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ADs updated daily at www.Tdata.com
PROPOSED AD TRANSPORT AND COMMUTER CATEGORY AIRPLANES: Docket No. FAA-2022-1647; Project Identifier AD-2022-01379-T.
(a) COMMENTS DUE DATE

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

(b) AFFECTED ADS

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

(c) APPLICABILITY

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

(1) The Boeing Company

(2) Airbus SAS

(3) Bombardier Inc.

(4) Embraer S.A.

(5) Gulfstream Aerospace Corporation

(6) Gulfstream Aerospace LP

(7) Textron Aviation Inc.

(8) Pilatus Aircraft Limited

(9) Fokker Services B.V.

(10) Saab AB, Support and Services

(11) DeHavilland Aircraft of Canada Limited

(12) Airbus Canada Limited Partnership

(13) ATR-GIE Avions de Transport Regional

(14) Yabora Industria Aeronautica S.A.

(15) MHI RJ Aviation ULC

(16) BAE Systems (Operations) Limited

(17) Lockheed Martin Corporation/Lockheed Martin Aeronautics Company

(18) Viking Air Limited

(19) Dassault Aviation

(d) SUBJECT

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

(e) UNSAFE CONDITION

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

(f) COMPLIANCE

    Comply with this AD within the compliance times specified,  unless al-
    ready done.

(g) DEFINITIONS

(1) For purposes of this AD,  a "5G C-Band mitigated airport"  (5G CMA) is
    an airport at  which the telecommunications  companies have agreed  to
    voluntarily limit their  5G deployment at  the request of  the FAA, as
    identified by an FAA Domestic Notice.

(2) For purposes of this AD,  a "radio altimeter tolerant airplane" is one
    for which the radio altimeter,  as installed,  demonstrates the toler-
    ances specified in paragraphs (g)(2)(i) and (ii) of this AD,  using  a
    method approved by the FAA.

(i) Tolerance to radio altimeter interference  at or above the power spec-
    tral density (PSD) curve threshold  specified in figure 1 to paragraph
    (g)(2) of this AD.

(ii) Tolerance to an aggregate base station  conducted  spurious  emission
     level of -48 dBm/MHz in the 4200-4400 MHz radio altimeter band.

                            ILLUSTRATION (Figure 1)

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

(h) AIRPLANE/AIRCRAFT FLIGHT MANUAL (AFM) REVISION FOR ALL AIRPLANES

    Before further flight: Revise the Limitations Section of the  existing
    AFM  by  incorporating  the  limitations  specified  in  figure  2  to
    paragraph (h)  of this  AD. This  may be  done by  inserting a copy of
    figure 2  to paragraph  (h) of  this AD  into the  existing AFM. If an
    operator has complied with paragraph (g) of AD 2021-23-12, that action
    satisfies the requirements of this paragraph.

                   FIGURE 2 TO PARAGRAPH (H)  AFM REVISION
    ______________________________________________________________________
                                               (Required by AD 20**-**-**)
    Radio Altimeter Flight Restrictions

    When operating in U.S. airspace,  the following  operations  requiring
    radio altimeter are prohibited  in the presence  of 5G C-Band wireless
    broadband interference  as identified by NOTAM  (NOTAMs will be issued
    to state the specific airports where the radio altimeter is unreliable
    due to the presence of 5G C-Band wireless broadband interference):
    - Instrument Landing System (ILS) Instrument Approach Procedures (IAP)
      SA CAT I, SA CAT II, CAT II, and CAT III
    - Automatic Landing operations
    - Manual Flight Control Guidance System operations to landing/head -up
      display (HUD) to touchdown operation
    - Use of Enhanced Flight Vision System  (EFVS)  to  touchdown under 14
      CFR 91.176(a)
    ______________________________________________________________________

(i) AFM REVISION FOR NON-RADIO ALTIMETER TOLERANT AIRPLANES

    For non-radio altimeter tolerant airplanes,  do the actions  specified
    in paragraphs (i)(1) and (2) of this AD.

