The Federal Aviation Administration will give airlines one more year to fix or replace aircraft altimeters that fail to filter out cellular transmissions outside of their allocated frequencies. In a notice of proposed rulemaking (NPRM) released today, the FAA proposed a deadline of February 1, 2024, to replace or upgrade faulty altimeters used by aircraft to measure altitude.
Of the 7,993 aircraft on the US registry, the FAA said, “approximately 180 aircraft will require radio altimeter modification and 820 aircraft will require the addition of radio altimeter filters to comply with the proposed modification requirement.” The total estimated cost of compliance is $26 million.
The requirement could finally end a dispute between the aviation and wireless industries that has prevented AT&T and Verizon from fully deploying 5G in the C-Band spectrum licenses the wireless carriers bought for $69 billion. Aircraft altimeters rely on the 4.2 GHz to 4.4 GHz spectrum, but some cannot filter out 5G transmissions from carriers’ spectrum in the 3.7 to 3.98 GHz range.
“Some radio altimeters may already be able to tolerate 5G C-Band emissions without modification,” the FAA said. “Some may need to install filters between the radio altimeter and the antenna to increase the radio altimeter’s tolerance. For others, adding a filter will not be enough to eliminate susceptibility to interference; therefore, the radio altimeter should be replaced with an improved altimeter. radio altimeter.”
“Faulty system warnings” are a problem
The FAA said it “anticipates erroneous system alerts due to a malfunctioning radio altimeter that would cause the flight crew to become desensitized to system alerts. Such desensitization negates the safety benefits of the alert itself and could lead to a catastrophic event.”
The FAA said in June 2022 that airlines must replace or improve faulty altimeter gauges “as soon as possible.” But today’s notice says Feb. 1, 2024, is “the date the FAA has determined as soon as reasonably practicable consistent with FAA policy.”
The FAA will receive public comment for 30 days before finalizing the new proposal. Airlines “are working diligently to ensure fleets are equipped with the appropriate radio altimeters, but global supply chains continue to lag behind current demand. Any government term must take this reality into account,” Bloomberg’s report said, citing lobby group Airlines for America.
The proposed rule would affect aircraft operating under Part 121 certificates. The FAA states that these certificates generally apply to carriers with scheduled air service, which include “major, US-based airlines, regional air carriers and all cargo operators.”
“To minimize the number of erroneous system messages and the hazardous conditions they create, the FAA is proposing to require all airplanes operating under part 121 to meet PSD requirements. [power spectral density] the performance curve will operate in the contiguous United States after February 1, 2024,” the notice said.
Between now and a deadline of 2024, planes that continue to use faulty altimeters must follow some restrictions when landing. In December 2021, the Airworthiness Directive “prohibits[ed] Certain operations that require radio altimeter data when there is 5G C-Band interference.”
Older altimeters “ignore assigned spectrum boundaries”
The Federal Communications Commission approved cellular use in the C-Band in February 2020 after hearing complaints from the airline industry. The FCC concluded that harmful interference to altimeters is unlikely to occur “under reasonable scenarios” because of the 220 MHz guard band between 5G and altimeter operations and the FCC’s power limits required for C-Band transmission.
But airlines were not ready for the C-Band rollouts, which were originally scheduled to begin in December 2021. AT&T and Verizon agreed to a nationwide delay of about six weeks and restrictions on deployment near airports. Accommodation restrictions around airports were originally supposed to end in July 2022, but carriers later agreed to maintain some restrictions on airport grounds until July 2023.
Altimeters were originally developed decades ago when nearby spectrum bands contained only low-power operations such as satellites. Expert Dennis Roberson explained at a congressional hearing in February 2022: “This caused the early designers of altimeters to actually decide to ignore the assigned spectrum boundaries, and as a result, they allowed energy far outside their band to be transmitted to the receiver.”
The altimeter problems prompted the FCC to launch an investigation into poorly designed wireless devices that receive transmissions outside of their assigned frequencies. The survey may result in new regulations for wireless receivers; The FCC has traditionally only imposed restrictions on transmitters.