This summer’s travel season is expected to be the busiest on record for the U.S. airline industry. But last Friday, U.S. Transportation Secretary Pete Buttigieg warned that about 20% of the country’s domestic airline fleet still has not upgraded its radio altimeters to avoid interference from 5G wireless usage in the C-band spectrum.

Altimeters measure the distance a plane is from the ground. And they’re important when a plane is landing and doesn’t have good visibility. The problem with a lot of older altimeters is that they weren’t engineered to stay strictly in their spectrum lane. And they can pick up interference from C-band spectrum, even though there’s a hefty spectrum guard band between the different spectrum lanes.

The altimeter issue has been ongoing since late 2021 when the FAA sounded a warning that deployment of newly purchased C-band spectrum by AT&T, Verizon and US Cellular for their 5G networks could potentially cause plane crashes because many older aircraft didn’t have updated altimeters.

The wireless carriers have been collaborating with the FAA ever since to arrive at a solution. And they’ve voluntarily delayed some C-band deployments around airports to ensure airline safety. In April this year, Verizon and AT&T agreed to give the airline industry until July 1 to complete its altimeter upgrades.

But now, it appears the airline industry won’t meet that deadline for 100% of its fleet. And the onus is now on it as the wireless carriers are moving forward with their full C- band deployments.

Buttigieg said that more than 80% of the domestic fleet and 65% of international carriers that serve U.S. airports have updated altimeters, according to a letter he wrote last week to Airlines for America (A4A), which was reviewed by the Wall Street Journal.

“We continue to see a significant number of aircraft still awaiting retrofit, including many operated by foreign air carriers,” he wrote. “This means on bad-weather, low-visibility days in particular, there could be increased delays and cancellations.”

For the airplanes that don’t have updated altimeters, the FAA has issued a directive, mandating that they not be allowed to make low-visibility landings.

Reuters reported that the trade organization A4A said carriers are working to update altimeters “but global supply chains continue to lag behind current demand…. A4A member carriers are confident in their ability to maintain the integrity of their schedules, despite the impending deadline.”

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