A broad coalition of aviation groups is criticizing the FAA’s proposed airworthiness directive (AD) on 5G C-band interference mitigation on aircraft systems, saying it is technically flawed and ignores Part 91 and Part 135 operations. The coalition—including Helicopter Association International, NBAA, and 21 other aviation OEMs, companies, and associations—said the proposal “falls short of the goal of preserving the safe operation of airplanes in the National Airspace System (NAS),” calling it “incomplete” and charging that it would adversely impact aircraft operations.

The FAA issued an NPRM on the proposed AD on January 11 along with a 30-day comment period. The proposed AD supersedes all previously issued guidance and ADs (including 2021-23-12) dealing with the interference issue. The FAA said “additional limitations are needed” to deal with the 5G issue as wireless companies withdraw their voluntary signal mitigation around airports and more broadly roll out 5G C-band nationwide. The FAA said it expects 19 additional wireless companies to join AT&T and Verizon in rolling out 5G C-band. The agency said it has received reports of 420 possible instances of 5G C-band radio altimeter interference over the last year but eliminated 75 percent of those reports as unrelated.

AFM Changes

Nevertheless, the FAA proposes to mandate modification of aircraft flight manuals (AFM) no later than July 1, 2023, to prohibit certain operations in the presence of 5G C-band (4.2-4.4 GHz) interference. Those operations include ILS approach procedures SA CAT I, SA CAT II, CAT II, and CAT III; automatic landing operations; manual flight control guidance systems operations to landing including head-up display (HUD) to touchdown; and use of enhanced flight vision system (EFVS) to touchdown under 14 CFR 91.176(a). Those operations would be allowed at airports the FAA identifies as 5G C-band mitigation airports (CMA), provided the aircraft are equipped with “a radio altimeter [interference] tolerant airplane.” The approved airports would be identified in an FAA Domestic Notice, as opposed to individual notams, which the FAA considers overly burdensome and unworkable.

The AD would also prohibit all Part 121 (airline) operations after Feb. 1, 2024, unless performed with a radio altimeter-tolerant airplane.

The FAA calls the proposed AD an “interim action” subject to change once a Technical Standard Order (TSO) for radio altimeters is established that meets international minimum operational performance standards (MOPS). Once equipped with these new radio altimeters, aircraft will be able to operate at 5G CMA and non-CMA airports, the FAA said.

The FAA estimates the new AD would impact 7,993 airplanes on the U.S. registry and also interpolates that 7,000 U.S. aircraft already are equipped or have been retrofitted to address radio altimeter interference with new radio altimeters or filters, or are not used for Part 121 operations, and therefore can be brought into compliance by simply modifying the AFM. The FAA estimates that of the remaining Part 121 fleet, 180 aircraft would need to be fitted with new radio altimeters while another 820 would need filtering. It estimates the cost of the AD at $26,049,810—$24,691,000 for new radio altimeters and filtering and the remainder for costs associated with AFM revisions.

Deficient AD

The coalition unified in opposition to the proposed AD found it deficient in three major ways:

1.)
It is an incomplete solution for maintaining the safety of the NAS while preserving existing aviation operations as 5G service is expanded.

2.)
It does not identify “sufficiently robust controls” to ensure continued aviation safety as aviation operations and wireless services “co-evolve.”

3.)
It effectively reallocates responsibility for maintaining the safety of the NAS from the FAA to individual operators.

The coalition further noted that the FAA’s definition of a “radio altimeter tolerant airplane” was murky and lacked “sufficient information for radio altimeter and airplane manufacturers.” Specifically, the group took issue with the FAA’s methods and measurement standards used to calculate specific levels of interference. It was also critical of the FAA’s determination of just what constituted a CMA, saying that the FAA was merely labeling airports as such where AT&T and Verizon had voluntarily agreed to limit emissions, noting, “These voluntary agreements provide insufficient assurance upon which to base coexistence strategies when compared to controls established in regulation.” In particular, “Conditions of the [voluntary] agreements have not been made available for review by the aviation industry to assess the impact on operations at non-mitigated airports.” Concern also arose over the issue of what to do about wireless operators who do not enter into these voluntary agreements.

Shift To Operators

The proposed AD’s effect of shifting partial responsibility for the safety of the NAS from the FAA to individual operators drew particularly blunt commentary. The coalition noted the FAA’s primacy in ensuring the current high level of aviation safety in the U.S. based on its “uncompromising approach of establishing rules that identify and mitigate potential safety hazards to acceptably low levels of occurrence,” but warned that the proposed AD “does not adequately address safety concerns for operations at airports not identified on the FAA’s 5G CMA Domestic Notice, which has the effect of pushing this responsibility to the airplane operators.”

Degraded aircraft performance as the result of unmitigated C-band emissions near airports could also force OEMs and operators to limit allowable payloads and operational capabilities even after new radio altimeters are developed, given that the FAA is powerless to determine where 5G C-band transmitters are located near non-CMA airports, the coalition said. “It is not clear that the new radio altimeter TSO will be able to tolerate C-band 5G within the U.S. such that radio altimeter equipment will continue to function correctly up to the point where an aircraft contacts the transmitter.

“It is unclear…how new TSO/MOPS requirements that obviate AFM limitations for non-CMA operations are to be determined without additional information on the 5G sources.”

The coalition recommended that the FAA make several changes to the proposed AD. Among those recommendations: study the specific impact of 5G C-band interference on the performance of safety-critical linked systems including TAWS Class A, TCAS II, and windshear safety systems—both in and outside the runway environment—and refine frequency tolerance definitions; recognize the addition of radio altimeter filters as minor changes to type design; clarify how voluntary 5G limitations will be defined, accomplished, and enforced; and clarify the process for inclusion or subtraction to the CMA list.

Coalition members, while pledging further cooperation with the FAA on 5G, concluded, “We find significant deficiencies in the existing text to adequately provide equivalent levels of safety for airplane operations in the NAS given the rapid growth of 5G C-band emissions in the United States.”