(1) On  or  before June 30, 2023,  revise  the  Limitations Section of the
    existing AFM by incorporating the limitations specified in figure 3 to
    paragraph (i)  of this  AD.  This  may be done by  inserting a copy of
    figure  3  to  paragraph  (i)  of  this  AD  into  the  existing  AFM.
    Incorporating the AFM revision  required by this paragraph  terminates
    the AFM revision required by paragraph (h) of this AD.

(2) Before further flight after incorporating the limitations specified in
    figure 3 to paragraph (i) of this AD, remove the AFM revision required
    by paragraph (h) of this AD.

       FIGURE 3 TO PARAGRAPH (I)  AFM REVISION FOR NON-RADIO ALTIMETER
                                   TOLERANT AIRPLANES                  
    ______________________________________________________________________
                                               (Required by AD 20**-**-**)
    Radio Altimeter Flight Restrictions

    Due to the presence of 5G C-Band wireless broadband interference, when
    operating in the contiguous U.S. airspace,  the  following  operations
    requiring radio altimeter are prohibited:
    - Instrument Landing System (ILS) Instrument Approach Procedures (IAP)
      SA CAT I, SA CAT II, CAT II, and CAT III
    - Automatic Landing operations
    - Manual Flight Control Guidance System operations to landing/head -up
      display (HUD) to touchdown operation
    - Use of Enhanced Flight Vision System  (EFVS)  to  touchdown under 14
      CFR 91.176(a).
    ______________________________________________________________________

(j) AFM REVISION FOR RADIO ALTIMETER TOLERANT AIRPLANES

    For radio altimeter tolerant airplanes,  do  the  actions specified in
    paragraphs (j)(1) and (2) of this AD.

(1) On  or  before June 30, 2023,  revise  the  Limitations Section of the
    existing AFM by incorporating the limitations specified in figure 4 to
    paragraph (j)  of this  AD. This  may be  done by  inserting a copy of
    figure  4  to  paragraph  (j)  of  this  AD  into  the  existing  AFM.
    Incorporating the AFM revision  required by this paragraph  terminates
    the AFM revision required by paragraph (h) of this AD.

(2) Before further flight after incorporating the limitations specified in
    figure 4 to paragraph (j) of this AD, remove the AFM revision required
    by paragraph (h) of this AD.

    FIGURE 4 TO PARAGRAPH (J)  AFM REVISION FOR RADIO ALTIMETER TOLERANT
                                AIRPLANES                                
    ______________________________________________________________________
                                               (Required by AD 20**-**-**)
    Radio Altimeter Flight Restrictions

    Due to the presence of 5G C-Band wireless broadband interference, when
    operating in the contiguous U.S. airspace,  the  following  operations
    requiring radio altimeter  are  prohibited  unless  operating  at a 5G
    C-Band mitigated airport as identified in an FAA Domestic Notice:
    - Instrument Landing System (ILS) Instrument Approach Procedures (IAP)
      SA CAT I, SA CAT II, CAT II, and CAT III
    - Automatic Landing operations
    - Manual Flight Control Guidance System operations to landing/head -up
      display (HUD) to touchdown operation
    - Use of Enhanced Flight Vision System  (EFVS)  to  touchdown under 14
      CFR 91.176(a).
    ______________________________________________________________________

(k) MODIFICATION

(1) For non-radio altimeter tolerant airplanes operating under 14 CFR part
    121: On or before February 1, 2024,  modify  each  airplane to a radio
    altimeter tolerant airplane  and  accomplish  the actions specified in
    paragraphs (k)(i) and (ii) of this AD.

(i) Revise the Limitations Section  of  the  existing AFM by incorporating
    the limitations  specified  in figure 4  to paragraph (j)  of this AD.
    This may be done  by inserting a copy of figure 4  to paragraph (j) of
    this AD into the existing AFM.

(ii) Remove the AFM revision required by paragraph (i) of this AD.

(2) For non-radio altimeter tolerant airplanes  not  operating  under part
    121, accomplishing the modification and AFM revision specified in par-
    agraph (k)(1) of this AD is optional.

(l) ALTERNATIVE METHODS OF COMPLIANCE (AMOCS)

(1) The  Manager,  Operational Safety Branch,  FAA,  has  the authority to
    approve AMOCs for this AD, if requested using the procedures found  in
    14 CFR 39.19. In  accordance with 14 CFR  39.19, send your request  to
    your principal  inspector or  responsible Flight  Standards Office, as
    appropriate. If  sending information  directly to  the manager  of the
    Operational Safety  Branch, send  it to  the attention  of the  person
    identified in paragraph (m) of this AD. Information may be emailed to:
    AMOC@faa.gov.

(2) Before using any approved AMOC,  notify your appropriate principal in-
    spector, or lacking a principal inspector,  the manager of the respon-
    sible Flight Standards Office.

(3) AMOCs approved for AD 2021-23-12 are approved as AMOCs for the require
    -ments specified in paragraph (h) of this AD.

(m) RELATED INFORMATION

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

(n) MATERIAL INCORPORATED BY REFERENCE

    None.

Issued on January 6, 2023.  Gaetano A. Sciortino, Acting Director, Compli-
ance & Airworthiness Division, Aircraft Certification Service.

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

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

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

Airworthiness Directives; Transport and Commuter Category
Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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

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

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

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

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

SUPPLEMENTARY INFORMATION:

Comments Invited


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

Confidential Business Information

CBI is commercial or financial information that is both customarily
and actually treated as private by its owner. Under the Freedom of
Information Act (FOIA) (5 U.S.C. 552), CBI is exempt from public
disclosure. If your comments responsive to this NPRM contain commercial
or financial information that is customarily treated as private, that
you actually treat as private, and that is relevant or responsive to
this NPRM, it is important that you clearly designate the submitted
comments as CBI. Please mark each page of your submission containing
CBI as ``PROPIN.'' The FAA will treat such marked submissions as
confidential under the FOIA, and they will not be placed in the public
docket of this NPRM. Submissions containing CBI should be sent to Brett
Portwood, Continued Operational Safety Technical Advisor, COS Program
Management Section, Operational Safety Branch, FAA, 3960 Paramount
Boulevard, Lakewood, CA 90712-4137; phone: 817-222-5390; email:
operationalsafety@faa.gov. Any commentary that the FAA receives that is
not specifically designated as CBI will be placed in the public docket
for this rulemaking.

Background

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

Actions Since AD 2021-23-12

Airplane Capability and Alterations: Since issuing AD 2021-23-12,
the FAA has reviewed data from dozens of alternative method of
compliance (AMOC) requests, demonstrating that these radio altimeters
can be relied upon to perform their intended function when operating
beyond a certain protection radius around 5G C-Band transmitters. The
iterative AMOC process allowed the FAA to gain insight into 5G C-Band
transmission impacts to runway safety zones ¹ in a progressively more
sophisticated manner. At first, the FAA made conservative assumptions
about the potential for impact on radio altimeters from 5G C-Band
transmissions and applied them to all airport environments. During the
FAA's initial analyses of AMOC requests, the FAA looked to protect
against 5G C-Band interference during the most critical phases of
flight (takeoffs and landings) by protecting a 2-nautical mile circle
around the ends of runways. After some time and an improved
understanding of the C-Band signals and their effects on specific radio
altimeters, the FAA was able to reduce the protected area around the
ends of runways and instead define a rectangular airspace area to
protect around runways. The rectangular area was further refined into a
trapezoidal area, which allowed for geographically expanded 5G C-Band
transmissions that would not affect radio altimeter functions within
the area. The FAA is now able to assess the 5G C-Band transmissions'
impact to aviation operations in a specific area, taking into account
the particularities of the signal and the airport environment. This
assessment process is the Signal in Space (SiS) analysis. It includes a
3-dimensional model for the runway safety zone and considers base
station heights and terrain around the airport.
---------------------------------------------------------------------------

¹ Runway safety zones are those areas around a runway where
radio altimeters on transport and commuter category airplanes must
function accurately and reliably during critical phases of flight
where radio altimeter interference is most likely to result in a
catastrophic accident.
---------------------------------------------------------------------------

The AMOC process also provided data about the varying levels of
interference tolerance for a majority of radio altimeters on the
market, allowing the FAA to understand the overall susceptibility to
interference of the existing fleet of transport airplanes. In addition,
the FAA learned about the aircraft alterations that can be accomplished
quickly to improve a radio altimeter's tolerance to transmissions in
adjacent or nearby spectrum bands. Now that the FAA better understands
the performance of specific radio altimeters and the means to make them
more tolerant of transmissions in adjacent or nearby spectrum bands,
the FAA is proposing the updated corrective action presented in this
proposed AD.
5G Compatibility: AMOCs allowing operations otherwise prohibited by
AD 2021-23-12 were based on voluntary operational mitigations
undertaken by AT&T and Verizon, 5G C-Band licensees. The FAA, AT&T, and
Verizon have collaborated extensively to ensure 5G C-Band radio
frequency transmissions and aircraft operations can safely co-exist. In
early January 2022, the FAA progressively tailored runway protection
zones around airports to envelop only the airspace areas where critical
phases of flight occur. The FAA has worked with AT&T and Verizon to
improve the precision of the FAA's interference analyses used during
the AMOC process. In turn, AT&T and Verizon coordinated their
deployment around 5G C-Band mitigated airports (5G CMAs),² including
in some cases reducing emission power around airports and committing to
antenna pointing angles in the vertical plane to limit the potential
for interference within the tailored runway safety zones. This
collaborative work has allowed safe transport and commuter airplane
operations to continue in the short term.
---------------------------------------------------------------------------

² For purposes of this proposed AD, a ``5G C-Band mitigated
airport'' is an airport at which AT&T and Verizon have agreed to
voluntarily limit their 5G deployment at the request of the FAA. The
FAA will provide a list of these airports in the United States
through the FAA Domestic Notice system. More information about
Domestic Notices can be found on the FAA website at https://www.faa.gov/air_traffic/publications/domesticnotices/dom1_foreword.html.
---------------------------------------------------------------------------

Update to Safety Determination: The FAA's initial determination
that radio altimeters cannot be relied upon to perform their intended
function if they experience interference from wireless broadband
operations in the 5G C-Band remains unchanged. Therefore, this proposed
AD would continue prohibiting the use of the same operations identified
in the original AD (AD 2021-23-12) except for the prohibition of
Required Navigation Performance with Authorization Required (RNP AR)
Instrument Approach Procedures (IAP). After further analysis, the FAA
has determined that 5G C-Band interference does not create an unsafe
condition for an airplane conducting RNP AR IAPs because RNP AR
operations do not rely on direct radio altimeter inputs to determine
arrival at altitude minimums or the flight path of the airplane.
Therefore, this proposed AD would no longer prohibit RNP AR IAPs.
The FAA has also gained a better understanding of 5G C-Band
interference beyond its effect on the operations prohibited by AD 2021-
23-12. Since 5G C-band deployment began, the FAA has solicited reports
of radio altimeter anomalies from aircraft pilots and operators.³ The
FAA has received over 420 reports of radio altimeter anomalies
occurring within a known location of a 5G C-Band deployment.
Approximately 315 of these reports were determined to not be related to
5G C-Band interference and were resolved through normal continued
operational safety procedures. But for roughly 100 of the anomaly
reports occurring within NOTAM areas, the FAA has excluded other
potential causes for the anomaly, but could not rule out 5G C-Band
interference as the potential source of the radio altimeter anomalies.
These approximately 100 incidents included various flight deck effects
such as erroneous Terrain Awareness and Warning System (TAWS) warnings,
erroneous Traffic Collision Avoidance System (TCAS) warnings, erroneous
landing gear warnings, and the erroneous display of radio altimeter
data. Although these flight deck effects are less severe than the
hazards associated with low-visibility landings, the FAA is concerned
that to the extent 5G C-Band operations contributed to such events, the
effects will occur more frequently as telecommunication companies
continue to deploy 5G C-Band services throughout the country. The FAA
has assessed the cumulative effects of increasing numbers of erroneous
warnings across the fleet of transport and commuter airplanes. Although
they may seem minor in isolation such that some may consider them a
mere nuisance, these warnings have safety implications over time. The
erroneous warnings increase flightcrew workload as they try to
ascertain the validity of the warning. Repeated determinations that the
warning occurred in error will lead to flightcrew desensitization to
warnings from these safety systems.\4\ In other words, as the
flightcrew becomes more desensitized to erroneous warnings, they are
less likely to react to an accurate warning, negating the safety
benefits of the warning altogether and likely leading to a catastrophic
incident.
---------------------------------------------------------------------------

³ Special Airworthiness Information Bulletin AIR-21-18R1 and
subsequent revisions encouraged pilots to submit detailed reports of
radio altimeter anomalies using the Radio Altimeter Anomaly
Reporting Form available on the FAA website at www.faa.gov/air_traffic/nas/RADALT_reports/.
\4\ FAA research on nuisance alerts in the air traffic control
(ATC) environment has shown that nuisance alerts can desensitize
people toward the alert and lead to slower responses to real alerts.
When people experience frequent false or low-urgency alerts, they
tend to respond less quickly and less accurately to real and high-
urgency alerts. Further, when there is a high incidence of nuisance
alerts, people may suppress the alert before determining its actual
status or may no longer treat the alert as mandatory. In both cases,
overall alarm compliance decreases and they may stop responding to
every alert. See Nuisance Alerts in Operational ATC Environments:
Classification and Frequencies, Friedman-Berg, Allendoerfer, and Pai
(2008). A copy of this paper can be found on the FAA website at
https://hf.tc.faa.gov/publications/2008-nuisance-alerts-in-operational-atc-environments/full_text.pdf.
---------------------------------------------------------------------------

AD 2021-23-12 does not mitigate the hazards associated with
erroneous system warnings, focusing instead on the potentially more
severe hazards associated with certain low visibility operations.
Additionally, AD 2021-23-12 does not address other operations near
airports, such as Category I instrument landing system (ILS) or visual
flight rule (VFR) approaches. Therefore, the FAA has determined that
additional corrective action is required to address this unsafe
condition and proposes to supersede AD 2021-23-12.
Why New Corrective Action is Needed: In addition to the hazards due
to the cumulative effects of nuisance warnings described earlier, the
FAA expects an increase in the number of 5G C-Band base stations around
airports in the national airspace system (NAS) and expects these
stations to transmit in the entire 5G C-Band frequency band (from 3.7
to 3.98 GHz). Since the FAA issued AD 2021-23-12, which focused solely
on the airport environment, 5G C-Band base stations have increasingly
begun transmission in other areas of the country. Whereas 5G
transmissions were initially limited to 3.7 to 3.8 GHz, these
transmissions have also begun to expand to 3.8 to 3.98 GHz, and the FAA
expects deployment at the higher end of the frequency range to expand
after July 1, 2023.5 These higher frequencies are nearer to the
spectrum allocation where radio altimeters operate (4.2 to 4.4 GHz),
which means that the potential for interference to radio altimeters
from in-band and spurious 6 emissions may be more likely. In
addition, the FAA expects approximately 19 additional telecommunication
companies in addition to AT&T and Verizon will begin transmitting in
the C-Band at some point after June 2023.7 As the 21
telecommunication companies authorized to transmit 5G C-Band continue
to expand transmissions throughout the country, using NOTAMs to
identify affected areas and assessing proposed AMOCs will become
untenable. NOTAMs are temporary means of disseminating information
until the information can be publicized by other means. Given 5G C-Band
signals are not expected to be temporary and that 5G signals will cover
the contiguous U.S., NOTAMs are no longer the best means of
communicating the location of the 5G C-Band environment. In addition,
given the information gleaned over the past year, the FAA is now able
to identify the conditions under which radio altimeters can be relied
on to perform their intended function in the presence of a 5G C-Band
environment. Therefore, case-by-case AMOC approvals that allow
performing certain operations otherwise prohibited by an AD are no
longer the most efficient way for airplane operators to show that their
radio altimeters perform their intended function in the 5G C-Band
environment.
---------------------------------------------------------------------------

5 FCC licenses authorized 5G transmissions from 3.7 to 3.98
GHz.
6 The tolerance to 5G spurious emissions is the level of
aggregate interference in the radio altimeter band below which the
installed radio altimeter system will meet its performance standards
and perform its intended function.
7 The additional 19 telecommunications companies will have
access to the FCC-licensed spectrum after current users vacate use
of the frequencies.
---------------------------------------------------------------------------

Determination of Airplane Radio Altimeter Tolerance Requirements:
The FAA is proposing interference tolerance requirements for radio
altimeters that can be used across the affected fleet. Airplanes
meeting these proposed minimum performance levels would be allowed to
use the prohibited operations at the airports identified by an FAA
Domestic Notice 8 after July 1, 2023. Airplanes operating under 14
CFR part 121 would also be required to have a radio altimeter that
meets the proposed minimum performance standards (i.e., tolerance
requirements) on or before February 1, 2024.
---------------------------------------------------------------------------

8 Domestic Notices publish special notices or notices
containing graphics pertaining to almost every aspect of aviation,
such as military training areas, large scale sporting events, air
show information, Special Management Programs (STMPs), and airport-
specific information.
---------------------------------------------------------------------------

The FAA determined the proposed interference tolerance requirements
by using the fuller understanding of specific radio altimeter
capabilities the FAA gained during the AMOC process. This process
revealed the radio altimeter modifications that would not require a
substantial system redesign, allowing aircraft operators to readily
replace radio altimeters or install filters that allowed the aircraft
to operate safely in a mitigated 5G environment.
The interference tolerance requirements are represented by a power
spectral density (PSD) curve. The PSD curve, as depicted in figure 1 to
paragraph (g)(2) of this proposed AD, represents the height over the
ground and received power from a 5G C-Band emitter, at or below which
the radio altimeter is expected to function reliably, measured in
decibels per megahertz. These measurements are limited to the 5G CMAs
that will be listed in an FAA Domestic Notice. For purposes of this
proposed AD, a ``radio altimeter tolerant airplane'' (also known within
industry as a Group 4 airplane) is one for which the radio altimeter,
as installed, demonstrates tolerance to radio altimeter interference at
or above PSD curve threshold specified in figure 1 to paragraph (g)(2)
of this proposed AD. A radio altimeter tolerant airplane also
demonstrates tolerance to a spurious emission level of -48 dBm/MHz in
the 4200-4400 MHz radio altimeter band. For purposes of this proposed
AD, a ``non-radio altimeter tolerant airplane'' (also known in industry
as a Group 1, 2, or 3 airplane) is one for which the radio altimeter,
as installed, does not demonstrate those tolerances. Some radio
altimeters may already demonstrate tolerance to the 5G C-Band emissions
without modification. Some may need to install filters between the
radio altimeter and antenna to increase a radio altimeter's tolerance.
For others, the addition of a filter will not be sufficient to address
interference susceptibility; therefore, the radio altimeter will need
to be replaced with an upgraded radio altimeter. The FAA has determined
that radio altimeter tolerant airplanes will not experience the unsafe
condition at any airport identified by the FAA as a 5G CMA in an FAA
Domestic Notice.
Areas of Operation: Over the past year, the FAA and the aviation
industry, using data voluntarily provided by AT&T and Verizon, have
identified maximum power levels for 5G C-Band transmissions that would
permit safe aircraft operations. These power levels were identified
using a SiS analysis that considers factors specific to an airport.
That is, the SiS analysis considers specific 5G C-Band base station
data to predict the 5G signal characteristics in the runway safety
zone. The base station data includes 5G C-Band tower or antenna
locations, fundamental transmission power levels, and antenna height.
Using this analysis, the FAA has found that airplanes meeting the
proposed standards as represented by the PSD curve can safely perform
the prohibited operations specified in this proposed AD at 5G CMAs. These
operations are safe for radio altimeter tolerant airplanes to perform
at these airports as long as telecommunication companies transmit at
parameters under the current voluntary agreements with the FAA and FCC.
Compatibility with 5G C-Band Providers: The FAA has determined that
any 5G C-Band provider that maintains the mitigated actions will not
have an effect on the safety of transport and commuter airplanes with
radio altimeters that meet the interference tolerance requirements at
5G CMAs. The FAA will assess the effects of any changes to transmission
parameters at 5G CMAs to determine whether they would result in a
hazard to air navigation. If the transmission changes negatively affect
the safe operation of a radio altimeter tolerant airplane at that
airport, the FAA will remove that airport from the 5G CMAs list.
Therefore, the FAA has determined that an unsafe condition exists
when performing certain operations in the presence of 5G C-Band
transmissions affecting the proper function of radio altimeters. For
that reason, operators would be required to revise their existing AFM
to prohibit these operations unless operating a radio altimeter
tolerant airplane at a 5G CMA. This proposed requirement would take
effect on July 1, 2023.
In addition, the FAA proposes to prohibit operations under part 121
in the U.S. after February 1, 2024, unless such operations are
conducted with a radio altimeter tolerant airplane. As explained
earlier, the FAA expects erroneous system warnings due to a
malfunctioning radio altimeter to lead to flightcrew becoming
desensitized to system warnings. Such desensitization negates the
safety benefits of the warning itself and can lead to a catastrophic
event. To minimize the number of erroneous system messages and the
unsafe condition they produce, the FAA is proposing to require all
airplanes operating under part 121 meet the PSD performance curve to
operate in the contiguous U.S. after February 1, 2024. This is the date
the FAA has determined to be as soon as reasonably practical,
consistent with FAA policy.
9 Non-radio altimeter tolerant airplanes
can operate under part 121 subject to the revised AFM limitations until
February 1, 2024, without meeting the radio altimeter performance
requirements proposed in this AD. If this AD is finalized as proposed,
after February 1, 2024, airplanes operating under part 121 must meet
the radio altimeter tolerant requirements specified in figure 1 to
paragraph (g)(2) of this proposed AD.
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9 PS-ANM-25-05, Risk Assessment Methodology for Transport
Category Airplanes, available at drs.faa.gov/browse/excelExternalWindow/4E5AE8707164674A862579510061F96B.0001.
---------------------------------------------------------------------------

FAA's Determination

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

Proposed AD Requirements in This NPRM

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

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

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

Interim Action

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

Costs of Compliance


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

Estimated Costs

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

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

Authority for This Rulemaking

Title 49 of the United States Code specifies the FAA's authority to
issue rules on aviation safety. Subtitle I, section 106, describes the
authority of the FAA Administrator. Subtitle VII, Aviation Programs,
describes in more detail the scope of the Agency's authority.
The FAA is issuing this rulemaking under the authority described in
Subtitle VII, Part A, Subpart III, Section 44701: General requirements.
Under that section, Congress charges the FAA with promoting safe flight
of civil aircraft in air commerce by prescribing regulations for
practices, methods, and procedures the Administrator finds necessary
for safety in air commerce. This regulation is within the scope of that
authority because it addresses an unsafe condition that is likely to
exist or develop on products identified in this rulemaking action.

Regulatory Findings

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

Regulatory Flexibility Act

The Regulatory Flexibility Act (RFA) of 1980, Public Law 96-354, 94
Stat. 1164 (5 U.S.C. 601-612), as amended by the Small Business
Regulatory Enforcement Fairness Act of 1996 (Pub. L. 104-121, 110 Stat.
857, Mar. 29, 1996) and the Small Business Jobs Act of 2010 (Pub. L.
111-240, 124 Stat. 2504, Sept. 27, 2010), requires Federal agencies to
consider the effects of the regulatory action on small business and
other small entities and to minimize any significant economic impact.
The term ``small entities'' comprises small businesses and not-for-
profit organizations that are independently owned and operated and are
not dominant in their fields, and governmental jurisdictions with
populations of less than 50,000.
The FAA is publishing this Initial Regulatory Flexibility Analysis
(IRFA) to aid the public in commenting on the potential impacts to
small entities from this proposal. The FAA invites interested parties
to submit data and information regarding the potential economic impact
that would result from the proposal. The FAA will consider comments
when making a determination or when completing a Final Regulatory
Flexibility Assessment. An IRFA contains the following:
(1) A description of the reasons why the action by the agency is
being considered;
(2) A succinct statement of the objective of, and legal basis for,
the proposed rule;
(3) A description of and, where feasible, an estimate of the number
of small entities to which the proposed rule will apply;
(4) A description of the projected reporting, recordkeeping, and
other compliance requirements of the proposed rule, including an
estimate of the classes of small entities which will be subject to the
requirement and the type of professional skills necessary for
preparation of the report or record;
(5) An identification, to the extent practicable, of all relevant
Federal rules that may duplicate, overlap, or conflict with the
proposed rule; and
(6) A description of any significant alternatives to the proposed
rule which accomplish the stated objectives of applicable statutes and
which minimize any significant economic impact of the proposed rule on
small entities.

Reasons the Action Is Being Considered

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

Objectives of the Proposed Rule

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

Description and Estimate of the Number of Small Entities

The FAA used the definition of small entities in the RFA for this
analysis. The RFA defines small entities as small businesses, small
governmental jurisdictions, or small organizations. In 5 U.S.C. 601(3),
the RFA defines ``small business'' to have the same meaning as ``small
business concern'' under section 3 of the Small Business Act. The Small
Business Act authorizes the Small Business Administration (SBA) to
define ``small business'' by issuing regulations.
The SBA established size standards for various types of economic
activities, or industries, under the North American Industry
Classification System (NAICS).
10 These size standards generally
define small businesses based on the number of employees or annual
receipts. The following table shows the SBA size standards for
certificate holders as an example. Note that the SBA definition of a
small business applies to the parent company and all affiliates as a
single entity.
---------------------------------------------------------------------------

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

Small Business Size Standards: Air Transportation

NAICS code
Description
SBA size standard
481111
Scheduled Passenger Air Transportation
1,500 employees
481112
Scheduled Freight Air Transportation
1,500 employees
481211
Nonscheduled Chartered Passenger Air Transportation
1,500 employees
481212
Nonscheduled Chartered Freight Air Transportation
1,500 employees
481219
Other Nonscheduled Air Transportation
$25 million

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

Estimated Number of Small Entities

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

Projected Reporting, Recordkeeping, and Other Compliance Requirements

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

All Federal Rules That May Duplicate, Overlap, or Conflict

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

Significant Alternatives Considered

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

List of Subjects in 14 CFR Part 39

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

The Proposed Amendment

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

PART 39--AIRWORTHINESS DIRECTIVES

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: