Airline travellers who have suffered thousands of weather-related flight delays this week could face a new source of disruptions starting Saturday when wireless providers are expected to power up new 5G systems near major airports. Is.

Aviation groups have warned for years that 5G signals could interfere with aircraft equipment, especially equipment that uses radio waves to measure distance above the ground and which is critical when planes land in low visibility.

Predictions that the interference would lead to the mass grounding of flights did not come true last year when telecommunications companies began rolling out the new service.

They then agreed to limit signal strength around busy airports, giving airlines an extra year to upgrade their planes.

The leader of the country’s largest pilots’ union said crews would be able to handle the effects of 5G, but criticized the way wireless licenses are awarded, saying it added unnecessary risk to aviation.

Transportation Secretary Pete Buttigieg recently told airlines that flights could be disrupted because a small portion of the nation’s fleet hasn’t been upgraded to protect against radio interference.

Most major US airlines say they are ready. American, Southwest, Alaska, Frontier and United say all of their planes have altitude measuring devices, called radio altimeters, that are protected from 5G interference.

Delta Air Lines is the big exception. Delta says it has 190 aircraft, including the smallest aircraft, that still lack advanced altimeters because its supplier has been unable to provide them fast enough.

Delta said Friday that the airline does not expect to cancel any flights because of the issue.

The airline plans to carefully route 190 aircraft to limit the risk of flight cancellations or diversion away from airports where visibility is low due to fog or low clouds.

Delta planes that have not been retrofitted include several models of Airbus jets: all of its A220s, most of its A319s and A320s, and some of its A321s.

The airline said the airline’s Boeing jets have upgraded altimeters, as have all Delta Connection planes, which are operated by Endeavor Air, Republic Airways and SkyWest Airlines.

JetBlue did not respond to requests for comment but told The Wall Street Journal that it expected to reinstate 17 small Airbus jets with a possible “limited impact” over a few days in Boston by October.

Wireless carriers including Verizon and AT&T use a part of the radio spectrum called C-band for their new 5G service, which is close to the frequencies used by radio altimeters.

The Federal Communications Commission granted them a license for the C-band spectrum and dismissed any risk of interference, saying that there was a sufficient buffer between the C-band and altimeter frequencies.

When the Federal Aviation Administration sided with the airlines and objected, the wireless companies halted the rollout of their new service.

In a deal brokered by the Biden administration, wireless carriers agreed not to power 5G signals near about 50 busy airports. That moratorium ends on Saturday.

AT&T declined to comment. Verizon did not immediately respond to a question about its plans.

Buttigieg reminded the head of the trade group Airlines for the Americas of the deadline in a letter last week, warning that only planes with retrofitted altimeters would be allowed to land in low-visibility conditions.

He said more than 80 per cent of the US fleet has been retrofitted, but a large number of aircraft, including many operated by foreign airlines, have not been upgraded.

“This means that inclement weather, especially on days with low visibility, could increase delays and cancellations,” Buttigieg wrote. He said airlines whose planes are awaiting retrofitting should adjust their schedules to avoid passengers being stranded.

Airlines say the FAA was slow to approve standards to upgrade radio altimeters and that supply-chain problems have made it difficult for manufacturers to produce enough devices.

Nicolas Calio, the head of Airlines for the Americas, complained about a race to modify the planes “under pressure from telecommunications companies”.

Jason Ambrose, a Delta pilot and president of the Air Line Pilots Association, accused the FCC of granting 5G licenses without consulting aviation interests, saying it “puts the world’s safest aviation system at risk.” But, he added, “eventually, we will be able to deal with the effects of 5G.”