Document ID: FAA-2023-0938-0012
Agency: faa
Document Type: Rule
Title: Demonstration of Radio Altimeter Tolerant Aircraft
Posted Date: 2023-07-19T04:00Z

[Federal Register Volume 88, Number 137 (Wednesday, July 19, 2023)]
[Rules and Regulations]
[Pages 46055-46057]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2023-14927]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 21

[Docket No. FAA-2023-0938]

Demonstration of Radio Altimeter Tolerant Aircraft

AGENCY: Federal Aviation Administration, DOT

ACTION: Notice of availability; final policy and disposition of 
comments.

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SUMMARY: The Federal Aviation Administration (FAA) announces Policy 
Statement PS-AIR-600-39-01 for demonstrating an aircraft is a ``radio 
altimeter tolerant airplane'' or a ``radio altimeter tolerant 
rotorcraft'' using a method approved by the FAA.

DATES:  This policy is effective July 19, 2023.

FOR FURTHER INFORMATION CONTACT: For technical questions concerning 
this policy statement, contact Barbara Clark, Supervisory Aviation 
Safety Specialist, Avionics Navigation & Flight Deck Unit (AIR-626B), 
800 Independence Ave. SW, Washington, DC 20591; telephone: 202-267-
8569; email: [email protected].

SUPPLEMENTARY INFORMATION: 

Background

    The current performance standards for radio altimeters (also known 
as radar altimeters) are based on the presumption that no occupancy of 
an adjacent radio frequency spectrum would cause interference with 
radio altimeters. During 2021, the radio frequency (RF) operating 
environment surrounding radio altimeters substantially changed when 
wireless telecommunication service providers began offering 5G C-Band 
services near the 4.2-4.4 GHz band. In both the U.S. and 
internationally, this band is allocated on a primary basis for 
aeronautical radionavigation service, which is used by aviation radio 
altimeters. The FAA subsequently determined that radio altimeters could 
not be relied upon to perform their intended function if they 
experience interference from 5G wireless broadband operations in the C-
Band.
    Deployment of the new 5G C-Band services prompted the FAA to 
address the risks posed by RF interference to radio altimeters. On 
December 7, 2021, the FAA issued airworthiness directive (AD) 2021-23-
12 \1\ for transport and commuter category airplanes equipped with a 
radio altimeter and AD 2021-23-13 \2\ for helicopters equipped with a 
radio altimeter. AD 2021-23-12 and AD 2021-23-13 prohibit certain 
flight operations requiring radio altimeter data when flying in the 
presence of 5G C-Band interference as identified by Notices to Air 
Missions (NOTAMs). In response to AD 2021-23-12, the aviation industry 
developed a method to show compatibility with 5G emissions in the 
United States national airspace system for the initial 5G deployment, 
which was limited to 3.7-3.8 GHz, and the 5G spurious emissions in the 
radio altimeter band (4.2-4.4 GHz). The FAA accepted this method as 
support for proposals for alternative methods of compliance (AMOCs) 
with AD 2021-23-12 and AD 2021-23-13. These AMOCs used standardized 
assessment parameters, values, and methods to estimate an installed 
altimeter system protection radii or distance. Aircraft with an 
altimeter operating beyond this distance from all 5G base stations 
would not expect harmful effects from RF incompatibility and indeed 
could depend upon the radio altimeter system to perform fully its 
intended function. These AMOCs were based on interference thresholds of 
specific individual radio altimeter transceivers. That is, each 
transceiver was tested to benchmark their performance in the presence 
of out-of-band and in-band C-Band signals.\3\ The thresholds were then 
modified and tailored to installation factors specific to the installed 
platform (e.g., measured antenna gains and cable losses). These values 
were then used to determine the necessary mitigations to protect the 
airport airspace most critical for the safety of operations. The 
mitigations included actions by wireless providers as well as flight 
limitations imposed by the FAA for the airspace areas identified by 
NOTAM, unless operating under an approved AMOC.
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    \1\ Amendment 39-21810, 86 FR 69984, December 9, 2021.
    \2\ Amendment 39-21811, 86 FR 69992, December 9, 2021.
    \3\ ``In-band signals'' have frequencies in the radio altimeter 
band of 4.2-4.4 GHz. The frequencies of ``out-of-band'' signals are 
outside of the radio altimeter band.
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    The deployment of new 5G C-Band stations continues. Their signals 
are expected to cover most of the contiguous United States at 
transmission frequencies between 3.7-3.98 GHz.\4\
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    \4\ Federal Communications Commission (FCC) Report and Order 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, see 
https://www.fcc.gov.
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    On May 26, 2023, the FAA superseded AD 2021-23-12 with AD 2023-10-
02.\5\ The flight limitations imposed by AD 2023-10-02 depend on 
whether an airplane has a radio altimeter system that demonstrates the 
tolerances specified in paragraph (g)(1) of the AD using a method 
approved by the FAA (i.e., whether the aircraft is a ``radio altimeter 
tolerant airplane'').\6\
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    \5\ Amendment 39-22438, 88 FR 34065, May 26, 2023.
    \6\ The FAA subsequently issued several ADs to address 5G 
interference for specific Boeing airplane models: AD 2023-12-05, AD 
2023-12-10, AD 2023-12-11, AD 2023-12-12, AD 2023-12-13, AD 2023-12-
14, and AD 2023-12-15.
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    On June 22, 2023, the FAA superseded AD 2021-23-13 with AD 2023-11-
07.\7\ The flight limitations imposed by AD 2023-11-07 depend on 
whether a rotorcraft has a radio altimeter system that demonstrates the 
tolerances specified in paragraph (g)(1) of the AD using a method 
approved by the FAA (i.e., whether the aircraft is a ``radio altimeter 
tolerant rotorcraft'').
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    \7\ Amendment 39-22453, 88 FR 40685, June 22, 2023.
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    The FAA published a notice of availability and request for comments 
on proposed guidance for demonstrating an aircraft is a ``radio 
altimeter tolerant aircraft'' in the Federal Register on May 8, 2023 
(88 FR 29554). The public comment period for the notice closed on June 
7, 2023.

[[Page 46056]]

Discussion of Comments

    The FAA received comments from eight organizations: Thales Group, 
Airlines for America (A4A), MHI RJ Aviation ULC, Dassault Aviation 
(Dassault), Embraer S.A. (Embraer), the Cargo Airline Association 
(CAA), Gulfstream Aerospace Corporation (Gulfstream), and the Aviation 
Coalition. Comments fell into broad categories to include requests 
regarding how to show compliance; suggestions to harmonize language 
between the policy and the newly published ADs; statements regarding 5G 
bandwidth, and recommendations on content, editing, and formatting. One 
commenter urged the FAA to withdraw the policy.
    A copy of the FAA's disposition of the public comments received is 
also available at regulations.gov in Docket No. FAA-2023-0938.

A. Showing Compliance

    The FAA received several comments regarding how to show compliance 
with the policy statement and with AD 2023-10-02 or AD 2023-11-07.
    Thales Group asked if performance justifications submitted for a 
prior AD must be re-submitted as evidence to support compliance with AD 
2023-10-02 or AD 2023-11-07. Performance justification evidence must be 
re-submitted to the FAA to show compliance. Performance justifications 
for prior ADs were approved by the FAA before the method in the policy 
statement was defined. Although the FAA expects that aircraft with 
AMOCs approved for prior ADs may be able to meet the definition of a 
``radio altimeter tolerant airplane'' or ``radio altimeter tolerant 
rotorcraft,'' design approval holders (DAHs) or operators will need to 
provide the FAA with data showing explicitly that the aircraft meets 
the tolerances in paragraph (g)(1) of AD 2023-10-02 or AD 2023-11-07 
before the FAA will approve the method they propose to use. However, 
the FAA has updated the policy statement to clarify that data 
previously submitted for AMOCs with AD 2021-23-12 or AD 2021-23-13 may 
be referenced to support the method in PS-AIR-600-39-01 without re-
submitting the referenced documents themselves.
    Gulfstream commented that the policy allows for the use of existing 
data and analysis, which manufacturers collected for AMOCs with AD 
2021-23-12 or AD 2021-23-13, to demonstrate compliance with the new AD. 
The FAA acknowledges that while the use of existing data is supported, 
additional data and analysis is not precluded. The use of power 
spectral density (PSD) curves for compliance demonstration is 
different.
    Gulfstream and CAA requested that the FAA identify the radio 
altimeter technologies that meet the AD requirements. Gulfstream stated 
the policy creates a duplicative effort and burden on the aviation 
community in that it asks for data the FAA already possesses based on 
prior AMOC approvals. CAA stated that the policy creates an undue 
burden on operators to coordinate with DAHs and radio altimeter 
manufacturers.
    The FAA disagrees. The FAA approved AMOCs for AD 2021-23-12 and AD 
2021-23-13 before the radio altimeter tolerant PSD curves were defined. 
Although the FAA expects that the aircraft with AMOCs approved for AD 
2021-23-12 or AD 2021-23-13 may be able to meet the definition of 
``radio altimeter tolerant'' aircraft, DAHs will need to provide the 
FAA with data showing explicitly that the aircraft meets the tolerances 
in AD 2023-10-02 or AD 2023-11-07 before the FAA will approve the 
method they propose to use. Additionally, the FAA does not maintain a 
list of tolerant radio altimeters; the determination of a radio 
altimeter tolerant aircraft must consider the installation details, 
which vary from aircraft to aircraft. However, the FAA has added 
guidance to the policy to assist with obtaining FAA approval 
expeditiously.
    CAA requested the FAA accept requests for AMOCs for operators to 
continue to operate without restrictions after July 1, 2023, given the 
same level of safety could be achieved. Operators will not need an AMOC 
provided their aircraft meets the fundamental and spurious emissions 
PSD curve thresholds specified in AD 2023-10-02 and AD 2023-11-07. No 
flight restrictions are mandated by AD 2023-10-02 and AD 2023-11-07 for 
radio altimeter tolerant aircraft.
    Dassault requested the FAA confirm whether bench tests performed by 
the transceiver manufacturer would be sufficient for substantiation 
without additional tests. The FAA partially agrees. Bench tests of 
multiple units of a given transceiver model; antenna patterns, both in-
band and out-of-band; and an analysis of the installed system to 
determine the appropriate cable loss are all necessary for 
substantiation. A test of the system (transceiver, antenna(s), and 
cabling) when installed on the aircraft is not necessary.

B. Harmonize Guidance

    The guidance for spurious emissions in the proposed policy 
statement was based on a spurious emission level. When the FAA issued 
AD 2023-10-02 and AD 2023-11-07, the FAA replaced the proposed fixed 
emission level with a spurious PSD tolerance curve.
    Embraer, Dassault, and the Aviation Coalition requested the FAA 
revise numerous references in the policy from the spurious level to the 
spurious emissions PSD curve to be consistent with AD 2023-10-02 and AD 
2023-22-07. The FAA agrees and has revised the policy statement 
accordingly.

C. 5G Bandwidth and Interference

    The Aviation Coalition requested the FAA clarify whether actual 5G 
C-band transmissions will present a condition where cumulative 5G 
signaling bandwidths in excess of 100 MHz will be in practice after 
July 1, 2023. It commented that the cumulative impact of multiple 100 
MHz bands is not fully characterized for existing altimeters. The 
Aviation Coalition further stated the current accepted practice 
reflects the use of 100 MHz 5G signaling bandwidth for the purposes of 
compatibility assessment between 5G and radio altimeters. The FAA does 
not expect cumulative 5G signaling bandwidths in excess of 100 MHz for 
a given frequency at any one location in practice, based on 
communication with the FCC.
    The Aviation Coalition asked why the policy specifies an 
interference tolerance threshold (ITT) measurement since the 
superseding ADs establish a curve for fundamental tolerance thresholds. 
The commenter stated it should be sufficient for an applicant to test 
to the appropriate tolerance levels of the curve and show that the 
radio altimeter performance at those levels is not unacceptably 
degraded. The FAA agrees that compliance with the curve can be 
determined without necessarily determining the transceiver's 
performance limit. The FAA changed the policy to reflect an 
interference tolerance (IT) measurement instead of an ITT measurement 
and updated the definition accordingly.
    Dassault requested the FAA add the exact 5G fundamental frequency 
bandwidth to the guidance. The FAA agrees and has revised the document 
accordingly.

D. Definitions, Editing, and Formatting

    The FAA agreed with multiple requests from Dassault and the 
Aviation Coalition for editorial and formatting changes and 
reorganization, and revised the proposed policy statement accordingly, 
including the following:

 The FAA added introductory text to the beginning of sections 1 
and 2.

[[Page 46057]]

 The FAA added a diagram as figure 1 for clarity and improve 
understanding.
 The FAA added a definition of the ``stair-step method'' to 
sections 1 and 2.
 The FAA changed ``the log-linear interpolation'' to ``a log-
linear interpolation,'' as both terms are equivalent.
 The FAA changed ``line losses'' to ``cable losses'' for 
consistency in terminology.
 The FAA moved the discussion of the 5G spurious PSD formula 
from paragraph 2.c to the introductory part of section 2.

    Dassault asked whether the performance criteria in the policy 
section only applies to the transceiver. The performance criteria 
applies to the installed radio altimeter system. The FAA notes that the 
equations include terms to characterize the performance of the entire 
system. The FAA has replaced four instances of ``radio altimeter'' with 
``radio altimeter system'' to clarify.
    The FAA disagreed with the Aviation Coalition's request to revise 
the language in paragraph 2.a regarding base stations. The statement is 
correct as written, as it is a factual definition of 5G base station 
and aircraft compatibility. For clarity, the FAA moved the statement to 
the policy section before section 1 as background information.

E. Request for the FAA To Withdraw the Policy

    A4A requested the FAA withdraw the proposed policy because 
operators who are not DAHs do not have the data and information to show 
compliance with the methods in the policy without significant 
assistance from aircraft original equipment manufacturers (OEMs) and 
radio altimeter manufacturers. A4A stated this would be infeasible 
given the short compliance timeframe, as well as duplicative since the 
same data and information associated with aircraft type and radio 
altimeter technology combination will have already been submitted to 
the FAA by the DAH/OEM. Lastly, A4A stated the FAA does not have the 
appropriate resources to timely coordinate and evaluate every 
operator's submissions while simultaneously reviewing data submitted by 
the DAH/OEM. Alternatively, A4A requested that operators who are not 
DAHs be permitted to submit a letter of compliance to their principal 
avionics inspector, citing either an FAA-published list of compliant 
aircraft model/radio altimeter combinations or a list from the DAH/OEM.
    The FAA disagrees with withdrawing the policy, as it provides 
guidance for obtaining FAA approval of a method showing compliance with 
AD 2023-10-02 and AD 2023-11-07. However, the FAA has added guidance to 
the policy to assist with obtaining FAA approval expeditiously.

F. Request Regarding Horizontal Separation for Rotorcraft

    The Aviation Coalition noted that language in the proposed policy 
referring to horizontal separation distance by wing span may be 
appropriate for airplanes, but not for rotorcraft. Because of other 
changes made to the policy statement (replacing the proposed spurious 
emissions level with a spurious PSD curve), the language noted by the 
commenter has been removed from the document. As a result, no change to 
the policy is necessary.

G. Request To Include ``Should''

    The Aviation Coalition requested that the FAA add the word 
``should'' to several places throughout the policy statement, to be 
consistent with nature of the policy as a guidance document. The FAA 
disagrees as the specified language identifies how to use the guidance 
in this policy as a means of compliance. In some instances, the 
language specified by the commenter defines a certain value and 
therefore the addition of ``should'' would be inappropriate.

H. Request To Clarify

    MHI RJ Aviation ULC requested the FAA clarify an apparent 
inconsistency between figure 1 in AD 2023-10-02 and AD 2023-11-07 and 
the section of the policy on 5G spurious tolerance. The FAA understands 
the commenter to be comparing the fundamental PSD curve in AD 2023-10-
02 with guidance for spurious tolerance in the policy statement.

Policy

    The FAA's policy statement provides guidance for operators and 
manufacturers to demonstrate an aircraft is a radio altimeter tolerant 
aircraft, as defined in AD 2023-10-02 and AD 2023-11-07.You may view 
the final policy statement, PS-AIR-600-39-01, Demonstration of Radio 
Altimeter Tolerant Aircraft, at regulations.gov in Docket No. FAA-2023-
0938 or on the FAA's Dynamic Regulatory System website at drs.faa.gov.

    Issued in Des Moines, Washington on July 10, 2023.
Suzanne A. Masterson,
Acting Manager, Technical Innovation Policy Branch, Policy and 
Innovation Division, Aircraft Certification Service.
[FR Doc. 2023-14927 Filed 7-18-23; 8:45 am]
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