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Federal Register / Vol. 86, No. 234 / Thursday, December 9, 2021 / Rules and Regulations 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.
FAA20210954; 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:
Dave Swartz, Continued Operational Safety Technical Advisor, COS Program Management Section, Operational Safety Branch, FAA, 222 W 7th Ave., M/S 14 Anchorage, AK 99513; phone:
8172225390; email:
operationalsafety@faa.gov.
SUPPLEMENTARY INFORMATION:

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Background In March 2020, the United States Federal Communications Commission FCC adopted final rules authorizing flexible use of the 3.73.98 GHz band for next generation services, including 5G and other advanced spectrum-based services.1 Pursuant to these rules, CBand wireless broadband deployment is permitted to occur in phases with the opportunity for operations in the lower 100 megahertz of the band 3.73.8 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.
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 3.73.98 GHz band, as well as a detailed assessment of the risk of such interference occurring and 1 The FCCs 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 3.73.98
GHz.

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impacting aviation safety. 3 Based on the work of the task force, RTCA
published a report, which concluded 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 aircraftincluding commercial transport airplanes;
business, regional, and general aviation airplanes; and both transport and general aviation helicopters. 4
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 2022,5
which identifies radio frequencies and power level conditions for the new CBand 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 RTCAs 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.
Additionally, the deployment of CBand wireless broadband networks is occurring globally. In certain countries, deployment has already occurred in CBand frequencies. In some countries, temporary technical, regulatory, and operational mitigations on C-Band 3 RTCA Paper No. 27420/PMC2073, Assessment of C-Band Mobile Telecommunications Interference Impact on Low Range Radar Altimeter Options, dated October 7, 2020 RTCA Paper No.
27420/PMC2073, page i. This document is available in Docket No. FAA20210954, and at https www.rtca.org/wp-content/uploads/2020/10/
SC-239-5G-Interference-Assessment-Report_274-20PMC-2073_accepted_changes.pdf.
4 RTCA Paper No. 27420/PMC2073, page i.
5 FCC Report and Order R&O FCC 2022 in the Matter of Expanding Flexible Use of the 3.74.2
GHz Band, adopted February 28, 2020, and released March 3, 2020. This document is available in Docket No. FAA20210954, and at https
www.fcc.gov/document/fcc-expands-flexible-use-cband-5g-0.

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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.
The radio altimeter is an important aircraft instrument, and its intended function is to provide direct heightabove-terrain/water information to a variety of aircraft systems. Commercial aviation radio altimeters operate in the 4.24.4 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 autohover or other low altitude operations. The receiver on the radio altimeter typically is highly accurate, however it may deliver erroneous results in the presence of out-of-band radio frequency 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 without visual reference to the landing 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 helicopters facilitate low visibility operations and rely on accurate radio altimeter inputs. These inputs may provide height data for landing and takeoff for Category A and Category B
operations. 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.

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