Since the 1990s, aviation organizations have issued warnings that 5G transmissions may interfere with aircraft equipment.

On Saturday, wireless carriers are anticipated to turn on new 5G systems close to major airports, posing a fresh source of interruptions for airline customers who have already experienced tens of thousands of weather-related flight delays this week.

Since years, aviation organizations have warned that 5G signals may interfere with aircraft equipment, particularly with devices that use radio waves to calculate distance above the ground and are essential for landing in low visibility.

When telecom operators started implementing the new service last year, predictions that interference would result in widespread flight cancellations failed to materialize. After that, they decided to restrict the signal strength near congested airports, giving airlines an extra year to update their aircraft.

The head of the biggest pilots union in the country predicted that crews will be able to handle the effects of 5G, but he criticized the process for awarding wireless licenses, saying it had increased unneeded risk in the aviation industry.

Airlines may see delays due to a small fraction of the country’s fleet not being modernized to prevent radio interference, according to Transportation Secretary Pete Buttigieg.

Most of the big American airlines claim to be prepared. According to American, Southwest, Alaska, Frontier, and United, all of their aircraft feature radio altimeters and height-measuring equipment that is immune to 5G interference.

In a big way, Delta Air Lines is an exception. According to Delta, 190 of its aircraft, including the majority of its smaller models, still do not have new altimeters since the supplier has not been able to provide them quickly enough.

According to Delta, the airline does not anticipate canceling any flights as a result of the problem. The airline intends to carefully schedule the 190 aircraft to reduce the likelihood that flights will be cancelled or forced to divert from locations where visibility is poor due to fog or low clouds.

All of Delta’s A220 aircraft, the majority of its A319 and A320 aircraft, and some of its A321 aircraft are among the Airbus aircraft that have not undergone retrofitting. All Delta Connection aircraft, including those flown by Endeavour Air, Republic Airways, and SkyWest Airlines, as well as the company’s Boeing aircraft, have improved altimeters, according to the airline.

JetBlue declined to comment, but it did tell The Wall Street Journal that it planned to adapt 17 smaller Airbus planes by October, with a potential “limited impact” on select days in Boston.

For their new 5G service, wireless companies like Verizon and AT&T use the C-Band, a region of the radio spectrum that is near the frequencies used by radio altimeters.

They received licenses for the C-Band spectrum from the Federal Communications Commission, which disregarded any chance of interference because there was enough room between the C-Band and altimeter frequencies.

The telecom providers delayed the introduction of their new service after the Federal Aviation Administration raised concerns and took the side of the airlines. The telecom providers then consented to refrain from activating 5G signals close to approximately 50 of the busiest airports as part of a settlement mediated by the Biden administration. This adjournment expires on Saturday.

AT&T opted not to respond. When asked about its plans, Verizon did not react right away.

In a letter sent last week, Buttigieg reminded the leader of the industry association Airlines for America of the impending deadline and cautioned that only aircraft with modified altimeters would be permitted to land in low-visibility situations.

He claimed that more than 80% of the American fleet had undergone retrofits, but a sizable portion of aircraft, including many flown by foreign airlines, had not.

This suggests that there may be more delays and cancellations, especially on days with poor visibility and heavy weather, Buttigieg noted.

He advised airlines to modify their itineraries in order to prevent leaving people stranded while their planes are being retrofitted.

Airlines claim that the FAA took too long to establish rules for modernizing radio altimeters and that supply-chain issues have made it challenging for manufacturers to create enough of the devices.

The leader of Airlines for America, Nicholas Calio, lamented the hurry to change aircraft “under pressure from the telecommunications companies.”

The FCC was charged with giving 5G licenses without consulting aviation groups, according to Jason Ambrosi, a Delta pilot, and head of the Air Line Pilots Association.

He claimed this “has left the safest aviation system in the world at increased risk.” On the other hand, he added, “Ultimately, we will be able to address the impacts of 5G.”