Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
AIR-21-18 R3
May 24, 2023
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin informs aircraft
manufacturers, radio altimeter manufacturers, operators, and pilots of
the continued deployment of wireless broadband networks in the 3.7-3.98
GHz bands (C-Band).1
C-Band wireless broadband deployment, which began in January 2022, is
continuing to occur in phases for operations in the contiguous U.S.
This SAIB recommends that radio altimeter manufacturers, aircraft
manufacturers, and operators continue to voluntarily provide to federal
authorities specific information related to altimeter design and
functionality, specifics on deployment and usage of radio altimeters in
aircraft, and that they test and assess their equipment in conjunction
with federal authorities. Results from that testing and assessment
should be reported to the appropriate civil aviation authorities (CAAs)
and spectrum regulators. The FAA is collaborating with the Federal
Communications Commission (FCC) and the National Telecommunications and
Information Administration (NTIA) to assess the need for mitigation
beyond the recommended action in this SAIB and the required actions in
airworthiness directives (ADs) addressing 5G C-Band interference on
transport and commuter category airplanes and helicopters equipped with
radio altimeters.
Background
For the past 15 years, the 3 GHz band globally has been the subject of
harmonization activity for mobile broadband, both at the International
Telecommunication Union and within regional groups. A number of
countries already have allocated and assigned spectrum for wireless
deployments in the 3 GHz range and additional countries are following
suit. Many countries around the world are already deploying wireless
networks in the bands from 3.3-4.2 GHz; some countries have implemented
temporary technical, regulatory and operational mitigations, including
temporary proximity and power restrictions, on wireless broadband
networks operating in bands ranging from 3.7 -4.2 GHz. There have not
yet been proven reports of harmful interference due to wireless
broadband operations internationally, although this issue is continuing
to be studied. In the United States, there has been wireless broadband
deployment in the 3.65-3.7 GHz band since 2007. The FCC started a
proceeding to authorize mobile broadband service in the 3.55-3.7 GHz
band in December 2012 and adopted final rules in April 2015 and October
2018. Commercial deployment started in September 2019, with no known
issues for altimeters to date.
With respect to 3.7-4.2 GHz, the FCC first sought comment on
introducing mobile broadband into the band in 2017. In March 2018, the
MOBILE NOW Act directed the FCC to evaluate the feasibility of
commercial wireless deployments in the 3.7-4.2 GHz range. The FCC
started a proceeding in May 2018 and adopted the C-band Report and
Order authorizing flexible use of the 3.7-3.98 GHz band in March 2020.
In February 2021, the FCC completed an auction of the 3.7–3.98 GHz
frequency band and subsequently issued licenses to several wireless
network providers subject to license conditions about deployment timing
and parameters. Under the FCC’s rules, actual wireless broadband
deployment started to occur in phases with operations beginning only in
the lower 100 megahertz of the band (3.7-3.8 GHz) and only in 46
markets in January 2022. (See 47 CFR § 27.1412(b)(1), which
specifies the 46 market areas as Partial Economic Areas (PEAs) 1-4,
6-10, 12- 19, 21-41, and 43-50. A list of PEAs is available at
https://www.fcc.gov/oet/maps/areas).
Radio altimeters operate between 4.2-4.4 GHz. Currently, the FAA’s
approval process for radar altimeters is based on FAA Technical
Standard Order TSO-C87A, Airborne Low-Range Radio Altimeter. TSO-C87A
does not provide criteria for compatibility with adjacent band
operations, including potential impacts associated with wireless
communications system deployments. RTCA formed a task force to assess
the interference impact of wireless broadband operations in the 3.7
-3.98 GHz band on radio altimeters. Based on the work of the task
force, RTCA published a report entitled, “Assessment of C-Band Mobile
Telecommunications Interference Impact on Low Range Radar Altimeter
Operations” (RTCA Paper No. 274-20/PMC-2073). Additional information
about RTCA can be found at https://www.rtca.org/about/. The FAA is
actively participating in the RTCA/SC-239 committee, which is
developing adjacent band compatible minimum operational performance
standards (MOPS) for future radio altimeter designs. The FAA encourages
radio altimeter manufacturers to participate in the international radio
altimeter standards development activity conducted jointly by RTCA and
EUROCAE to establish a common set of performance requirements and
compliance demonstration methods to promote compatibility with the
international adjacent-band spectrum environment.
The FAA conducted a risk assessment to ascertain whether further
mitigation is warranted in addition to the recommended actions in this
SAIB. As a result, the FAA issued ADs addressing 5G C-Band interference
on certain airplanes and helicopters equipped with radio altimeters.
The FAA based its risk assessment on the conditions in the FCC Report
and Order (R&O) in the Matter of Expanding Flexible Use of the
3.7-4.2 GHz Band (FCC 20-22) and 3GPP specification TS 38.104 V17.2.0
(2021-06). Further, in the event that wireless broadband deployment
conditions evolve, if the FAA becomes aware of further relevant
information or guidance, or if additional information further validates
or invalidates concerns of potential harmful interference, the FAA will
update its risk assessment and may issue additional rulemaking.
Therefore, with the ongoing deployments that began in the 3.7-3.8 GHz
band and subsequently expanded to 3.7-3.98 GHz, the FAA recommends the
following actions, in addition to the required actions of the
previously mentioned ADs, and requests that affected parties continue
to voluntarily submit information for consideration by the FAA, FCC and
NTIA during the ongoing wireless network deployments:
Radio Altimeter Manufacturers
1. We request that radio altimeter manufacturers submit receiver
radiofrequency (RF) selectivity, interference tolerance masks, and
baseline operational specifications for each model number in production
or still in use, and approximate numbers of each radio altimeter model
currently in service in the United States; and, if appropriate, mark
submissions as proprietary. This information can be submitted to the
FAA at OperationalSafety@faa.gov and should indicate "Radio Altimeter
SAIB" in the subject line. The FAA will provide any information you
submit to the FCC and NTIA subject to those agencies’ processes for
protection of confidential information, where such protection is
requested.
2. We recommend radio altimeter manufacturers complete analysis or
testing of each model number either in production, supported, or still
being employed, to determine the susceptibility to interference from
fundamental emissions in the full 3.7-3.98 GHz band, as well as
potential spurious emissions in the 4.2-4.4 GHz band, and assess this
susceptibility for compatibility with the adjacent spectrum AIR-21-18R3
environment in accordance with the FCC R&O and 3GPP specification,
as well as the environment in other countries in which each radio
altimeter could be operated. We encourage radio altimeter manufacturers
to work with the FAA to facilitate this process.
3. Report instances of erroneous altimeter performance to the FAA and
FCC, or other appropriate CAAs and spectrum regulators. If the
equipment is susceptible to harmful interference, determine what design
changes are necessary to remediate.
4. Collaborate with aircraft manufacturers on design changes and issue
guidance to operators on potential retrofit plans for radio altimeters
susceptible to harmful interference, if necessary.
5. Determine any necessary operational restrictions or actions needed
to address in-flight radio altimeter anomalies in order to maintain
safe operations with radio altimeter equipment susceptible to harmful
interference and communicate this information to CAAs, operators, and
pilots.
6. Collect radio altimeter retrofit plans, timelines, and completion
information from operators and share this information with MITRE
Corporation. MITRE will collect, aggregate, and de-identify proprietary
information.
Aircraft Manufacturers
1. We request that aircraft manufacturers submit approximate numbers of
each radio altimeter model installed on each aircraft currently in
service in the United States; and, if appropriate, mark submissions as
proprietary. This information can be submitted to the FAA at
OperationalSafety@faa.gov and should indicate "Radio Altimeter SAIB" in
the subject line. The FAA will provide any information you submit to
the FCC and NTIA subject to those agencies’ processes for protection of
confidential information, where such protection is requested.
2. Communicate with radio altimeter suppliers and the FAA to ascertain
equipment susceptibility to harmful interference caused by fundamental
emissions in the 3.7-3.98 GHz band, as well as spurious emissions in
the 4.2-4.4 GHz band. We encourage aircraft manufacturers to work with
the FAA to facilitate this process.
3. Conduct testing and analysis of the effects of loss of function, and
erroneous or misleading radio altimeter data from potential harmful
interference caused by fundamental emissions in the 3.7-3.98 GHz band,
as well as spurious emissions in the 4.2-4.4 GHz band, due to wireless
broadband deployment. We encourage aircraft manufacturers to work with
the FAA to facilitate this process.
4. Evaluate the potential for harmful interference on the different
types of aircraft in the manufacturer’s fleet based on aircraft
architecture and radio altimeter integration, including cascading
effects.
5. Determine any operational restrictions necessary or actions to take
to maintain safe operations with radio altimeter equipment susceptible
to harmful interference due to wireless broadband operation in the
3.7-3.98 GHz band. Restrictions and other actions should address all
systems that use the radio altimeter. Information regarding such
restrictions or other actions should be communicated to operators,
pilots, and each CAA for the country in which the aircraft operates.
Revise aircraft flight manual procedures, if appropriate.
6. Update operators’ anomaly reporting process to ensure accurate
recording and reporting of erroneous radio altimeter readings to the
aircraft manufacturer.
7. Collaborate with radio altimeter manufacturers and issue guidance to
operators on retrofit plans for radio altimeters susceptible to harmful
interference due to wireless broadband operation in the 3.7-3.98 GHz
band. Collect radio altimeter retrofit plans, timelines, and completion
information from operators and share this information with MITRE
Corporation. MITRE will collect, aggregate, and de-identify proprietary
information.
Operators and Pilots
1. Review the 14 CFR Part 5 requirements for Part 121 certificate
holders, if appropriate. Other certificate holders should use the
Safety Management Systems (SMS) tools to assess the risk to each type
of radio altimeter configuration and how it impacts typical flight
operations.
2. Remind passengers that all portable electronic devices allowed for
transport in checked baggage (including smartphones and other devices)
should be turned off and protected from accidental activation and that
compliance is mandatory for lithium battery-powered portable electronic
devices. See 49 CFR 175.10(a)(18).
3. Remind passengers to set all portable electronic devices in the
cabin and any carried on the aircraft to a non-transmitting mode or
turn them off. See 14 CFR 91.21, 121.306, 125.204 and 135.144.
4. Seek information from the manufacturers of the aircraft and the
radio altimeter on possible effects of harmful interference due to
wireless broadband deployment in the 3.7-3.98 GHz band and possible
pilot interventions. Provide radio altimeter retrofit plans, timelines,
and completion information to your original equipment manufacturer and
directly to the FAA at 9-AWA-AFS400-RADALT@faa.gov.
5. Operators should ensure their pilots are aware of the potential
degradation of the radio altimeter capabilities and any means to
compensate for in-flight radio altimeter anomalies. Consider both
erroneous altimeter readings and loss of altimeter function.
6. Operators should ensure their pilots are aware of the potential
degradation to the capabilities of safety systems and other equipment
dependent upon radio altimeters and any means to compensate for
resulting anomalies. Consider both the loss of function of the safety
systems and other dependent systems and the manners in which they may
malfunction.
7. Operators should consider the potential loss of pilot trust in
dependent aircraft safety systems in the assessment of existing and the
development of new crew procedures.
8. Operators and pilots who experience radio altimeter anomalies should
notify air traffic control, as soon as practical. Post flight, pilots
are encouraged to submit detailed reports of radio altimeter
disruptions or interference events, as soon as practical, using the
Radio Altimeter Anomaly Reporting Form available on the FAA website at
https://www.faa.gov/air_traffic/nas/RADALT_reports/.
9. Coordinate with aircraft and radio altimeter OEMs to determine
retrofit options that are available or under development and develop an
implementation timeline that aligns with any applicable ADs.
Paperwork Reduction Act Burden Statement
A federal agency may not conduct or sponsor, and a person is not
required to respond to, nor shall a person be subject to a penalty for
failure to comply with a collection of information subject to the
requirements of the Paperwork Reduction Act unless that collection of
information displays a currently valid OMB Control Number. The OMB
Control Number for this information collection is AIR-21-18R3
2120-0731. Public reporting for this collection of information is
estimated to be approximately 1 hour per response, including the time
for reviewing instructions, searching existing data sources, gathering
and maintaining the data needed, completing and reviewing the
collection of information. All responses to this collection of
information are voluntary; the nature and extent of confidentiality to
be provided, if any. Send comments regarding this burden estimate or
any other aspect of this collection of information, including
suggestions for reducing this burden to: Information Collection
Clearance Officer, Federal Aviation Administration, 10101 Hillwood
Parkway, Fort Worth, TX 76177-1524.
For Further Information Contact
Jeffrey Palmer, COS Program Manager, Operational Safety Branch, 3960
Paramount Boulevard, Lakewood, CA 90712-4137, e-mail:
OperationalSafety@faa.gov.
1 This
revision has been updated to reflect the 5G C-Band wireless broadband
ongoing deployment and the introduction of retrofit related actions.