DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2021-0953; Project Identifier AD-2021-01169-T;
Amendment 39-21810; AD 2021-23-12]
RIN 2120-AA64

Airworthiness Directives; Transport and Commuter Category
Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule; request for comments.

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SUMMARY: The FAA is adopting a new airworthiness directive (AD) for all
transport and commuter category airplanes equipped with a radio (also
known as radar) altimeter. This AD was prompted by a determination that
radio altimeters cannot be relied upon to perform their intended
function if they experience interference from wireless broadband
operations in the 3.7-3.98 GHz frequency band (5G C-Band). This AD
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 FAA is issuing this AD to address the unsafe condition on
these products.

DATES: This AD is effective December 9, 2021.
The FAA must receive comments on this AD by January 24, 2022.

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 https://www.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.

Examining the AD Docket

You may examine the AD docket at https://www.regulations.gov by
searching for and locating Docket No. FAA-2021-0953; or in person at
Docket Operations between 9 a.m. and 5 p.m., Monday through Friday,
except Federal holidays. The AD docket contains this final rule, any
comments received, and other information. The street address for the
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:

Background

In March 2020, the United States Federal Communications Commission
(FCC) adopted final rules authorizing flexible use of the 3.7-3.98 GHz
band for next generation services, including 5G and other advanced
spectrum-based services.¹ Pursuant to these rules, C-Band wireless
broadband deployment is permitted to occur in phases with the
opportunity for operations in the lower 100 megahertz of the band (3.7-
3.8 GHz) in 46 markets beginning as soon as December 5, 2021; however,
the FAA does not expect actual deployment to commence until January 5,
2022. This AD refers to ``5G C-Band'' interference, but wireless
broadband technologies, other than 5G, may use the same frequency
band.² These other uses of the same frequency band are within the
scope of this AD since they would introduce the same risk of radio
altimeter interference as 5G C-Band.
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¹ The FCC's rules did not make C-Band wireless broadband
available in Alaska, Hawaii, and the U.S. Territories.
² The regulatory text of the AD uses the term ``5G C-Band''
which, for purposes of this AD, has the same meaning as ``5G'', ``C-
Band'' and ``3.7-3.98 GHz''
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In April 2020, RTCA formed a 5G Task Force, including members from
RTCA, the FAA, aircraft and radio altimeter manufacturers, European
Organisation for Civil Aviation Equipment (EUROCAE), industry
organizations, and operators, to perform ``a quantitative evaluation of
radar altimeter performance regarding RF interference from expected 5G
emissions in the 3.7-3.98 GHz band, as well as a detailed assessment of
the risk of such interference occurring and impacting aviation
safety.'' ³ Based on the work of the task force, RTCA published a
report which concludes that there is ``a major risk that 5G
telecommunications systems in the 3.7-3.98 GHz band will cause harmful
interference to radar altimeters on all types of civil aircraft--
including commercial transport airplanes; business, regional, and
general aviation airplanes; and both transport and general aviation
helicopters.'' 4
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³ RTCA Paper No. 274-20/PMC-2073, Assessment of C-Band Mobile
Telecommunications Interference Impact on Low Range Radar Altimeter
Options, dated October 7, 2020 (RTCA Paper No. 274-20/PMC-2073),
page i. This document is available in Docket No. FAA-2021-0953, and
at https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-Interference-
Assessment-Report_274-20-PMC-2073_accepted_changes.pdf.
4 RTCA Paper No. 274-20/PMC-2073, page i.
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The report further concludes that the likelihood and severity of
radio frequency interference increases for operations at lower
altitudes. That interference could cause the radio altimeter to either
become inoperable or present misleading information, and/or also affect
associated systems on civil aircraft. The RTCA report refers to FCC
Report and Order (R&O) FCC 20-22,5 which identifies radio frequencies
and power level conditions for the new C-Band services. The RTCA report
identified the possibility of interference from both wireless emitters
(on base stations, for example) as well as onboard user handsets. The
RTCA report and conclusions remain under review, including by federal
spectrum regulators. The FAA risk assessment included consideration of
the RTCA report, public comments to the RTCA report, and analyses from
radio altimeter manufacturers and aircraft manufacturers in support of
the safety risk determination. The analyses FAA considered were
consistent with RTCA's conclusions pertaining to radio altimeter
interference from C-Band emissions. The FAA determined that, at this time, no information
has been presented that shows radio altimeters are not susceptible to
interference caused by C-Band emissions permitted in the United States.
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5 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-2021-0953, and at https://www.fcc.gov/document/fcc-expands-
flexible-use-c-band-5g-0.
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Additionally, the deployment of C-Band wireless broadband networks
is occurring globally. In certain countries, deployment has already
occurred in C-Band frequencies. In some countries, temporary technical,
regulatory, and operational mitigations on C-Band systems have been
implemented while aviation authorities complete their safety
assessments. Under the FCC rules adopted in 2020, base stations in
rural areas of the United States are permitted to emit at higher levels
in comparison to other countries which may affect radio altimeter
equipment accuracy and reliability.
The radio altimeter is an important aircraft instrument, and its
intended function is to provide direct height-above-terrain/water
information to a variety of aircraft systems. Commercial aviation radio
altimeters operate in the 4.2-4.4 GHz band, which is separated by 220
megahertz from the C-Band telecommunication systems in the 3.7-3.98 GHz
band. The radio altimeter is more precise than a barometric altimeter
and for that reason is used where aircraft height over the ground needs
to be precisely measured, such as autoland or other low altitude
operations. The receiver on the radio altimeter is typically highly
accurate, however it may deliver erroneous results in the presence of
out-of-band radiofrequency emissions from other frequency bands. The
radio altimeter must detect faint signals reflected off the ground to
measure altitude, in a manner similar to radar. Out-of-band signals
could significantly degrade radio altimeter functions during critical
phases of flight, if the altimeter is unable to sufficiently reject
those signals.
Many operators need to be able to land in low visibility
conditions. These operators employ specially certified equipment and
flightcrew training in order to be able to fly closer to the ground
during approach in instrument conditions, in some cases all the way
through the landing phase, without visual reference to the runway
environment. These operations can only be conducted with reference to
actual height above the ground, as measured by a radio altimeter.
Additionally, automatic and/or manual flight guidance systems on
airplanes facilitate low visibility operations and rely on accurate
radio altimeter inputs. These inputs determine when and where the
aircraft flares for landing, when power reductions are made for
landing, and when automated crosswind controls and other control inputs
are made. Anomalous (missing or erroneous) radio altimeter inputs to
these systems may cause the aircraft to be maneuvered in an unexpected
or hazardous manner during the final stages of approach and landing,
and may not be detectable by the pilot in time to maintain continued
safe flight and landing. Inaccurate radio altimeter data can result in
pilots not trusting their instruments, eroding the foundation on which
all instrument flight training is built.
Although the FAA has determined the operations immediately at risk
are those requiring a radio altimeter to land in low visibility
conditions, a wide range of other automated safety systems rely on
radio altimeter data. Harmful interference to the radio altimeter could
cause these systems to operate in an unexpected way. The FAA continues
to work with inter-agency and industry stakeholders to collect data on
potential effects to these systems to determine whether additional
mitigations are necessary. The FAA determined, however, that mandatory
action is not immediately required for these systems.
The FAA plans to use data provided by telecommunications providers
to determine which airports within the United States have or will have
C-Band base stations or other devices that could potentially impact
airplane systems. NOTAMs will be issued, as necessary, to state the
specific airports where the data from a radio altimeter may be
unreliable due to the presence of 5G C-Band wireless broadband
signals.6 For this reason, this AD requires flight manual limitations
that prohibit certain operations requiring radio altimeter data at
locations that will be identified by NOTAMs. Due to the dynamic nature
of both the base station activation and the ongoing process of
identifying the resulting affected airspace, including potential
consideration for variability in C-Band deployment conditions such as
radiated power levels and locations, the FAA has determined that NOTAMs
are the best means to communicate changes in restrictions at affected
airports.
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6 The FAA's process for issuing NOTAMs is described in FAA
Order 7930.2S, Notices to Air Missions (NOTAM), December 2, 2021.
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Finally, the FAA notes that in accordance with paragraph (h) of
this AD, any person may propose and request FAA approval of an
alternative method of compliance (AMOC). The proposed AMOC must include
specific conditions that would address the unsafe condition (e.g., by
providing information substantiating that certain aircraft or altimeter
models are not susceptible to C-Band radiofrequency interference).

FAA's Determination

The FAA is issuing this AD because the agency has determined the
unsafe condition as described previously is likely to exist or develop
in transport and commuter category airplanes with a radio altimeter as
part of their type design.

AD Requirements

This AD 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 wireless
broadband signals as identified by NOTAM. These limitations could
prevent dispatch of flights to certain locations with low visibility,
and could also result in flight diversions.

Compliance With AFM Revisions

Section 91.9 prohibits any person from operating a civil aircraft
without complying with the operating limitations specified in the AFM.
FAA regulations also require operators to furnish pilots with any
changes to the AFM (14 CFR 121.137) and pilots in command to be
familiar with the AFM (14 CFR 91.505).

Interim Action

The FAA considers this AD to be an interim action. If final action
is later identified, the FAA might consider further rulemaking.

Justification for Immediate Adoption and Determination of the Effective
Date

Section 553(b)(3)(B) of the Administrative Procedure Act (APA) (5
U.S.C. 551 et seq.) authorizes agencies to dispense with notice and
comment procedures for rules when the agency, for ``good cause,'' finds
that those procedures are ``impracticable, unnecessary, or contrary to
the public interest.'' Under this section, an agency, upon finding good
cause, may issue a final rule without providing notice and seeking
comment prior to issuance. Further, section 553(d) of the APA
authorizes agencies to make rules effective in less than thirty days,
upon a finding of good cause.
An unsafe condition exists that requires the immediate adoption of
this AD without providing an opportunity for public comments prior to
adoption. The FAA has found that the risk to the flying public
justifies forgoing notice and comment prior to adoption of this
rule because 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. The urgency is based on C-Band wireless broadband deployment,
which is expected to occur in phases with operations beginning as soon
as January 5, 2022. Accordingly, notice and opportunity for prior
public comment are impracticable and contrary to the public interest
pursuant to 5 U.S.C. 553(b)(3)(B).
In addition, the FAA finds that good cause exists pursuant to 5
U.S.C. 553(d) for making this amendment effective in less than 30 days,
for the same reasons the FAA found good cause to forgo notice and
comment.

Comments Invited

The FAA invites you to send any written data, views, or arguments
about this final rule. Send your comments to an address listed under
ADDRESSES. Include ``Docket No. FAA-2021-0953 and Project Identifier
AD-2021-01169-T'' at the beginning of your comments. The most helpful
comments reference a specific portion of the final rule, explain the
reason for any recommended change, and include supporting data. The FAA
will consider all comments received by the closing date and may amend
this final rule because of those comments.
Except for Confidential Business Information (CBI) as described in
the following paragraph, and other information as described in 14 CFR
11.35, the FAA will post all comments received, without change, to
https://www.regulations.gov, including any personal information you
provide. The agency will also post a report summarizing each
substantive verbal contact received about this final rule.

Confidential Business Information

CBI is commercial or financial information that is both customarily
and actually treated as private by its owner. Under the Freedom of
Information Act (FOIA) (5 U.S.C. 552), CBI is exempt from public
disclosure. If your comments responsive to this AD contain commercial
or financial information that is customarily treated as private, that
you actually treat as private, and that is relevant or responsive to
this AD, it is important that you clearly designate the submitted
comments as CBI. Please mark each page of your submission containing
CBI as ``PROPIN.'' The FAA will treat such marked submissions as
confidential under the FOIA, and they will not be placed in the public
docket of this AD. Submissions containing CBI should be sent to 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.

Regulatory Flexibility Act

The requirements of the Regulatory Flexibility Act (RFA) do not
apply when an agency finds good cause pursuant to 5 U.S.C. 553 to adopt
a rule without prior notice and comment. Because the FAA has determined
that it has good cause to adopt this rule without notice and comment,
RFA analysis is not required.

Impact on Intrastate Aviation in Alaska

For the reasons discussed above, this AD will not affect intrastate
aviation in Alaska.

Costs of Compliance

The FAA estimates that this AD affects 6,834 airplanes of U.S.
registry. The FAA estimates the following costs to comply with this AD:

Estimated Costs

As previously discussed, there may be other impacts to aviation;
however there remains uncertainty as to cost due to various factors
such as which airports within the United States have, or will have,
base stations or other devices that could interfere with aircraft radio
altimeters.

Authority for This Rulemaking

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

Regulatory Findings

This AD will not have federalism implications under Executive Order
13132. This AD will not have a substantial direct effect on the States,
on the relationship between the national government and the States, or
on the distribution of power and responsibilities among the various
levels of government.

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 adding the following new airworthiness
directive: