US wireless providers are on schedule to boost 5G service strength on July 1, but that expanded coverage may come at a cost. As Secretary of Transportation Pete Buttigieg warned over the weekend, some airline companies who haven’t yet upgraded their fleets’ radio altimeters to withstand an increased 5G C-Band data could soon face new flight delays, adding to an already frequently chaotic airscape

In an interview with The Wall Street Journal on Friday, Secretary Buttigieg explained that portions of some airline companies’ fleets could still experience radio altimeter issues stemming from 5G interference despite an 18 month advance warning.

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“There’s a real risk of delays or cancellations,” Buttigieg said. “This represents one of the biggest—probably the biggest—foreseeable problem affecting performance this summer.”

A plane’s radio altimeter uses radio waves to calculate a plane’s height above the ground—an often crucial measurement when needing to land during inclement weather conditions. Although overseas wireless companies use different radio frequencies, their American counterparts can overlap within the telecommunication industry’s chosen spectrum. As such, travelers could potentially begin seeing an influx of delays and cancellations as the Federal Aviation Administration (FAA) bars pilots from landing non-updated planes during low-visibility circumstances.

Although The WSJ explained the necessary altimeter retrofits are relatively easy to implement, the process still takes time, and overhauling active fleets of planes adds more complications. Buttigieg confirmed that over 80 percent of domestic airlines and roughly 65 percent of international planes have already retrofitted their radar altimeters to handle boosted 5G signals. United and Southwest Airlines have both stated their mainline fleets are ready ahead of the deadline, while American Airlines told The WSJ its own retrofits will be completed by the end of the month.

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Meanwhile, however, companies such as Delta claim supply-chain issues are preventing it from finishing its retrofits of roughly 190 narrow-body planes, including all of its Airbus A220 jets and some of its Airbus fleet. JetBlue doesn’t expect all of its 17 A220 jets to be upgraded until October, but believes the delay will only incur a “limited impact” to its services.

The rapidly approaching deadline is only the latest in a longstanding spat that has seen the Federal Communications Commission and FAA face off against telecommunication companies such as Verizon and AT&T, who insist 5G signals pose minimal issues to planes. As The Verge noted on Sunday, a full expansion to 5G C-Band coverage was first delayed to January 2022 before regulators kicked the deadline back to July 5, 2022—and yet again to the current July 1 cutoff.

Airlines currently face a hard deadline of February 2024 for finishing their radio altimeter retrofits, barring any additional delays.