DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2022-1647; Project Identifier AD-2022-01379-T;
Amendment 39-22438; AD 2023-10-02]
RIN 2120-AA64

Airworthiness Directives; Transport and Commuter Category Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: The FAA is superseding Airworthiness Directive (AD) 2021-23-
12, which applied to all transport and commuter category airplanes
equipped with a radio (also known as radar) altimeter. AD 2021-23-12
required 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). 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 stations whose signals are expected to cover most of the
contiguous United States at transmission frequencies between 3.7-3.98
GHz. For certain airplanes, this AD requires 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 AD also requires modifying certain
airplanes to allow safe operations in the U.S. 5G C-Band radio
frequency environment. The FAA is issuing this AD to address the unsafe
condition on these products.

DATES: This AD is effective May 26, 2023.

ADDRESSES: 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 final rule, any comments
received, and other information. The address for Docket Operations is
U.S. Department of Transportation, Docket Operations, M-30, West
Building Ground Floor, Room W12-140, 1200 New Jersey Avenue SE,
Washington, DC 20590.

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: operationalsafety@faa.gov.

SUPPLEMENTARY INFORMATION:

Background

The FAA issued a notice of proposed rulemaking (NPRM) to amend 14
CFR part 39 to supersede AD 2021-23-12, Amendment 39-21810 (86 FR
69984, December 9, 2021) (AD 2021-23-12). AD 2021-23-12 applied to all
transport and commuter category airplanes equipped with a radio (also
known as radar) altimeter. The NPRM published in the Federal Register
on January 11, 2023 (88 FR 1520). The NPRM was prompted by the
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 to maintain the requirements of AD
2021-23-12, except for the limitations pertaining to Required
Navigation Performance with Authorization Required (RNP AR) instrument
approach procedures (IAPs), by requiring revising the existing AFM to
incorporate limitations prohibiting certain operations in the presence
of 5G C-Band wireless broadband interference as identified by NOTAM.
Alternatively, the FAA proposed to allow operators to retain the AFM
revision required by paragraph (g) of AD 2021-23-12. The FAA also
proposed, on or before June 30, 2023, to 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 C-Band mitigated airports (5G CMAs) as identified in an
FAA Domestic Notice.
Lastly, the FAA proposed, on or before February 1, 2024, to require
that airplanes operating under 14 CFR part 121 be modified from a non-
radio altimeter tolerant airplane to a radio altimeter tolerant
airplane. The FAA proposed 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.

Discussion of Final Airworthiness Directive

Comments

The FAA provided the public with an opportunity to comment on the
proposed AD and received 82 submissions to Docket No. FAA-2022-1647.
The FAA received comments from individual commenters as well as from
organizations. The majority of the comments were from organizations
such as the International Air Transport Association (IATA), Airlines
for America (A4A), the Cargo Airline Association, the Aerospace
Industries Association (AIA), the Air Line Pilots Association,
International (ALPA), the Allied Pilots Association, the Regional
Airline Association (RAA), CTIA-The Wireless Association (CTIA), and
the American Association of Airport Executives (AAAE); manufacturers
such as Airbus DS (Airbus Defence and Space), Airbus SAS (Airbus), The
Boeing Company (Boeing), MHI RJ Aviation ULC (MHI RJ), Gulfstream
Aerospace Corporation (Gulfstream), Bombardier Inc. (Bombardier),
Textron Aviation (Textron), and Thales; and operators such as Atlas
Air, Inc. (Atlas), Frontier Airlines (Frontier), Southwest Airlines,
and Virgin Atlantic Airways.
The following summarizes the comments received on the NPRM, and
provides the FAA's responses.

A. Support for the NPRM

CTIA supported the NPRM without change.

B. Alternative Methods of Compliance (AMOCs)

1. Request for Continued NOTAMs and AMOCs
Comment summary: Some commenters asked if the FAA will continue
approving AMOCs for radio altimeter tolerant airplanes at non-5G CMAs
consistent with the process used for AD 2021-23-12. Airbus Defence and
Space asked whether the FAA will still allow AMOCs between July 1,
2023, and January 31, 2024. One commenter asked whether the FAA will
take into account the availability of a certified solution before
ceasing to process new AMOCs and, if not, when will FAA stop processing
AMOCs for non-radio altimeter tolerant airplanes. Other commenters
requested that the FAA continue to use NOTAMs and AMOCs until relevant
airplanes are retrofitted. Airbus asked when the FAA will stop issuing
NOTAMs for identification of the 5G environment.
FAA response: Since the publication of the NPRM, the FAA has
conducted further analysis of possible 5G C-Band interference to radio
altimeter tolerant airplanes at non-5G CMAs and has determined that the
risks associated with category (CAT) I autoland, CAT I head-up display
(HUD) to touchdown, and enhanced flight vision system (EFVS) to
touchdown operations, are mitigated to an acceptable level. The FAA
found a lower-than-expected likelihood of interference because of
current tower locations, a high percentage of flat terrain around
airports, and the expectation that future tower locations will impose
no additional interference than current towers do. Risks associated
with CAT II/III, SA CAT I, and SA CAT II have been mitigated at non-5G
CMAs because all current CAT II/III and SA CAT I/II operations are only
at 5G CMAs.\1\ Therefore, the FAA has determined that radio altimeter
tolerant airplanes may conduct these operations to all airports in the
contiguous U.S. without limitation.\2\ As a result, there is no need to
use a domestic notice to identify specific airports where radio
altimeter tolerant airplanes can perform these procedures. The FAA has
removed the references to 5G CMAs and Domestic Notices from the
regulatory requirements of this final rule.
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\1\ Locations of 5G CMAs can be found on the FCC's website at:
https://www.fcc.gov/ecfs/search/search-filings/filing/1033142661477.
\2\ This determination applies only to the unsafe condition
identified in this AD, and not to the model-specific unsafe
conditions addressed in AD 2022-02-16, AD 2022-03-05, AD 2022-03-20,
AD 2022-04-05, AD 2022-05-04, AD 2022-06-16, AD 2022-09-18, AD 2023-
03-06, and AD 2023-06-13. Copies of those ADs may be found on the
FAA's Dynamic Regulatory System website at www.drs.faa.gov.
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NOTAMs identifying the 5G environment are no longer practical
because the environment is expected to cover most of the contiguous
U.S. In addition, limitations required by this AD apply to non-radio
altimeter tolerant airplanes at all airports in the contiguous U.S. For
those airplanes, the FAA has determined that the AMOC process used for
AD 2021-23-12, which included generating monthly cleared runway lists
based on base station data for non-5G CMAs, will be untenable beyond
June 30, 2023, due to complexities associated with the continued
operational expansion of 5G C-Band emissions.
2. Request for Alternative Mitigation
Comment summary: Thales requested that the proposed AD be revised
to allow for other mitigations at the airplane level, based on
airplane-level architecture, including alerts and crew procedures
related to radio altimeter NCD (no computed data) or failure. Thales
stated that radio altimeter compliance with the tolerances specified in
paragraphs (g)(2)(i) and (ii) of the proposed AD is not the only way to
prevent the unsafe condition. Additionally, Airbus Defence and Space
stated that it expected stronger operations limitations for non-radio
altimeter tolerant airplanes, rather than full prohibition of the
operations.
FAA response: Although the FAA acknowledges that there may be other
ways to prevent the unsafe condition, the alternatives proposed by the
commenters must be evaluated on a case-by-case basis to determine an
acceptable level of safety. Because including such language to address
all airplane type designs would not be feasible in this AD, anyone may
propose alternative actions to address the unsafe condition under the
AMOC procedures referenced in paragraph (k) of this AD.
3. Request To Clarify Credit for Prior AMOCs
Comment summary: Bombardier stated that the existing AMOC
methodology remains valid, and therefore radio altimeter/airplane
configurations that receive approved AMOCs for 5G CMAs in June 2023
would meet the definition of radio altimeter tolerant airplanes.
Bombardier requested that the FAA clarify whether an FAA-approved AMOC
for AD 2021-23-12 is a ``method approved by the FAA'' for demonstrating
that an airplane is a radio altimeter tolerant airplane for purposes of
the AFM limitations that would otherwise be required by paragraph (i)
of the proposed AD.
FAA response: In the NPRM, the FAA proposed that AMOCs approved for
AD 2021-23-12 would only be approved for the AFM revision in paragraph
(h) of the proposed AD. The FAA approved AMOCs for AD 2021-23-12 before
the radio altimeter tolerant PSD (power spectral density) curve
proposed in the NPRM was defined. Although the FAA expects that the
airplanes with AMOCs approved for AD 2021-23-12 will be able to meet
the definition of ``radio altimeter tolerant airplane,'' operators will
need to provide the FAA with data showing explicitly that the airplane
meets the tolerances in paragraph (g)(2) before the FAA will approve
the method they propose to use.

