Transportation Secretary Pete Buttigieg warned the nation’s airlines Friday that flights could be disrupted starting this week because some planes lack updated equipment to prevent interference from transmissions by wireless companies.

Buttigieg said that only planes retrofitted with the right equipment will be allowed to land when visibility is poor, such as during bad weather.

The warning — in a letter from Buttigieg to trade group Airlines for America — comes just before AT&T, Verizon and other wireless carriers will be free to boost the power of their C-Band, 5G signals on July 1.

Airlines have told the government they are having trouble getting equipment to retrofit planes because of supply-chain problems. Still, the industry trade group said airlines are confident they will avoid disruptions.

Some aviation experts believe that C-Band signals are too close on the radio spectrum to the frequencies used by radio altimeters, which measure the height of a plane above the ground. Newer altimeters are protected from interference, but some airlines have complained that a shortage of the devices has prevented them from upgrading all their planes.

It’s unclear whether the spectrum conflict could cause major travel disruptions. When the issue arose early last year, predictions of widespread problems turned out to be wrong, although a small number of flights were canceled or diverted.

Delta Air Lines said about 190 of its more than 900 planes won’t be equipped with updated radio altimeters by the deadline and could face restrictions operating in bad weather. The airline said it will route them carefully to limit disruptions while it works with a supplier to retrofit more planes through the summer.

American Airlines, Southwest Airlines and Frontier Airlines said they have retrofitted all their planes and do not expect problems. United Airlines said it expects to meet the deadline for all its “mainline” jets, although it referred questions about United Express planes to the smaller carriers that operate them.

The Federal Communications Commission, which granted the 5G licenses to the wireless companies, contends that there is no risk of interference, while the Federal Aviation Administration has taken the airlines’ side. Under pressure from the Biden administration, the wireless companies agreed to delay the full rollout of their new networks around major airports until July 1.

The Transportation Department, relying on information from airlines, says more than 80% of the U.S. fleet has been retrofitted, but Buttigieg said Friday that “some operators still have work to do.”

Buttigieg threatened to sanction airlines for deceptive trade practices if they schedule more flights than they can operate with retrofitted planes.

Airlines for America, which represents the largest U.S. carriers, said its members are working hard to equip planes with up-to-date radio altimeters, but there is a shortage because of global supply-chains problems.

“Carriers have repeatedly communicated this reality to the government,” said Marli Collier, a spokeswoman for the group. “Nevertheless, thanks to careful planning, A4A member carriers are confident in their ability to maintain the integrity of their schedules, despite the impending deadline.”