Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
AIR-21-18 R1
December 23, 2021
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin (SAIB) informs aircraft
manufacturers, radio altimeter manufacturers, operators, and pilots of
the planned deployment of wireless broadband networks in the 3700-3980
MHz bands (C-Band).1 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 SAIB recommends that radio
altimeter manufacturers, aircraft manufacturers, and operators
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
currently 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.
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 3300-4200 MHz; 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 3700-4200
MHz. 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 will occur in phases with operations beginning only in the
lower 100 megahertz of the band (3700-3800 MHz) and only in 46 markets
as soon as December 5, 2021. (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 4200-4400 MHz. 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
3700-3980 MHz 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 RTCA report
is under review. 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 is currently conducting a risk assessment to ascertain whether
further mitigation is warranted in addition to the recommended actions
in this SAIB. The FAA is basing its risk assessment on the conditions
in the FCC Report and Order (R&O) in the Matter of Expanding
Flexible Use of the 3700-4200 MHz 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.
Therefore, in preparation for the December 2021 wireless network
deployment in the 3700-3800 MHz band, the FAA recommends the following
actions and requests that affected parties voluntarily submit
information for consideration by the FAA, FCC and NTIA prior to the
planned 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 3700-3800 MHz which is available for wireless
broadband in December of 2021 and the full 3700-3980 MHz band which
will become available later, as well as potential spurious emissions in
the 4200-4400 MHz band, and assess this susceptibility for
compatibility with the adjacent spectrum 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 civil aviation authorities (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.
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 3700-3800 MHz and 3700-3980 MHz band, as well as
spurious emissions in the 4200-4400 MHz 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 3700-3800 MHz and
3700-3980 MHz band, as well as spurious emissions in the 4200-4400 MHz
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
3700-3800 MHz 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 3700-3800 MHz
band.
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 3700-3800 MHz band and possible
pilot interventions.
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./
For Further Information, Contact: Charisse R. Green, Aerospace
Engineer, Aircraft Information Systems Section, 950 L'Enfant Plaza SW,
5th Floor, Washington, DC 20024, e-mail: OperationalSafety@faa.gov.
1 This revision clarifies the radio altimeter anomaly reporting request.