C. AFM Limitations

1. Request To Change AFM Limitations
Comment summary: Frontier requested that the AD include language
allowing operators to omit any portion of the radio altimeter flight
restrictions that is not applicable to the operator, such as HUD and
EFVS. Frontier stated that this would eliminate confusion when the
specified equipment is not installed in the airplane or the operator is
not authorized to utilize the equipment.
FAA response: The FAA disagrees, as operators may change or add
equipment and approvals at a future time. If an airplane is not
equipped or approved for an approach, then the operational restrictions
would still broadly apply, but would have no impact to the operator.
2. AFM Limitations Inappropriate for General Operational Restrictions
Comment summary: MHI RJ and Air Wisconsin Airlines stated that the
AFM limitations section was not the appropriate area to document
operational restrictions not related to a specific airplane. An
individual commenter suggested that the proposed AFM revision does not
follow ``FAA AFM criteria.'' Gulfstream stated that the proposed
requirement to revise the AFM with limitations places an unnecessary
burden on original equipment manufacturers (OEMs). Gulfstream requested
that instead the FAA require that operators obtain Letters of
Authorization or operations specifications.
FAA response: The FAA disagrees. 14 CFR 91.9 prohibits any person
from operating a civil aircraft without complying with the operating
limitations specified in the AFM. The FAA routinely issues ADs to
mandate changes to the limitations section of an FAA-approved AFM for airplanes
in service.
3. Requests for Different Method of Incorporating AFM Limitations
Comment summary: Bombardier and Gulfstream requested the FAA allow
other options for incorporating the proposed limitations into the AFM,
due to the complexity of updating entire catalogs of flight manual
documentation and authoring, approving, and publishing customized
limitations based on the unique configuration and characteristics of
each airplane model. Specifically, Bombardier requested that the AD
include language to automatically delegate approval of AFM changes to
civil aviation authorities; automatically recognize AFM changes that
have been approved by Transport Canada Civil Aviation Authority for
Bombardier airplane models; and state that airplanes with specific AFM
revisions meet the intent of the proposed AD. Lastly, Bombardier
requested that the proposed AD allow compliance by either incorporating
or referencing an electronic or paper copy of the AD, since Bombardier
plans on making an electronic copy of the FAA AD available through the
Bombardier flight deck application's supplemental documents function.
FAA response: The FAA acknowledges that each owner/operator may
have a different method for incorporating revisions into the AFM for
its airplanes. This is why the FAA did not propose a specific method of
complying with this requirement. As long as the language added to the
limitations section of the AFM is identical to the language specified
in the applicable figure, owners/operators may make the revision
electronically, with pen-and-ink changes, by inserting a copy of the
AD, by inserting a copy of the applicable figure, by adopting the OEM's
AFM revision, or by any other method. To provide clarification, the FAA
has changed paragraphs (h), (i)(1), and (j)(1) of this AD to require
including ``the information'' specified in the figure instead of ``the
limitations'' specified in the figure. With regard to Bombardier's
request that the proposed AD be revised to state that Bombardier
airplanes meet the intent of paragraph (h) of this AD if they have
incorporated certain AFM revisions, the FAA disagrees. Although the
requested changes to the proposed AD may minimize some requests for
AMOC approvals, including language specific to all possible current and
future state-of-design 5G C-Band-related ADs, is out of the scope of
the intent of this AD.
Comment summary: In order to minimize unnecessary revisions to the
AFM language in the future, Bombardier asked the FAA to clarify why the
flight restrictions in the figures required by June 30, 2023, are
limited to the contiguous U.S. airspace and whether the situation will
evolve as various telecommunications companies deploy 5G services in
the C-Band outside the contiguous U.S.
FAA response: The FAA limited the flight restrictions in the
proposed figures to the contiguous U.S. based on Federal Communications
Commission (FCC) Report and Order FCC 20-22,\3\ which identifies radio
frequencies and power level conditions for the new C-Band services only
in the contiguous lower-48 states. In the event the FCC updates the
report and order to include additional states and U.S. territories, the
FAA might consider future rulemaking.
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\3\ 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-2022-1647, and at https://www.fcc.gov/document/fcc-expands-flexible-use-cband-5g-0.
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D. Effect of Winglets on Accomplishment of the Proposed Actions

Comment summary: Aviation Partners Boeing stated that installing
winglets under supplemental type certificate (STC) ST00830SE, STC
ST01219SE, STC ST01518SE, and STC ST01920SE on applicable Boeing models
does not affect accomplishment of the actions specified in the proposed
AD.
FAA response: The FAA agrees. The FAA has not changed this AD in
this regard.

E. Clarifications

1. Request To Clarify Terminology
Comment summary: In the NPRM preamble, the FAA explained that if
the unsafe condition is not addressed, it may result in a catastrophic
accident, incident, or event. Airbus stated that because
``catastrophic'' is part of the analysis conducted under 14 CFR
25.1309, the FAA's use of it in the NPRM could be misleading. Textron
requested that the FAA add language to the unsafe condition statement
in paragraph (e) of the proposed AD to clarify the severity of possible
failure conditions (catastrophic, hazardous) associated with 5G C-Band
interference.
FAA response: The FAA used the term ``catastrophic'' in the
preamble of the NPRM to indicate an event that would result in multiple
fatalities, usually with loss of the airplane. The unsafe condition
statement in paragraph (e) of this AD, which states that radio
altimeter anomalies could result in loss of continued safe flight and
landing or increased flightcrew workload and desensitization to
warnings, is sufficiently clear. No change to this AD is necessary
based on these comments.
2. Request To Clarify Relaxation on Non-Precision Approaches (NPAs) to
Certain Airports
Comment summary: Qatar Airways referenced the statement in the NPRM
that the proposed AD would no longer prohibit RNP AR IAPs and asked
whether the FAA was relaxing NPAs for non-radio altimeter-tolerant
airplanes other than RNP AR operations to airports with potential 5G C-
Band interference.
FAA response: NPAs were not included in the list of prohibited
operations in AD 2021-23-12, since an NPA is an instrument approach
that provides lateral guidance only, and does not rely on radio
altimeter inputs. Therefore, this AD does not address NPAs.
3. Request To Clarify Whether Compliance With AD 2021-23-12 Satisfies
AFM Revision Requirement
Comment summary: Qatar Airways asked the FAA to clarify the
statement in paragraph (h) of the proposed AD that ``If an operator has
complied with paragraph (g) of AD 2021-23-12, that action satisfies the
requirements of this paragraph.'' The commenter noted that RNP AR IAPs
are included in the list of prohibited operations in paragraph (g) of
AD 2021-23-12; however, the NPRM states that, after further FAA
analysis, those operations would no longer be prohibited by the
proposed AD.
FAA response: The commenter is correct that the prohibition in
paragraph (g) of AD 2021-23-12 includes RNP AR approaches, and the
prohibition in paragraph (h) of this AD does not. However, since all of
the requirements of paragraph (h) of this AD are included in paragraph
(g) of AD 2021-23-12, operators have the option of retaining the AFM
revision required AD 2021-23-12 instead of revising the AFM again to
comply with paragraph (h) of this AD even though it prohibits RNP AR
approaches that are not required by this AD. No change to this AD is
necessary based on this comment.
4. Request To Clarify Limitations for Tolerant and Non-Tolerant
Airplanes
Comment summary: Singapore Airlines, Airbus Defence and Space,
Airbus, Qatar Airways, AIA, AFR, and Air France requested clarification
of the limitations for radio altimeter tolerant
airplanes and non-radio altimeter tolerant airplanes, as related to 5G
CMAs and non-5G CMAs. Airbus also requested clarification regarding
retrofitting with a 5G tolerant radio altimeter and the effect of a
future technical standard order (TSO).
FAA response: In the NPRM, the FAA proposed that radio altimeter
tolerant airplanes could perform the otherwise prohibited operations at
5G CMAs, while non-radio altimeter tolerant airplanes would be
prohibited from performing those operations at all airports. As
explained in section B.1. of this final rule, since the NPRM was
published, the FAA has determined that radio altimeter tolerant
airplanes may perform the prohibited operations at all airports in the
contiguous U.S., as long as the telecommunications companies continue
to transmit within mitigated parameters.\4\ As a result, the FAA has
removed paragraph (j) of the proposed AD from this final rule. The
FAA's determination that non-radio altimeter tolerant airplanes will
not be able to safely perform the four prohibited operations at any
airport remains unchanged.
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\4\ 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 (hereinafter
referred to as ``voluntary commitments'' or ``voluntary agreement
letter dated March 31, 2023'') is in Docket No. FAA-2022-1647 and
can be found on the FCC's website at: https://www.fcc.gov/ecfs/search/search-filings/filing/1033142661477.
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Some radio altimeters may already demonstrate tolerance to 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 radio altimeter will need to be replaced
with an upgraded radio altimeter as established by a new radio
altimeter TSO, which will follow the existing international technical
consensus on the establishment of the minimum operational performance
standards (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.
5. Request To Clarify Multiple AFM Limitations
Comment summary: Textron asked the FAA to clarify the relationship
between the AFM limitation requirements in paragraphs (h) and (i) of
the proposed AD.
FAA response: As explained in section B.1. of this final rule,
since the NPRM was published, the FAA has determined that radio
altimeter tolerant airplanes may perform the prohibited operations at
all airports in the contiguous U.S. As a result, the FAA has revised
this AD so that all of the AFM revisions are required only for non-
radio altimeter tolerant airplanes. Those airplanes must incorporate
either the limitations in paragraph (h) of this AD or paragraph (g) of
AD 2021-23-12 until June 30, 2023. After June 30, 2023, non-radio
altimeter tolerant airplanes must replace those limitations with the
limitations in paragraph (i) of this AD. For operators of radio
altimeter tolerant airplanes, this AD terminates the AFM limitations
required by AD 2021-23-12.
6. Request To Clarify Applicability for Military Airplanes
Comment summary: The Department of Defense requested that the FAA
revise the NPRM to clarify that military aircraft, including civil
derivatives, are exempt. The commenter stated that the NPRM's reference
to part 121 operations creates confusion as to whether the AD applies
to civil derivative airplanes operated by the Department of Defense.
FAA response: The FAA disagrees. This AD applies to all airplanes
with an FAA type certificate in the transport or commuter category,
including military surplus airplanes and civil derivatives of military
airplanes. To the extent that the AFM revisions required by this AD
impose operational restrictions that apply only to civil aircraft,
those restrictions do apply to Department of Defense airplanes used in
civil operations in the national airspace system.
7. Factors Affecting Accomplishment of Required Actions
Comment summary: AIA requested that the NPRM preamble be revised to
acknowledge that quickly accomplishing alterations depends on many
factors, including adequate specification of the replacement equipment
and availability of updated equipment.
FAA response: The FAA acknowledges that factors including those
cited by the commenter impact the ability to accomplish the
modification of the radio altimeters as required by this AD. Since the
language from the NRPM that the commenter cited does not appear in this
final rule, no change to the AD is necessary.

F. 5G CMAs

Comment summary: The FAA received many comments concerning the
airports that will be included as a 5G CMA. All Nippon Airways
requested the FAA establish a system that allows radio altimeter
tolerant airplanes to operate at all U.S. airports without
restrictions. MHI RJ, Thales, Airbus, Qatar Airlines, Japan Airlines,
ALPA, and two individual commenters asked for information about the
list of 5G CMAs. Aerologic, Emirates Airline, Atlas, the Department of
Defense, and the Cargo Airline Association requested the FAA expand the
list to include as many airports as possible. Multiple commenters,
including A4A, Boeing, Airbus, AIA, ALPA, RAA, and the Cargo Airline
Association, requested clarification of the criteria used to determine
the 5G CMAs. Thales, Airbus, Allied Pilots Association, AIA, ALPA,
Gulfstream, and AAAE requested guidance for safe aviation operations at
non-5G CMAs.
FAA response: As mentioned in section B.1. of this final rule,
since the NPRM was published, the FAA conducted further analysis of
possible 5G C-Band interference with radio altimeter tolerant airplanes
and determined that radio altimeter tolerant airplanes are not
susceptible to the 5G C-Band interference this AD is addressing.
Therefore, this AD will not require operators of radio altimeter
tolerant airplanes to revise their AFM to prohibit the low-visibility
operations proposed in the NPRM. The FAA has revised this final rule
accordingly.

G. Compliance Time

Comment summary: While CTIA agreed with the FAA's proposed
compliance times, China Airlines, SkyWest Airlines, Embraer S.A.,
Airbus Defence and Space, the Association of Asia Pacific Airlines,
Qatar Airways, Endeavor Air, Virgin Atlantic Airways, Atlas Air, ATR,
Gulf Air Group, the Cargo Airline Association, Air Wisconsin Airlines,
Lynden Air Cargo, EVA Airways, AAAE, A4A, RAA, and an individual
commenter expressed concern, with many stating that modification of the
fleet would not be achievable by June 30, 2023, or by February 1, 2024.
The commenters requested extensions ranging from three months to two
years, based on the size of each operator's fleet and availability of
parts.
FAA response: The FAA carefully considered the impact of the loss
of low-visibility operations on the remaining 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 their FCC-authorized levels for a year and a half,
wireless companies are now able to operate at higher levels, yet still
not at the levels authorized.

Specifically, wireless companies expect to operate their networks in
urban areas with minimal restrictions due to the completion of
retrofits.\5\ Additionally, the FAA anticipates 19 additional
telecommunication 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 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.
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\5\ See the FAA website faa.gov/newsroom/faa-statements-5g.
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The FAA re-evaluated the February 1, 2024, date based on the latest
radio altimeter equipage data and determined that an extension is not
justified. The only airplanes operating under part 121 that are
forecast to be at risk of not being equipped by February 1, 2024, are
approximately 164 transport category airplanes that have older radio
altimeters with no support from the airplane OEMs or radio altimeter
manufacturers. Operators of those airplanes will need to make a
business decision to equip with later model radio altimeters or retire
those airplanes from part 121 operations, as after February 1, 2024,
this AD prohibits unmodified airplanes from operating under part 121 in
the contiguous U.S. The FAA and its foreign civil aviation authority
partners plan to expedite radio altimeter approvals for both part 121
and part 129 operators, and the FAA has used means such as approved
model list (AML) STCs to help with equipage.
In addition, because some airplanes operate under part 121 solely
outside of the contiguous U.S. airspace where the AD's requirements do
not apply, the FAA has revised figure 4 to paragraph (i) of this AD to
include a prohibition that states, ``As of February 1, 2024, [non-radio
altimeter tolerant airplanes] must not operate under 14 CFR part 121 in
the contiguous U.S.'' The FAA has also revised paragraph (k) of the
NPRM (paragraph (j) of this AD) from a modification requirement to a
terminating action for airplanes that have been modified to radio
altimeter tolerant airplanes by allowing for the removal of the
limitations from the AFM.

H. Costs

1. Small Business Status for Business Airplanes
Comment summary: One commenter stated that the vast majority of
business airplane operators under part 91 are small businesses as
defined by the Small Business Administration (SBA). The commenter
requested that the FAA not underestimate the choice small businesses
will have to make between an $80,000 retrofit and loss of utility of
the airplane during adverse weather conditions.
FAA response: The FAA has complied with the Regulatory Flexibility
Act for this AD and analyzed its impact on small businesses. However,
the FAA has identified an unsafe condition for which the agency could
not identify an appropriate alternative that sufficiently addresses the
safety problem. Further information regarding that analysis is provided
in section 2. of the Regulatory Flexibility Determination of the
preamble of this final rule.
2. Costs Underestimated for Legacy Airplanes
Comment summary: Lynden Air Cargo commented that the NPRM
underestimates the cost of modification for legacy airplanes that are
no longer in the ``as-delivered'' configuration and therefore lack
support from the OEM. The commenter stated there are significant costs
associated with the research and development, approval, and type design
amendment for new equipment.
FAA response: The FAA acknowledges that the certification cost is
not included in the estimate in this final rule. The FAA appreciates
the impact on operators of legacy fleets that do not have the support
of the airplane OEM. The FAA has been issuing letters accepting 5G C-
Band-resistant test data from the holders of TSO authorizations (TSOAs)
in order to assist independent entities in seeking approval in
situations like these to mitigate the cost impact. These letters are
available through the TSOA holders.
Regarding Lynden Air Cargo's comment on additional significant
costs, that comment is addressed in section 2. of the Regulatory
Flexibility Determination in the preamble of this AD. The FAA did not
change this AD as a result of this comment.
3. Request To Include Indirect Costs
Comment summary: Some commenters requested that FAA include costs
associated with development and certification, as well as with
operational impacts of the proposed AD such as delayed and canceled
flights.
FAA response: These comments are addressed in section 2. of the
Regulatory Flexibility Determination in the preamble of this AD. The
FAA did not change this AD as a result of these comments.
4. Request To Consider Costs for Non-Part 121 Operations
Comment summary: Textron commented that the FAA's estimated costs
almost exclusively addressed part 121 operations. Textron asked whether
airplanes that do not operate under part 121 are affected by the
prohibited operations and requested that the FAA include those
airplanes in the cost analysis.
FAA response: While part 121 operators own most of the affected
airplanes and bear the greatest cost associated with this AD, the FAA
is aware of the impact on other operators who choose not to modify
their airplanes to become radio altimeter tolerant. Regarding Textron's
request to include the cost of the impact of restricted operations for
those airplanes, that comment is addressed in section 2. of the
Regulatory Flexibility Determination in the preamble of this AD. The
FAA did not change this AD as a result of these comments.
5. Work-Hours Underestimated for AFM Updates
Comment summary: Bombardier and an individual stated that the
estimated cost of 1 work-hour per airplane at $85 per hour for revising
an AFM was too low.
FAA response: These comments are addressed in section 2. of the
Regulatory Flexibility Determination in the preamble of this AD. The
FAA did not change this AD as a result of these comments.
6. Cost Impact on Part 129 Operators
Comment summary: IATA stated the FAA's cost estimate is vastly
understated because it does not include costs for airplanes operating
under part 129. Ten other commenters agreed with IATA's comments.
FAA response: Although the FAA acknowledges and appreciates the
costs of retrofit for part 129 operators, the FAA did not include costs
for airplanes operating under part 129 because this AD does not impose
any requirements on non-U.S. registered airplanes operating into the
United States under part 129. Under International Civil Aviation
Organization (ICAO) Annex 8, Airworthiness of Aircraft, the state of
registry of an airplane is the state responsible for its airworthiness.
For this reason, FAA ADs apply only to U.S.-registered airplanes.
7. Inquiry Regarding Payment for Additional Upgrades
Comment summary: Emirates requested that, in the event additional
upgrades are needed due to the telecommunications companies not
following their voluntary agreements, the telecommunications companies
should be responsible for the cost of the upgrades.
FAA response: The FAA, as a federal agency, is responsible for all
directives, policies, and mandates issued under its authority. The FAA
does not have the authority to require telecommunications companies to
bear costs incurred in modifying privately owned aircraft.
8. Request To Revise Costs for Filters
Comment summary: SkyWest Airlines requested that the FAA re-
evaluate the part and labor costs for filter installation.
FAA response: This comment is addressed in section 2. of the
Regulatory Flexibility Determination in the preamble of this final
rule.
9. Request To Revise Cost Estimate Including Equipment To Meet Tolerant
Criteria
Comment summary: Airbus and the Cargo Airline Association stated
the FAA's cost estimate is too low. These commenters requested that the
FAA reconsider the cost estimate, including conducting a regulatory
evaluation, but did not provide cost data. Bombardier requested that
the FAA include a cost estimate for operators who are not required to
equip with an updated radio altimeter but chose to voluntarily do so.
FAA response: The FAA's cost evaluation reflects both a cost per
product and an estimated fleet cost, which the agency based on feedback
from airplane manufacturers, radio altimeter manufacturers, and
airlines. The FAA did conduct a regulatory evaluation in both the NPRM
and this final rule. Further information regarding that analysis is
provided in the Regulatory Flexibility Determination section of the
preamble of this final rule. As explained in section 2. of the
Regulatory Flexibility Determination, based on feedback from other
commenters, the FAA has revised some of the cost estimates in this
final rule.

I. Domestic Notice

Comment summary: Many commenters expressed concern about the use of
a Domestic Notice in the proposed AD and the additional burden it would
create for operators, as there is no routine subscription and
notification process for Domestic Notices (as there is with NOTAMs).
British Airways, Qatar Airways, A4A, Boeing, AIA, ALPA, AAAE, and EVA
Airways requested guidance on the process and revision cycle for the
FAA 5G C-Band Domestic Notice. Qatar Airways and Virgin Atlantic
Airways asked how the FAA 5G C-Band Domestic Notice will be
disseminated to foreign-registered operators.
FAA response: As explained in section B.1. of this final rule,
since the NPRM published, the FAA performed additional analysis and
determined that radio altimeter tolerant airplanes may conduct
operations at all airports in the contiguous U.S. without the
limitations imposed by this AD. As a result, there is no need to
identify specific airports where the radio altimeter tolerant airplanes
may operate. The FAA has removed the references to the FAA 5G C-Band
Domestic Notice from the regulatory text of this final rule.

J. FCC Codification

Comment summary: Many commenters expressed concern that the FAA
does not have authority to enforce the voluntary agreements between the
FAA and the telecommunications companies and questioned the possible
impacts if those companies stop honoring the agreements or change their
position. Airbus and Bombardier requested the FAA provide additional
information about the time duration of the agreements. Several of these
commenters asked the FAA to verify that the additional 19
telecommunications companies will also voluntarily agree to these
mitigations. Several commenters urged the FAA, the FCC, and the
National Telecommunications and Information Administration (NTIA) to
work together to develop binding long-term agreements. CTIA, however,
stated that the voluntary and coordinated approach has proven
successful for this issue so far and noted that the wireless industry
will continue to engage with the FAA and aviation industry.
FAA response: The commenters 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,
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 allow the FAA to continue to
address aviation safety by coordinating 5G C-Band effective isotropic
radiated power (EIRP) reductions when analysis indicates that a
proposed base station will exceed the permitted PSD values in the
runway safety zone of a 5G CMA runway, which ensures the FAA can
protect SA CAT I, SA CAT II, CAT II, and CAT III approach operations
without limitations.
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, AT&T, T-Mobile, UScellular, and
Verizon's commitment will last until January 1, 2028, at which point it
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.

K. Special Flight Permit Provisions

Comment summary: Go Jet Airlines and RAA asked whether the FAA will
issue special flight permits to allow operators to ferry airplanes to a
location to perform a radio altimeter upgrade.
FAA response: As provided in 14 CFR 39.23, the FAA may issue a
special flight permit to allow operators to fly their airplane to a
repair facility to perform work required by an AD unless the AD states
otherwise. Because this AD does not prohibit or limit the issuance of
special flight permits, they are allowed.

L. Interference Reports

Comment summary: In the NPRM, the FAA stated it had received over
420 reports of radio altimeter anomalies within a known location of a
5G C-Band deployment. Airbus and CTIA requested the FAA provide the
number of reports of radio altimeter anomalies collected by the FAA in
the same period of time in a comparable area before the deployment of
5G base stations. IATA and CTIA requested the FAA share the
approximately 100 reports of possible radio altimeter interference so
carriers can better understand and address the unsafe condition. CTIA
suggested the public would benefit from understanding the connection
between the data and the nature and scope of any coexistence concerns.
CTIA further suggested it would be helpful to understand how those
factors have been evaluated, how often reports in other contexts are
found to be unattributable, and what findings the FAA makes in those
other circumstances.

FAA response: The FAA received the reports referenced in the NPRM
from various sources and first determined which reports were associated
with radio altimeter-related systems in the vicinity of 5G C-Band
emitters. The FAA then reviewed all supporting information (e.g.,
maintenance data, aircraft and airport trends, and event description),
and closed reports where the event was due to maintenance, other
interference, or insufficient data. Because the FAA lacks the means to
definitively attribute a particular event to 5G C-Band interference,
the FAA determined that for the remaining events, 5G C-Band
interference could not be ruled out.
To the extent some commenters requested comparable data from before
and after the deployment of 5G C-Band base stations, no such data
exists. The FAA did not collect 5G C-Band interference report data
prior to activation of the C-Band. Therefore, a direct comparison is
not possible.

M. Non-Part 121 Flights

1. Request To Clarify Requirements for Airplanes Not Operating Under
Part 121
Comment summary: Numerous commenters asked about the proposed
requirements for airplanes not operating under part 121. AIA and five
other commenters asked why the modification is not required for
airplanes not operating under part 121, as all airplanes will see
degraded capabilities if the radio altimeter is not retrofitted.
Singapore Airlines requested that the FAA explain figure 3 to paragraph
(i) of the proposed AD, and whether airplanes not operating under part
121 can perform Instrument Landing System (ILS) CAT I IAPs after
February 1, 2024. Gulfstream and Bahamasair requested clarification of
the FAA's intent for future rulemaking to impose a modification
requirement for part 91 and part 135 operators.
FAA response: After June 30, 2023, this AD prohibits all transport
and commuter category airplanes, regardless of the type of operation
(part 91, part 135, part 121, etc.), from performing certain low-
visibility landing operations at any airport (as specified in figure 4
to paragraph (i) of this AD) unless they have upgraded their radio
altimeters. Airplanes without upgraded radio altimeters will be able to
operate into any airport, but cannot fly the prohibited low-visibility
operations. For airplanes that do not operate under part 121, these
restrictions, as well as the option to equip with an upgraded radio
altimeter, remain unchanged after February 1, 2024.
Only airplanes operating under part 121, in the contiguous U.S.,
must have a 5G C-Band-compatible radio altimeter (or install a
retrofit) prior to February 1, 2024. The FAA proposed this requirement
to address the accumulating risk for systems that are less hazardous
than low-visibility landings (for example, repeated false warnings from
the collision avoidance system from erroneous radio altimeter data).
The FAA determined that this accumulating risk will reach unacceptable
levels for part 121 operations in the contiguous U.S. after February 1,
2024. The FAA does not anticipate future rulemaking until a TSO
standard for radio altimeters is established.
2. Request To Clarify Part 129 Requirements
Comment summary: Eleven commenters asked for clarification of the
proposed AD with regard to airplanes operating under 14 CFR part 129.
British Airways, Virgin Atlantic Airways, and Qatar Airways asked the
FAA to explain the proposed requirements for airplanes operating under
part 129. The Association of Asia Pacific Airlines requested that the
FAA extend the proposed compliance date for part 129 operators. Boeing
requested that the FAA require the proposed modification for part 129
operators. Singapore Airways commented that the risk and unsafe
condition described in the NPRM would likely prompt the FAA's foreign
counterparts to mandate the upgrade to a radio altimeter-tolerant
airplane when operating in U.S. airspace and asked for clarification
that non-radio altimeter tolerant airplanes operating under part 129
could continue to use CAT I ILS approaches after February 1, 2024.
Airbus Defence and Space asked what the process would be for foreign
manufacturers and operators if the FAA's foreign counterparts do not
adopt the FAA's AD. A4A stated concern that the FAA is considering
different standards for domestic operators versus foreign operators,
which does not reflect a ``safety first'' approach.
FAA response: This AD does not impose any requirements, including
CAT I ILS, on non-U.S.-registered airplanes operating into the U.S.
under part 129. Under ICAO Annex 8, Airworthiness of Aircraft, the
state of registry of an airplane is the state responsible for its
airworthiness. For this reason, FAA ADs apply only to U.S.-registered
airplanes. To the extent the FAA's bilateral partners agree with the
FAA's finding of an unsafe condition in U.S. airspace, the FAA
encourages those authorities to adopt the FAA AD or similar
requirements as mandatory continuing airworthiness instructions for
airplanes registered in other countries. The FAA also plans to publish
information in the FAA's Aeronautical Information Publication to alert
international operators to the 5G C-Band situation in the U.S.,
including the agency's use of Domestic Notices. The FAA strongly urges
operators of foreign-registered airplanes to voluntarily comply with
the actions required by this AD when operating in the contiguous U.S.
given the unsafe condition affects their airplanes as much as the
airplanes subject to this AD.
3. Burden of Modification Requirement on Part 129 Operators
Comment summary: IATA commented that the part 129 carriers are
being disadvantaged by the proposed requirement to retrofit airplanes
with an upgraded radio altimeter. Specifically, IATA stated that radio
altimeter manufacturers are understandably prioritizing the equipment
needs of the U.S. fleet over non-U.S. air carriers; IATA also referred
to the FAA's exclusion of part 129 carriers from the roundtable
discussions the FAA has held to consult with impacted carriers on the
overall issue of 5G C-Band. Lufthansa Group, A4A, and ten other
commenters (air carriers and trade associations) expressly agreed with
IATA or stated similar concerns. Singapore Airways stated that the
required modification will worsen the supply chain issue with upgraded
radio altimeters.
FAA response: Although supply chain disparities and issues are
business matters beyond the authority of the FAA, the agency has worked
with radio altimeter manufacturers and airplane operators to help
ensure that filters and replacement units are available as quickly as
possible. The FAA is aware of these issues and acknowledges the
concerns regarding supply chain disruptions; however, due to the
reliance on the radar altimeter inputs for low-visibility landings and
the impending changes discussed in this final rule, the FAA has
determined that the restrictions are necessary to correct the unsafe
condition discussed in this AD.
To the extent that the commenters expressed concern about the
roundtable discussions, those discussions have been an overall
collaboration among the many stakeholders affected by 5G C-Band
deployment (U.S. federal agencies, the aerospace industry, the
telecommunications companies, and foreign civil aviation authorities)
and have not been limited to the FAA's ADs. Participants in these
discussions varied at each meeting; however, IATA was represented at
some of the meetings.

N. Operator Involvement

Comment summary: The Cargo Airlines Association commented that in
addition to the airplane OEMs and radio altimeter manufacturers,
airlines should participate in any future radio altimeter standards
development activity.
FAA response: Although airline operators are not usually members of
a standards development activity, they have sometimes been members in
the past for certain standards that have been airline operator centric.
Individuals may apply for membership on a committee, and acceptance
will be based on the committee chair's evaluation of the applicant.

O. PSD Curve and Associated Compliance Policy

1. Request for Part Numbers/Criteria for Radio Altimeter Tolerance
Comment summary: Many commenters stated concern that the process
for determining whether a radio altimeter meets the fundamental PSD
curve, as specified in the proposed AD, is not well defined and that
the requirement in the proposed AD of ``using a method approved by the
FAA'' is not adequate. Thales requested that the FAA provide an Issue
Paper, Advisory Circular, or other publicly available means of
compliance document. Airbus requested that the FAA clarify where
operators could find specific part numbers of radio altimeters that
would meet the definition in the proposed AD. Fourteen commenters, as
well as IATA and A4A, requested that the FAA provide a list of all
radio altimeters by part number that are considered tolerant under the
criteria discussed in the proposed AD. Gulf Air Group stated that
developing an FAA-approved method to demonstrate that an airplane is
radio altimeter tolerant should be the responsibility of the OEM, radio
altimeter manufacturer, or system integrator.
FAA response: The FAA has developed a policy statement that
provides a means of compliance with this AD for all transport and
commuter category airplanes and rotorcraft equipped with a radio
altimeter. The FAA 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 rotorcraft
is radio altimeter tolerant. The policy discusses compliance methods
that should be applied to programs for type certificates, amended type
certificates, STCs, and amended STCs. Furthermore, the FAA does not
maintain a list of tolerant radio altimeters because the determination
of a radio altimeter tolerant airplane must consider the installation
details, which vary from airplane to airplane. The proposed policy
addresses how to assess 5G C-Band tolerance. Although most data
submitted to demonstrate compliance in accordance with the FAA policy
statement will be proposed by design approval holders, any person/
entity can propose a method to demonstrate compliance.
2. Request To Clarify Acceptability of External Filters
Comment summary: Thales requested that the proposed AD be revised
to clearly state that installations with external filters can also be
used for compliance.
FAA response: The FAA stated in the preamble of the NPRM that radio
altimeter installations with external filters may be acceptable for
compliance with the requirements of this AD. The FAA is not requiring a
specific type of radio altimeter installation; the AD requires only
that the radio altimeter installation meet the radio altimeter
tolerance PSD curves. No change to this AD based is necessary based on
this comment.
3. Request To Identify Spurious Emissions Data
Comment summary: To determine what action may be necessary to
ensure safe aviation operations in the U.S., Thales requested that the
proposed AD include necessary spurious data that 5G network operators
should disclose to the FAA.
FAA response: The FAA disagrees. The spurious PSD curve that
defines a radio altimeter tolerant airplane for purposes of this AD is
based on the spurious emission limits documented in the voluntary
agreement letter dated March 31, 2023.
4. Request To Clarify Figure 1
Comment summary: Textron Aviation requested the FAA clarify at what
reference point the PSD requirements apply. The commenter stated it
assumed that they apply at the radio altimeter receive antenna input,
such that antenna characteristics, coax loss, and filter
characteristics would be included in the determination.
FAA response: The FAA agrees and has changed the title of figure 1
to paragraph (g)(1)(i) of this AD to reflect that the PSD requirements
apply at the antenna input to the radio altimeter, and that the figure
applies to the outward facing side of the antenna.
5. Request To Provide Additional Information on the Spurious Emission
Tolerance
Comment summary: AIA requested that the FAA provide more
information about the tolerances for determination of whether an
airplane is radio altimeter tolerant. Several commenters requested that
the FAA add 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 replaced the proposed fixed
emission level with a spurious PSD tolerance curve in figure 2 to
paragraph (g)(1)(ii) of this AD. Subsequent figures have been
redesignated accordingly.
6. Request To Recognize Installations as Minor Changes
Comment summary: Two commenters requested that the FAA revise the
proposed AD to allow modification of the airplane to a radio altimeter
tolerant airplane as a minor change to type design, to help expedite
approvals and make 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 airplane-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 characteristics affecting
airworthiness. For this reason, the FAA determined that FAA approval of
the method used for the modification was necessary before operators
could show compliance with this AD.
7. Request To Revise Tolerance Definition
Comment summary: Textron and Embraer asked the FAA to add language
to the definition of radio altimeter tolerant airplane to indicate the
frequency band being referenced (3.7-3.98 GHz).
FAA response: The FAA agrees and has changed paragraphs (g)(1)(i)
and (ii) of this AD to include the applicable frequency bands.
8. Request To Add Certain PSD Limit
Comment summary: Airbus and Bombardier requested that the FAA
revise the table at the bottom of the proposed PSD curve to add the
limit for 2500 feet above ground level. The commenters stated that this
would be consistent with the maximum operating range of popular radio
altimeter models installed on many airplanes and would avoid
extrapolation errors.
FAA response: The FAA disagrees. The FAA developed the PSD curve to
cover all transport and commuter radio altimeters and has determined
that any extrapolation errors are sufficiently small and will not
affect compliance or compromise safety.
9. Request To Revise Tolerance Requirements for Certain Operations
Comment summary: One commenter stated the proposed PSD values are
not appropriate for some airplane operations. In support, the commenter
stated that CAT I-only qualified airplanes do not require radio
altimeter data, and that CAT II and CAT I qualified airplanes do not
use radio altimeter data below 100 feet.
FAA response: The FAA infers that the commenter is requesting that
the proposed AD be revised for operators that perform only CAT I and
CAT II approaches. The FAA disagrees. The unsafe condition identified
by the FAA is related not only to low-visibility operations but also to
the 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 the effects will occur more frequently
as 5G C-Band services continue to be deployed throughout the contiguous
U.S. 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. Meeting the radio altimeter
tolerant PSD curve will minimize erroneous flight deck warnings.
10. Request for Clarification of Spurious Emissions Limit
Comment summary: MHI RJ stated that demonstrating tolerance to the
aggregate base station conducted spurious emissions level is not
possible at an airplane level since the received signal will depend on
many other undefined factors, such as distance from base station and
base station antenna performance. An individual commenter stated the
spurious signal level of -48 dBm/MHz is not consistent with the FCC's
regulator limit and free air attenuation, as the spurious signal and
radio altimeter signals will attenuate as the airplanes gets farther
from the 5G C-Band station.
FAA response: As stated in section O.5. of this final rule, the FAA
has determined that a spurious emissions PSD curve is a more
appropriate method to define performance than a single aggregate
spurious emissions level and revised this final rule accordingly.
11. Request for Different PSD Criteria
Comment summary: An individual commenter stated the proposed AD
would establish PSD criteria as though the 5G C-Band transmitter is
located on the runway between threshold and touchdown zone, which is
not realistic given the FAA approach criteria. The Department of
Defense stated the PSD curve is lacking information to properly
determine the impact to radio altimeters.
FAA response: The FAA disagrees. The proposed PSD curve was
validated using the actual locations of 5G C-Band transmitters with
respect to runway safety zones at 5G CMAs.
12. Request To Revise Unit of Measurement
Comment summary: AIA and ATR requested the FAA correct the
references of dBm from ``decibels per megahertz'' to ``decibel-
milliwatts per megahertz.''
FAA response: The FAA agrees; however, because the cited reference
does not appear in this final rule, no change to the AD is necessary.
13. Request for AD Coverage of 65 dBm/MHz (Rural)
Comment summary: Thales requested that the radio altimeter
performance criteria specified in figure 1 to paragraph (g)(2) of the
proposed AD be revised to explicitly cover any 5G emitter station up to
65 dB/MHz in the applicable 3.7-3.98 GHz band.
FAA response: The FAA performed additional analysis, considering
both rural power levels (65 dBm/MHz) and urban power levels (62 dBm/
MHz), and determined that radio altimeter tolerant airplanes are safe
to fly to all airports in the contiguous U.S. However, no change to the
AD is necessary based on this comment.

P. RNP AR

1. Operation Under RNP AR IAP
Comment summary: Some commenters expressed concern over the FAA's
proposal to remove RNP AR IAPs from the list of prohibited operations.
Allied Pilots Association and AIA stated RNP AR approaches are commonly
used in high terrain environments where reliable TAWS functionality is
necessary. ALPA requested information on maintaining operational safety
while conducting RNP AR IAPs, especially at terrain-impacted runways.
FAA response: The FAA included RNP AR in the original list of
prohibited operations because it was unclear how 5G C-Band wireless
broadband interference would affect this operation. Unlike other
operations prohibited by the AD, RNP AR operations do not rely on
direct radio altimeter inputs to determine arrival at minimums or for
direct inputs that affect the flight path of the airplane. RNP AR
operations require operational TAWS equipment; however, TAWS is not
directly required for the procedure. An erroneous radio altimeter
output could affect maximum allowed bank angle, which could affect
course adherence. However, pilots would get an ``unable RNP'' message
and take appropriate action. After further analysis, the FAA determined
that 5G C-Band interference does not create an unsafe condition
specific to the conduct of an RNP AR IAP. While there is a risk of
erroneous TAWS warnings in the presence of 5G C-Band, that risk is not
limited to RNP AR operations, but rather applies to all operations. To
minimize the number of erroneous system messages and the unsafe
condition they produce, the FAA is requiring that all airplanes
operating under part 121 meet the PSD performance curves to operate in
the contiguous U.S. after February 1, 2024.
2. Request To Clarify AFM Prohibitions
Comment summary: Emirates stated that figure 3 to paragraph (i) and
figure 4 to paragraph (j) of the proposed AD contain prohibitions for
RNP AR IAPs and requested that the FAA clarify whether this is a
typographical error.
FAA response: In the NPRM, the FAA intentionally removed RNP AR
from the proposed figures referenced by the commenter. This AD does not
prohibit RNP AR IAPs.

Q. Additional Changes to NPRM

1. Request To Correct Paragraph Reference
Comment summary: Qatar Airways suggested that the reference to
``paragraphs (k)(i) and (ii)'' in paragraph (k)(1) of the proposed AD
be changed to ``paragraphs (k)(1)(i) and (ii).''
FAA response: The commenter correctly noted this error in the
proposed AD; however, because of other changes to paragraph (k)(1) of
the proposed AD (paragraph (j) of this final rule), as described in
section B.1. of this final rule, the requested change is not necessary.
2. Request To Remove Yabora From Applicability
Comment summary: An individual noted that the AD applicability
includes Yabora Industria Aeronautica S.A.
(Yabora), but the type certificate for Yabora models is
currently held by Embraer. The commenter suggested that Yabora
be removed from the applicability of this AD.
FAA response: The FAA has removed Yabora Industria
Aeronautica S.A. from the applicability of this AD and corrected
the clerical error by changing paragraph (c)(4) of this AD to state
that type certificates previously held by Yabora are now held by
Embraer S.A. However, because paragraph (c) of this AD uses the
language ``including, but not limited to,'' before listing the names of
various type certificate holders, the AD applies to any transport or
commuter category airplane equipped with a radio altimeter, regardless
of the name of the type certificate holder. In this case, the AD
applies to the airplanes whose type certificates were previously held
by Yabora that are now held by Embraer S.A.

R. Comments Outside Scope of NPRM

Comment summary and FAA response: The FAA also received and
reviewed several comments that were very general, stated the
commenter's viewpoint without a suggestion specific to the AD, or did
not make a request the FAA can act on. Some comments asked about other
Boeing-specific ADs or about the updated radio altimeter MOPS. These
comments are outside the scope of this AD.

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. None
of the changes will increase the economic burden on any operator.
Accordingly, the FAA is issuing this AD to address the unsafe condition
on these products.

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.

Effective Date

Section 553(d) of the Administrative Procedure Act (APA) (5 U.S.C.
551 et seq.) requires publication of a rule not less than 30 days
before its effective date. However, section 553(d) authorizes agencies
to make rules effective in less than 30 days when the agency finds
``good cause.'' Radio altimeter anomalies that are undetected by the
aircraft 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. 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. 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.
Therefore, the FAA finds that good cause exists pursuant to 5 U.S.C.
553(d) for making this amendment immediately effective.

Costs of Compliance

The FAA estimates that this AD affects approximately 1,000
airplanes of U.S. registry.
As of the date of publication of this AD, there are approximately
8,000 transport and commuter category airplanes of U.S. registry. 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 have already been equipped or are being retrofitted
to address radio altimeter interference tolerance, and thus will have
to take no actions to comply with this AD. Based on information
received, some operators will comply with the modification requirement
by replacing the radio altimeter with a new upgraded or modified radio
altimeter, and others will comply by installing an externally mounted
filter. The FAA estimates that approximately 180 airplanes will require
radio altimeter replacement and 820 airplanes will require addition of
radio altimeter filters to comply with the modification requirement. As
such, the FAA estimates the following costs to comply with this AD, for
a total U.S. fleet cost of compliance of up to $35,152,000.

Estimated Costs

The benefits of the AD 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 that have caused
accidents and fatalities during commercial air transportation in the
U.S. This AD will 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 AD will 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 Flexibility Determination

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.
Agencies must perform a review to determine whether a rule will
have a significant economic impact on a substantial number of small
entities. If the agency determines that it will, the agency must
prepare a regulatory flexibility analysis as described in the RFA.
The FAA published an Initial Regulatory Flexibility Analysis (IRFA)
(88 FR 1520, January 11, 2023) for Docket No. FAA-2022-1647, Project
Identifier AD-2022-01379-T, to aid the public in commenting on the
potential impacts to small entities. The FAA considered the public
comments in developing both the final rule and this Final Regulatory
Flexibility Analysis (FRFA). A FRFA must contain the following:
(1) A statement of the need for, and objectives of, the rule;
(2) A statement of the significant issues raised by the public
comments in response to the IRFA, a statement of the assessment of the
agency of such issues, and a statement of any changes made in the final
rule as a result of such comments;
(3) The response of the agency to any comments filed by the Chief
Counsel for Advocacy of the SBA in response to the proposed rule, and a
detailed statement of any change made in the final rule as a result of
the comments;
(4) A description of and an estimate of the number of small
entities to which the rule will apply or an explanation of why no such
estimate is available;
(5) A description of the projected reporting, recordkeeping, and
other compliance requirements of the proposed rule, including an
estimate of the classes of small entities that will be subject to the
requirement and the type of professional skills necessary for
preparation of the report or record; and
(6) A description of the steps the agency has taken to minimize the
significant economic impact on small entities consistent with the
stated objectives of applicable statutes, including a statement of the
factual, policy, and legal reasons for selecting the alternative
adopted in the final rule and why each of the other significant
alternatives to the rule considered by the agency that affect the
impact on small entities was rejected.

1. Need for and Objectives of the Rule

This AD replaces AD 2021-23-12 and requires revising the
limitations section of the existing AFM to incorporate limitations
prohibiting certain operations requiring radio altimeter data for
airplanes susceptible to 5G C-Band interference. This AD also requires
modifying certain airplanes to allow safe operations in the U.S. 5G C-
Band radio frequency environment by February 1, 2024. The more
restrictive limitations in this AD are needed due to the continued
deployment of new 5G C-Band base stations whose signals are expected to
cover most of the contiguous U.S. at transmission frequencies between
3.7-3.98 GHz. This AD addresses the unsafe condition resulting from the
continued deployment of 5G C-Band transmissions and their interference
to radio altimeters.
The FAA's legal basis for this AD is discussed in detail under the
``Authority for this Rulemaking'' section.

2. Significant Issues Raised in Public Comments

The FAA published an IRFA for Docket No. FAA-2022-1647, Project
Identifier AD-2022-01379-T, and requested comments.
One commenter stated that the vast majority of business airplane
operators under part 91 are small businesses as defined by the SBA. The
commenter requested that the FAA not underestimate the choice small
businesses will have to make between an $80,000 retrofit and loss of
utility of the airplane during adverse weather conditions.
As explained in more detail in section 4. of this Regulatory
Flexibility Determination, the FAA identified 31 small entities that
own and operate airplanes affected by this AD. Those entities fall
under North American Industry Classification System (NAICS) code
481111, 481112, 481211, or 481212 with a small business size standard
of a maximum of 1,500 employees, or under NAICS code 481219 with a
small business size standard of a maximum of $25 million in average
annual receipts, to be considered small business. The FAA did not
receive any comments with data concerning this part of the FAA's
regulatory analysis or concerning the estimated revenue impact for
small businesses to comply with this AD. The FAA determined that no
changes are necessary as a result of these comments.
Lynden Air Cargo commented that there are significant costs
associated with the research and development, approval, and type design
amendment for new equipment. Textron commented that the costs
associated with development and certification were not included in the
FAA's cost estimate. Textron, Atlas Air, A4A, and Bombardier requested
that the FAA include costs associated with impacts of the AD, such as
delayed and canceled flights and the costs of restricted operations.
The commenters are correct that these additional costs were not
included in the FAA's estimated costs. The cost analysis in FAA AD
rulemaking actions typically only contain the direct costs associated
with the specific actions
required by the AD. The FAA does not include secondary costs such as
the time necessary for planning or time necessitated by other
administrative actions, or indirect costs such as those resulting from
delayed or canceled flights and restricted operations. The FAA lacks
the data necessary to quantify those costs, which might vary
significantly among operators; the commenters did not provide such data
either.
Bombardier and an individual stated that the estimated cost of one
work-hour per airplane at $85 per hour for revising an AFM was too low
and omitted the costs of authoring the revisions, reviewing the
revisions, and briefing flight crews.
The FAA disagrees. The FAA uses one work-hour as a standard
estimate in ADs that require an administrative function such as a
revision to a flight manual. Operators and pilots must become familiar
with the AFM before beginning a flight because of other FAA
regulations, so that is not a cost associated with this AD.
SkyWest Airlines commented that its part and labor costs for filter
installation were nearly twice the costs specified in the NPRM and
requested the FAA re-evaluate the cost estimate.
The cost for filters specified in the NPRM was based on preliminary
estimates. Based on this comment, the FAA has revised the cost estimate
for the filter installation in this final rule.

3. Response to SBA Comments

The Chief Counsel for Advocacy of the SBA did not file any comments
in response to the NPRM.

4. Small Entities to Which the Rule Will Apply

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 SBA to define ``small business'' by issuing
regulations.
SBA has established size standards for various types of economic
activities, or industries, under the NAICS.\6\ These size standards
generally define small businesses based on the number of employees or
annual receipts.
---------------------------------------------------------------------------

\6\ SBA Table of Size Standards. Effective March 17, 2023.
https://www.sba.gov/document/support--table-size-standards.
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The following table shows the SBA size standards for FAA
certificate holders. Note that the SBA definition of a small business
applies to the parent company and all affiliates as a single entity.

Small Business Size Standards: Air Transportation

The modification costs of this AD affect certificate holders
authorized to conduct operations under 14 CFR part 121. To identify
which of those certificate holders may be 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 AD applies.

Estimated Number of Small Entities

Therefore, the FAA estimated that this AD impacts 31 small
entities.

5. Projected Reporting, Recordkeeping, and Other Compliance
Requirements

No new recordkeeping or reporting requirements are associated with
the AD. As discussed previously, the FAA estimates that the majority of
airplanes operated by small entities will already be equipped in a
manner that requires no actions to comply with this AD. For the
remaining number of airplanes, small entity compliance with the AD
would entail incorporation of AFM revisions at an approximate cost of
$170 per airplane. For the modification requirement of this AD, the FAA
anticipates that a small number of airplanes will need to have radio
altimeter filters installed (at an approximate cost of $10,040 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. 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 compliance
costs to their customers in the form of price increases.

6. Significant Alternatives Considered

As part of the FRFA, the FAA is required to consider regulatory
alternatives that may be less burdensome. The FAA did not find any
significant regulatory alternatives that would still accomplish the
safety objectives of this AD.
Operators may also propose a less burdensome method for mitigating
the unsafe condition using the AMOC procedures found in 14 CFR 39.19.

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, and
(2) Will not affect intrastate aviation in Alaska.

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

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]

2. The FAA amends Sec. 39.13 by:

a. Removing Airworthiness Directive (AD) 2021-23-12, Amendment 39-21810
(86 FR 69984, December 9, 2021); and

b. Adding the following new AD: