Getty Images | Daniel Garrido
The Federal Aviation Administration is giving airlines an additional year to repair or replace aircraft altimeters that can’t filter out cellular transmissions from outside their assigned frequencies. In a Notice of Proposed Rules and Regulations (NPRM) released today, the FAA proposed a deadline of February 1, 2024 to replace or retrofit faulty altimeters used by aircraft to measure altitude.
Of 7,993 aircraft on the US registry, the FAA said it “estimates that approximately 180 aircraft would require radio altimeter replacement and 820 aircraft would require the addition of radio altimeter filters to meet the proposed change request.” The estimated total cost of compliance is to $26 million.
The requirement could finally end a row between the airline and wireless industries that has prevented AT&T and Verizon from fully deploying 5G on the C-band spectrum licenses the wireless carriers acquired for a combined $69 billion. Aircraft altimeters rely on a 4.2 GHz to 4.4 GHz spectrum, but some cannot filter 5G transmissions from the spectrum of carriers in the 3.7 to 3.98 GHz range.
“Some radio altimeters may show tolerance to 5G C-band emissions even without modification,” the FAA said. “Some may need to install filters between the radio altimeter and the antenna to increase the tolerance of a radio altimeter. For others, adding a filter isn’t enough to fix the susceptibility to noise. Therefore, the radio altimeter must be replaced with an updated radio altimeter.”
“Error system alerts” are a problem
The FAA said it “expects that erroneous system warnings due to a faulty radio altimeter will cause the flight crew to become desensitized to system warnings.” Such desensitization negates the safety benefits of the alert itself and can result in a catastrophic event.”
In June 2022, the FAA stated that airlines must replace or retrofit defective altimeters “as soon as possible”. However, the notice issued today states that February 1, 2024 is “the date the FAA has determined to be as soon as possible, consistent with FAA policy.”
advertising
The FAA is taking 30 days of public comment on its new proposal before finalizing it. In a Bloomberg report, lobby group Airlines for America was quoted as saying that airlines are “working diligently to ensure their fleets are equipped with compliant radio altimeters, but global supply chains continue to lag behind current demand.” Any government term must take this reality into account.”
The proposed rule would affect airplanes operated under Part-121 certificates. These certificates apply to airlines with scheduled air services, which generally include “major U.S.-based airlines, regional airlines and all cargo carriers,” according to the FAA.
“To minimize the number of erroneous system messages and the unsafe condition they create, the FAA is proposing to require all aircraft operating under Part 121 to comply with the PSD [power spectral density] Performance curve for operations in the contiguous U.S. after February 1, 2024,” the release said.
By the 2024 deadline, aircraft that continue to use faulty altimeters will have to comply with some restrictions when landing. A December 2021 airworthiness directive “prohibited[ed] certain operations that require radio altimeter data when 5G C-band interference is present.”
Old Altimeters “Ignore Assigned Spectrum Limits”
The Federal Communications Commission approved C-band mobile use in February 2020 after reviewing airline industry complaints. The FCC concluded that harmful interference to altimeters is unlikely “under reasonable scenarios” due to the 220 MHz guard band between 5G and altimeter operations and power limitations that the FCC requires for C-band transmissions.
However, airlines were unprepared for the C-band launches originally scheduled for December 2021. AT&T and Verizon agreed to a nationwide delay of about six weeks and a limit on use near airports. Operational restrictions near airports were originally scheduled to expire in July 2022, but airlines later agreed to maintain some restrictions in airport areas until July 2023.
Altimeters were originally developed decades ago when adjacent frequency bands contained only low-power operations such as satellites. “This led early altimeter designers to decide that they could really ignore their assigned spectrum limits and, as a result, allow transmitted energy well outside of their band into the receiver,” expert Dennis Roberson explained at a February 2022 congressional hearing.
The altimeter problems prompted the FCC to open an investigation into poorly designed wireless devices receiving transmissions from outside of their assigned frequencies. The investigation could lead to new regulations for wireless receivers; The FCC has traditionally placed restrictions only on broadcasters.