FAA Issues Airworthiness Directives To Protect Against 5G

WASHINGTON—The FAA on Dec. 7 released airworthiness directives (AD) requiring operators of helicopters and passenger-carrying airplanes to prohibit certain operations requiring radio altimeter (RadAlt) data when in the presence of 5G wireless transmissions.

AD 2021-23-12, which applies to transport- and commuter-category airplanes, and AD 2021-23-13, which covers all helicopters equipped with RadAlts, were “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,” the agency states.

The directives require operators to revise their aircraft or rotorcraft flight manuals to prohibit operations requiring RadAlts in areas of 5G interference identified by Notices to Air Missions (NOTAMs), formerly called Notices to Airmen. The NOTAMs will be issued for specific airports where radio altimeter performance is considered unreliable.

Prohibited operations could include instrument landing system (ILS) approaches, required navigation performance (RNP) procedures, automatic landing operations and head-up display (HUD) or enhanced flight vision system (EFVS) use to touchdown on the runway.

RadAlts, also called radar altimeters, track an aircraft’s clearance height over terrain by measuring reflected signals from about 2,500 ft. above ground level, operating in the 4200-4400 MHz frequency band. The concern over 5G networks potentially interfering with RadAlts intensified earlier in 2021 with the Federal Communications Commission (FCC) auction of C-band spectrum at 3.7-3.98 GHz (3700-3980 MHz), within 220 MHz of the RadAlt operating range.

The FAA issued the ADs preemptively, before the planned Jan. 5, 2022 initial deployment of 5G networks by the winning bidders of the FCC auction. Those deployments were originally scheduled to start on Dec. 5. But telecommunications companies AT&T and Verizon voluntarily agreed to pause their plans by one month at the FAA’s request.

On Nov. 24, telecommunications companies submitted a proposal to the FCC to mitigate potential interference problems, including by reducing the power level at 5G base stations around airports and heliports, for a period of six months. But “an aviation (industry) assessment of that proposal decided the mitigations were insufficient to protect aviation operations,” said Clay Barber, a Garmin principal engineer who spoke Dec. 7 during an NBAA News Hour webinar on the 5G issue. 

A coalition of aviation organizations made a counter-proposal to the mitigation plan on Dec. 2, Barber said. “It retains some aspects of the telecom proposal but replaced others,” he explained. “It was intended to allow more flexibility in the 5G deployment that should benefit the telecoms. There is yet to be a response.”

Also speaking during the webinar, Andrew Roy, director of engineering services with Annapolis, Maryland-based Aviation Spectrum Resources, said telecommunications companies at this point have not provided maps of 5G tower locations.

“There was some thought about what this means for 5G,” Roy said. “There was a comment from the AT&T CEO the other week, [who] said that for the proposal Verizon and AT&T have put forward, his estimate was that it was single digits—maybe even less than that—as a percent of total [5G] towers that would be affected by this. Certainly, from the 5G perspective, we don’t think it’s a major issue to start considering at least around airports. Obviously, the helicopter concern is still there.”