio altimeter critical 
operations associated with missed approach points (MAPs) during point 
in space (PINS) approaches. Sikorsky further stated that radio 
altimeters are not operationally used above 1000 feet, and therefore 
there is little value for any PSD restrictions above 1000 feet 
operationally.
    FAA response: The FAA assumed an emitter base station height of 350 
feet based on consensus reached in rotorcraft industry group meetings. 
Additionally, FAA analysis of existing emitter base station height data 
shows that, while higher emitters do exist, they are infrequently 
adjacent to helipads. Second, the FAA assumed the rotorcraft was no 
closer than 50 feet above and 75 feet laterally from the emitter. The 
vertical distance was based on pilot feedback and the lateral distance 
was based on common lateral separation from the main rotor tip 
recommendations of 2 main rotor diameters from the tip of the rotor, 
selecting a 30 foot rotor as the smallest rotor in this class of 
aircraft with the antenna under the main rotor mast. An operator of an 
individual model rotorcraft with a larger rotor diameter may request 
approval of an AMOC to the AD on the basis of separation distance. 
Third, the FAA assumed that the emitter was active at the full rural 
power level of 65 dBm/MHz allowed in FCC Report and Order FCC 20-22.\2\ 
The FAA then evaluated the resulting curve against data from radio 
altimeters and found it to be achievable with current technology using 
filters that have been developed. Based on data from existing emitters, 
if the height reduction from 370 feet to 270 feet, as requested by the 
commenter, were adopted, it would result in many more landing scenarios 
unprotected from interference because emitters on towers at about 250 
feet as opposed to the accepted assumption of 350 feet are much more 
common. For this reason, the FAA did not adopt this request.
---------------------------------------------------------------------------

    \2\ FCC Report and Order (R&O) FCC 20-22 in the Matter of 
Expanding Flexible Use of the 3.7-4.2 GHz Band, adopted February 28, 
2020, and released March 3, 2020. This document is available in 
Docket No. FAA-2023-0668, and at https://www.fcc.gov/document/fcc-expands-flexible-use-cband-5g-0.
---------------------------------------------------------------------------

    The FAA has re-evaluated the need for PSD performance requirements 
above 1000 feet and agrees with the reasons provided by the commenter. 
The PSD curves specified in this AD have been changed accordingly.

Request for HTAWS Risk Assessment

    Comment summary: Bell Textron commented the helicopter terrain 
avoidance warning systems (HTAWS) discussion in the preamble of the 
NPRM mixes HTAWS functionality with separate radio altimeter 
functionality. The commenter requested that the NPRM discussion be 
revised to clearly distinguish between the stand-alone HTAWS and the 
hazards associated with the stand-alone radio altimeter and suggested 
the following language:

    Unlike the Terrain Awareness and Warnings Systems (TAWS) in 
transport airplanes, most Helicopter Terrain Avoidance Warning 
Systems (HTAWS) do not rely on radio altimeter inputs. Radio 
altimeter data is used for vertical situational awareness in low 
visibility conditions (i.e., snow and dust blown up by rotor down 
wash) and as an input into several procedures and automated system.

    FAA response: The FAA agrees the proposed language provides a 
clearer distinction between the stand-alone HTAWS and the hazards 
associated with the stand-alone radio altimeter. However, no change to 
the final rule is necessary since the language referenced by the 
commenter does not appear in the final rule.
    Comment summary: HAI requested that the FAA include in the final 
rule the particulars of the FAA's risk assessments on all rotorcraft 
systems where an incorrect radio altimeter reading would result in an 
unsafe condition, requiring mitigation.
    FAA response: The FAA assessed the risk of radio altimeter 
anomalies on operations coupled to automated functions and determined 
that these were the hazards that constituted an unsafe condition. The 
FAA provided additional information about its unsafe condition 
determination in the preamble of the NPRM and AD 2021-23-13. 
Additionally, the FAA acknowledges that radio altimeter anomalies could 
lead to increased flightcrew workload and flightcrew desensitization to 
warnings that could rise to an unsafe condition and result in 
additional rulemaking.

Request To Clarify Effect of Future TSO

    Comment summary: Thales requested clarification regarding 
retrofitting with a 5G tolerant radio altimeter and the effect of a 
future TSO.
    FAA response: Some rotorcraft may install filters between the radio 
altimeter and antenna or with an upgraded radio altimeter to become a 
radio altimeter tolerant rotorcraft. The FAA expects that the eventual 
new TSO will establish an internationally harmonized performance 
requirement, which will follow the existing international technical 
consensus on the establishment of the minimum operational performance 
standards

[[Page 40690]]

(MOPS). The FAA considers this AD an interim action because additional 
rulemaking may be necessary once a new radio altimeter TSO is 
developed, approved, and available.

Request To Change Unsafe Condition Statement

    Comment summary: In the NPRM, the FAA stated it was proposing the 
AD because radio altimeter anomalies that are undetected by the 
automation or pilot, particularly close to the ground, could lead to 
loss of continued safe flight and landing. Bell Textron commented that 
``continued safe flight and landing'' is associated with Category A 
rotorcraft and not Category B rotorcraft. The commenter requested the 
FAA revise that language to instead state that it could result in an 
unsafe condition.
    FAA response: The FAA disagrees. The FAA has evaluated the hazards 
associated with both Category A and B helicopter operations and 
determined that the severity of those hazards could lead to loss of 
continued safe flight and landing.

Comments Beyond Scope of NPRM

    Comment summary: The FAA also received and reviewed comments that 
were unrelated to the actions proposed in the NPRM or that did not make 
a request that the FAA can act on. These comments included questions 
about alternative technologies, which have not yet been developed, and 
future broadband towers outside of the locations authorized by the FCC. 
These comments are outside the scope of this AD.

Additional Change From the NPRM

    The FAA has added a terminating action in paragraph (j) of this AD 
for rotorcraft that have been modified to radio altimeter tolerant 
rotorcraft by allowing for the removal of the limitations from the RFM.

Conclusion

    The FAA reviewed the relevant data, considered any comments 
received, and determined that air safety requires adopting this AD as 
proposed in the NPRM, except for the changes described previously. 
Accordingly, the FAA is issuing this AD to address the unsafe condition 
on these products.

Interim Action

    The FAA considers this AD to be an interim action. The FAA 
considers that this AD is an interim action. Once the TSO standard for 
radio altimeters is established, which will follow the existing 
international technical consensus on the establishment of the MOPS, the 
FAA anticipates that the MOPS will be incorporated into the TSO. Once a 
new radio altimeter TSO is developed, approved, and available, the FAA 
might consider additional rulemaking.

Justification for Immediate Adoption and Determination of the Effective 
Date

    Section 553(b)(3)(B) of the Administrative Procedure Act (APA) (5 
U.S.C. 551 et seq.) authorizes agencies to dispense with notice and 
comment procedures for rules when the agency, for ``good cause,'' finds 
that those procedures are ``impracticable, unnecessary, or contrary to 
the public interest.'' Under this section, an agency, upon finding good 
cause, may issue a final rule without providing notice and seeking 
comment prior to issuance. Further, section 553(d) of the APA 
authorizes agencies to make rules effective in less than thirty days, 
upon a finding of good cause.
    Although the FAA provided the public with an opportunity to comment 
on the NPRM, as previously explained, this final rule contains a new 
requirement in figure 2 to paragraph (g)(1)(ii) of this AD for 
demonstrating a radio altimeter meets the tolerance for spurious 
emissions. The FAA is requesting public comments on this new 
requirement. However, the unsafe condition addressed by this AD 
requires the immediate adoption of this AD without providing this 
opportunity for public comments prior to adoption. Radio altimeter 
anomalies that are undetected by the aircraft automation or pilot, 
particularly close to the ground, could lead to loss of continued safe 
flight and landing. To address this unsafe condition, the actions 
required by this AD must be accomplished before the compliance date of 
June 30, 2023. The FAA based this date on the changes to the 5G C-Band 
environment beginning on July 1, 2023. These changes include increased 
wireless broadband deployment and transmissions closer to the 
parameters authorized by the FCC. Accordingly, notice and opportunity 
for prior public comment are impracticable and contrary to the public 
interest pursuant to 5 U.S.C. 553(b)(3)(B).
    The FAA finds that good cause exists pursuant to 5 U.S.C. 553(d) 
for making this amendment immediately effective for the same reasons 
the FAA found good cause to forgo notice and comment. In addition, the 
earlier operators learn of the requirements in this AD, the earlier 
they can take action to ensure compliance. An effective date less than 
30 days would ensure the AD is codified earlier, thereby increasing 
awareness of its requirements.

Comments Invited

    The FAA invites you to send any written data, views, or arguments 
about the spurious emissions PSD tolerance curve as specified in figure 
2 to paragraph (g)(1)(ii) of this final rule. Send your comments to an 
address listed under ADDRESSES. Include ``Docket No. FAA-2023-0668 and 
Project Identifier AD-2023-00199-T'' at the beginning of your comments. 
The most helpful comments reference a specific portion of the figure, 
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 this final rule 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 final rule.

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 AD 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 AD, 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 AD. Submissions containing CBI should be sent to David 
Swartz, Continued Operational Safety Technical Advisor, COS Program 
Management Section, Operational Safety Branch, FAA; 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.

Costs of Compliance

    The FAA estimates that there are approximately 5,500 helicopters of 
U.S. registry that are likely to have a radio

[[Page 40691]]

altimeter installed. The FAA also estimates that 1,128 of the 5,500 
helicopters of U.S. registry are equipped to be able to perform the 
operations prohibited by this AD for non-radio altimeter rotorcraft. 
The FAA estimates the following costs to comply with this AD:

                                                 Estimated Costs
----------------------------------------------------------------------------------------------------------------
                                                                                     Cost per      Cost on U.S.
                Action                         Labor cost           Parts cost        product        operators
----------------------------------------------------------------------------------------------------------------
RFM revision for non-radio altimeter    1 work-hour x $85 per                 $0             $85        $467,500
 tolerant rotorcraft (Retained action    hour = $85.
 from AD 2021-23-13).
RFM revision for non-radio altimeter    1 work-hour x $85 per                  0              85         467,500
 tolerant rotorcraft (new requirement).  hour = $85.
----------------------------------------------------------------------------------------------------------------

    The FAA estimates the following costs for operators who elect to 
modify their helicopter to be radio altimeter tolerant:

                                        Estimated Costs for Modification
----------------------------------------------------------------------------------------------------------------
               Action                         Labor cost                Parts cost           Cost per product
----------------------------------------------------------------------------------------------------------------
Modification (replacement).........  ...........................  ......................  Up to $40,000 per
                                                                                           helicopter
                                                                                          (includes labor and
                                                                                           parts).
Modification (filter)..............  Up to 24 work-hours x $85    Up to $8,000..........  Up to $10,040 per
                                      per hour = $2,040.                                   filter.
----------------------------------------------------------------------------------------------------------------

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

    This AD will not have federalism implications under Executive Order 
13132. This AD will 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 this AD:
    (1) Is not a ``significant regulatory action'' under Executive 
Order 12866,
    (2) Will not affect intrastate aviation in Alaska, and
    (3) Will not have a significant economic impact, positive or 
negative, on a substantial number of small entities under the criteria 
of the Regulatory Flexibility Act.

List of Subjects in 14 CFR Part 39

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

The Amendment

    Accordingly, under the authority delegated to me by the 
Administrator, the FAA amends 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) 2021-23-13, Amendment 39-21811 
(86 FR 69992, December 9, 2021), and
0
b. Adding the following new AD:

2023-11-07 Various Helicopters: Amendment 39-22453; Docket No. FAA-
2023-0668; Project Identifier AD-2023-00199-R.

(a) Effective Date

    This airworthiness directive (AD) is effective June 22, 2023.

(b) Affected ADs

    This AD replaces AD 2021-23-13, Amendment 39-21811 (86 FR 69992, 
December 9, 2021) (AD 2021-23-13).

(c) Applicability

    This AD applies to all helicopters, certificated in any 
category, equipped with a radio (also known as radar) altimeter. 
These radio altimeters are installed on various helicopter models 
including, but not limited to, the helicopters for which the design 
approval holder is identified in paragraphs (c)(1) through (20) of 
this AD.

(1) Airbus Helicopters
(2) Airbus Helicopters Deutschland GmbH
(3) Air Space Design and Manufacturing, LLC
(4) Bell Textron Canada Limited
(5) Bell Textron Inc.
(6) Brantly International, Inc.
(7) Centerpointe Aerospace Inc.
(8) Columbia Helicopters, Inc.
(9) The Enstrom Helicopter Corporation
(10) Erickson Air-Crane Incorporated, DBA Erickson Air-Crane
(11) Helicopteres Guimbal
(12) Siam Hiller Holdings, Inc.
(13) Kaman Aerospace Corporation
(14) Leonardo S.p.a.
(15) MD Helicopters Inc.
(16) PZL Swidnik S.A.
(17) Robinson Helicopter Company
(18) Schweizer RSG LLC
(19) Scotts-Bell 47 Inc.
(20) Sikorsky Aircraft Corporation

(d) Subject

    Air Transport Association (ATA) of America Code 3444, Ground 
Proximity System.

(e) Unsafe Condition

    This AD 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). The FAA is issuing this AD 
because radio altimeter anomalies that are undetected by the 
automation or pilot, particularly close to the

[[Page 40692]]

ground, could lead to loss of continued safe flight and landing.

(f) Compliance

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

(g) Definitions

    (1) For purposes of this AD, a ``radio altimeter tolerant 
rotorcraft'' is one for which the radio altimeter, as installed, 
demonstrates the tolerances specified in paragraphs (g)(1)(i) and 
(ii) of this AD, using a method approved by the FAA. No actions are 
required by this AD for radio altimeter tolerant rotorcraft.
    (i) Tolerance to radio altimeter interference, for the 
fundamental emissions (3.7-3.8 GHz), at or above the power spectral 
density (PSD) curve threshold specified in figure 1 to paragraph 
(g)(1)(i) of this AD.

Figure 1 to paragraph (g)(1)(i)--Fundamental Effective Isotropic PSD 
at Outside Interface of Rotorcraft Antenna
BILLING CODE 4910-13-P
[GRAPHIC] [TIFF OMITTED] TR22JN23.005

    (ii) Tolerance to radio altimeter interference, for the spurious 
emissions (4.2-4.4 GHz), at or above the PSD curve threshold 
specified in figure 2 to paragraph (g)(1)(ii) of this AD.

Figure 2 to paragraph (g)(1)(ii): Spurious Effective Isotropic PSD 
at Outside Interface of Rotorcraft Antenna

[[Page 40693]]

[GRAPHIC] [TIFF OMITTED] TR22JN23.006

    (2) For purposes of this AD, a ``non-radio altimeter tolerant 
rotorcraft'' is one for which the radio altimeter, as installed, 
does not demonstrate the tolerances specified in paragraphs 
(g)(1)(i) and (ii) of this AD.

(h) Retained Rotorcraft Flight Manual (RFM) Revision for Non-Radio 
Altimeter Tolerant Rotorcraft

    For non-radio altimeter tolerant rotorcraft: On or before 
January 4, 2022, revise the Limitations Section of the existing RFM 
for your helicopter by incorporating the limitations specified in 
figure 3 to paragraph (h) of this AD. This may be done by inserting 
a copy of this AD into the existing RFM for your helicopter. The 
action required by this paragraph may be performed by the owner/
operator (pilot) holding at least a private pilot certificate and 
must be entered into the aircraft records showing compliance with 
this AD in accordance with 14 CFR 43.9(a)(1) through (4) and 14 CFR 
91.417(a)(2)(v). The record must be maintained as required by 14 CFR 
91.417 or 14 CFR 135.439.

Figure 3 to paragraph (h)--RFM Revision

[[Page 40694]]

[GRAPHIC] [TIFF OMITTED] TR22JN23.007

(i) RFM Revision for Non-Radio Altimeter Tolerant Rotorcraft

    For non-radio altimeter tolerant rotorcraft, 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 RFM for your helicopter by including the information 
specified in figure 4 to paragraph (i) of this AD. This may be done 
by inserting a copy of this AD into the existing RFM for your 
helicopter. The action required by this paragraph may be performed 
by the owner/operator (pilot) holding at least a private pilot 
certificate and must be entered into the aircraft records showing 
compliance with this AD in accordance with 14 CFR 43.9(a)(1) through 
(4) and 14 CFR 91.417(a)(2)(v). The record must be maintained as 
required by 14 CFR 91.417 or 14 CFR 135.439. Incorporating the RFM 
revision required by this paragraph terminates the RFM revision 
required by paragraph (h) of this AD.
    (2) Before further flight after incorporating the limitations 
specified in figure 4 to paragraph (i) of this AD, remove the RFM 
revision required by paragraph (h) of this AD.

Figure 4 to paragraph (i)--RFM Revision for Non-Radio Altimeter 
Tolerant Rotorcraft
[GRAPHIC] [TIFF OMITTED] TR22JN23.008

(j) Terminating Action for RFM Limitations

    (1) Modifying the rotorcraft from a non-radio altimeter tolerant 
rotorcraft to a radio altimeter tolerant rotorcraft terminates the 
limitations in paragraph (i) of this AD for that rotorcraft.
    (2) After modifying the rotorcraft to a radio altimeter tolerant 
rotorcraft, the limitations specified by paragraph (i) of this AD 
may be removed from the RFM.

(k) 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 (l) 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 2021-23-13 are approved as AMOCs for 
the requirements specified in paragraph (h) of this AD until June 
30, 2023.

[[Page 40695]]

(l) Related Information

    For more information about this AD, contact David Swartz, 
Continued Operational Safety Technical Advisor, COS Program 
Management Section, Operational Safety Branch, FAA; phone: 817-222-
5390; email: [email protected].

(m) Material Incorporated by Reference

    None.

    Issued on June 15, 2023.
Michael Linegang, Acting Director,
Compliance & Airworthiness Division, Aircraft Certification Service.
[FR Doc. 2023-13319 Filed 6-16-23; 4:15 pm]
BILLING CODE 4910-13-C