The FAA has finalized policy for demonstrating radio altimeters can operate in areas where 5G C-band signals are present without fear of interference, rejecting calls for substantial changes and one request to withdraw the proposed standards.
Building on a draft document issued in May for a process that has been going for more than a year, the agency’s final policy—set to be released in the coming days, according to a Federal Register notice summarizing the move—made few changes to its initial proposals.
Among its clarifications: data submitted for alternative methods of compliance (AMOC) approvals prior to the draft policy must be re-submitted.
“Performance justifications for prior ADs were approved by the FAA before the method in the policy statement was defined,” the FAA said in response to an inquiry from Thales. “Although the FAA expects that aircraft with AMOCs approved for prior ADs may be able to meet the definition of a ‘radio altimeter tolerant airplane’ or ‘radio altimeter tolerant rotorcraft,’ design approval holders (DAHs) or operators will need to provide the FAA with data showing explicitly that the aircraft meets the tolerances” outlined in the latest ADs, the agency said.
The FAA rejected a similar request from Gulfstream and the Cargo Airline Association (CAA) that would have prevented industry from re-submitting data already presented to the agency.
“Gulfstream and CAA requested that the FAA identify the radio altimeter technologies that meet the AD requirements,” the agency said. The FAA noted the AMOC approvals came before the draft policy was developed. “Additionally, the FAA does not maintain a list of tolerant radio altimeters; the determination of a radio altimeter tolerant aircraft must consider the installation details, which vary from aircraft to aircraft,” it added.
The agency also said the final policy includes “guidance ... to assist with obtaining FAA approval expeditiously.”
Airlines For America (A4A) wanted the policy withdrawn, citing operators’ lack of control over what design approval holders have submitted to the FAA and the (then) pending deadline of July 1. FAA said the policy is needed to support the latest directives issued to prevent aircraft from encountering interference from 5G C-band deployments being rolled out by wireless telecommunications providers.
“A radio altimeter tolerant aircraft’s radio altimeter, as installed, must demonstrate tolerance to radio altimeter interference at or above the power spectral density (PSD) curve established in the ADs,” the draft policy said. A PSD curve is “the height over the ground and received power from a 5G C-Band emitter, at or below which the radio altimeter is expected to function reliably,” the agency explained.
Operators, manufacturers, and affected radio altimeter providers have been working to ensure their aircraft are “tolerant” in the new 5G environments for more than a year. A phased plan created by the FAA called for aircraft to be cleared or modified with filters by July 1 to align with telecommunications providers’ plans to continue their deployments.
A series of compromises between the FAA and telecommunications companies has pushed some deployments out, giving some areas in and around airports extra time before full 5G C-band is deployed. But the rollout is continuing, albeit at a slower pace than AT&T, Verizon Wireless, and others providing the new services envisioned.
U.S. carriers had cleared about 80% of their aircraft by late June, according to information provided to the U.S. Transportation Department. Airlines and manufacturers have cited supply chain issues and the challenge of developing upgraded equipment as primary hurdles.
Aircraft not cleared to operate in 5G environments face restrictions when operating in low-visibility weather. In addition, some systems that rely on radio altimeter data may be restricted.
Radio altimeters calculate the precise distance between an aircraft and the ground and use the 4.2-4.4 GHz frequency band of the radio spectrum. The new 5G services are allocated to use 3.7-3.98 GHz.
The 5G services using similar “C-band” spectrum are in place around the world with no reports of interference with aircraft. But the FAA is concerned that differences in the U.S. networks, including power settings on transmitters and antenna angles, introduce risk.
The final policy, “Demonstration of Radio Altimeter Tolerant Aircraft, Policy No. PS-AIR-600-39-01,” will be available in FAA’s Dynamic Regulatory System (drs.faa.gov) and in docket no. FAA-2023-0938.
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