FAA warns 5G-related landing restrictions could divert flights as snow hits airports

After a fairly smooth first day, the impact of new 5G cellular service on air travel is being put to the test by snow and other winter weather.

The Federal Aviation Administration has warned that it would limit landings in certain low-visibility conditions due to concerns that 5G signals could interfere with crucial aircraft equipment. As winter storms popped up Thursday, the FAA said it may have to divert some flights.

At the center of the issue is aircraft radio altimeters, which tell pilots how far the plane is from the ground. The altimeters use frequencies that sit next to those used for the new 5G service, raising concerns about aircraft receiving inaccurate data.

The new service began on Wednesday after two delays since December. Verizon and AT&T, at the last minute, agreed to temporarily delay the rollout near airports temporarily after airlines warned federal officials that the signals could cause widespread disruptions and “economic calamity.”

“Due to the nationwide expansion of 5G C-band and the potential for radio altimeter interference, [air traffic control] has identified airports and/or geographic regions that may be impacted by meteorological conditions leading to a diverted flight,” the FAA said.

Airports in Boston, Philadelphia, Baltimore and San Francisco were most affected, it said, noting that it could lead to traffic backups at airports in Detroit, Reno, Nevada, Chicago and Los Angeles.

“We simply don’t compromise on safety and when FAA is telling us it is not safe to land, one we don’t have any discretion in that, but two even if we did we wouldn’t do it,” United Airlines CEO Scott Kirby told CNBC’s “Squawk Box” on Thursday.

The FAA by late Wednesday had approved 62% of the U.S. fleet to land in low visibility, up from 45% over the weekend. The agency is planning to approve more as early as Thursday. Approved altimeters are on Boeing 717s, 737s, 747s, 757s, 767s and 777s as well as Airbus A310s, A320s, A321s, A350s and A380s.

Smaller regional aircraft are still waiting for clearance.

“We have been fortunate to enjoy favorable weather across most destinations on the first day of 5G implementation but have not received updates from the FAA on mitigations for our fleets,” regional carrier SkyWest Airlines said in a statement Wednesday. The carrier flies for American, United and Delta. “If weather deteriorates at any affected location, there remains potential for significant operational impact until full mitigations can be put into place for all commercial aircraft. As always, we will not compromise safety,” it said.

The first day of service brought few cancellations as airlines also had relatively clear weather. Some international airlines including Japan Airlines and Emirates Airline had canceled some U.S.-bound flights but reversed that decision after the FAA cleared 777 wide-body aircraft to land in low visibility. Those jets are normally used for long-haul international routes.