The Federal Aviation Administration (FAA) intends to require airlines and other operators to upgrade radio altimeters again to head off fresh interference threats posed by 5G cellular networks.
An FAA proposed rule released on 5 January would apply to all aircraft now equipped with radio altimeters and comes as the USA prepares to make another range of radio bandwidth available to cellular providers.
The FAA says the rule’s initial implementation date will likely fall between 2029 and 2032. It estimates US airlines alone would need to replace 17,033 radio altimeters to comply with the rule as proposed.
The agency had in 2023 set new radio altimeter standards after the cellular industry rolled out fifth-generation networks using transmissions in the 3.7-3.98GHz range, also known as the Lower C-band.
The FAA acted because those transmissions, being close to the 4.2-4.4GHz range used by radio altimeters, pose interference risks.
That rule, and the operating restrictions implemented to prevent interference, rocked the industry.
Now, airlines and other operators are facing another round of radio altimeter standards from the FAA – again in response to 5G networks.
The proposed rule comes as the cellular industry prepares to expand into the 3.98-4.2GHz radio range, called the Upper C-Band, which is even closer to the range used by radio altimeters.
The FAA says altimeter specifications mandated by its 2023 rule are “are insufficient to address the unsafe condition” created by Upper C-Band signals.
“Wireless signals in the neighbouring spectrum bands may interfere with [radio altimeters] and cause inaccurate altitude readings,” the FAA’s proposal says.
“New [radio altimeters] must be able to withstand interference from higher-powered wireless signals in neighbouring spectrum bands and spurious emissions.”
The FAA proposes two compliance deadlines. Operators flying under Part 121 rules – essentially US airlines – would be required to have conforming altimeters before the Federal Communications Commission (FCC) authorises wireless services in the 3.98-4.2GHz spectrum.
That same deadline would apply to foreign carriers operating under the FAA’s Part 129 rules flying aircraft with 30 or more passenger seats or payload capacities exceeding 3,402kg (7,500lb).
“FAA expects this initial… performance deadline to be sometime between 2029 and 2032,” it says.
All other aircraft equipped with radio altimeters would have two additional years to comply. The FAA says the mandate will apply to only a “minority of general aviation aircraft”, which often lack radio altimeters.
The agency is accepting comments for 60 days.
US airline trade group Airlines for America is “evaluating all of the details in the proposal” and plans to provide comments to the FAA.
”We have been working collaboratively with the telecommunications industry, the FAA and the FCC to identify solutions that ensure our nation’s airspace remains safe while allowing the spectrum to be used,” the group adds.
The US aviation industry scrambled last time around to meet the 2023 requirements. The FAA also responded then by limiting operations at some airports.
Erroneous altimeter readings can affect other cockpit systems like terrain avoidance and traffic warning systems, which rely on radio altimeter data.
As of August last year, “the FAA had received 659 reports of potential C-band interference, and 493 of these reports were associated with [radio altimeters] or related systems”, it says.
Story updated on 6 January 2026 to include comments from Airlines for America and to include the FAA’s estimate of how many aircraft operated by airlines may be affected.
