CEOs from major US airlines including American Airlines, Delta Air Lines, United Airlines, and Southwest Airlines warned that the impending deployment of new 5G spectrum on Wednesday, January 19th could cause “catastrophic disruption” for flights across the country. The warning, which was also signed by shipping companies including UPS Airlines and FedEx Express, was issued in a letter from the CEOs seen by Reuters, which said that both commercial and cargo flights could be affected.

The letter was sent to Transportation Secretary Pete Buttigieg, Federal Communications Commission (FCC) Chairwoman Jessica Rosenworcel, FAA Administrator Steve Dickson, and White House National Economic Council director Brian Deese, Reuters reports.

On Tuesday, ahead of the 5G spectrum push, some of that disruption became a reality — Emirates, Air India, All Nippon Airways, Japan Airlines, China Airlines, Korean Air Lines, Singapore Airlines, Cathay Pacific, Lufthansa, and Austrian Airlines all decided to cancel some flights or switch out some planes, after Boeing reportedly warned that its 777 and 747-8 aircraft would not be able to land at some airports under low visibility conditions. However, early Wednesday, Japan Airlines clamed that the FAA told it that “there is no longer a problem with the operation of the Boeing 777” and said it would resume flights to the US on January 20th. It’s not clear what changed.

The 5G spectrum in question is known as C-band, and AT&T and Verizon’s deployments went live on Wednesday, January 19th after suffering multiple previous delays. At the core of the issue are fears that these new C-band airwaves could interfere with sensitive radar altimeters on certain aircraft, my colleague Russell Brandom explained earlier this month. Altimeters are critical for landings especially when visibility is limited, and failures in this system were linked to a fatal 2009 Turkish Airlines crash.

“Airplane manufacturers have informed us that there are huge swaths of the operating fleet that may need to be indefinitely grounded” as a result of the new 5G deployments, the airlines wrote in their letter. “Immediate intervention is needed to avoid significant operational disruption to air passengers, shippers, supply chain and delivery of needed medical supplies,” that could cause “chaos” and strand “tens of thousands of Americans overseas.” 

“When deployed next to runways, the 5G signals could interfere with the key safety equipment that pilots rely on to take off and land in inclement weather,” United Airlines said in its own statement sent to The Verge. Without more safeguards, the rollout of this 5G spectrum could “result in significant restrictions on 787s, 777s, 737s and regional aircraft in major cities” and cause “hundreds of thousands of flight cancellations and disruptions for customers across the industry in 2022, [and] also the suspension of cargo flights into these locations.”

The FAA issued a rule in early December that keeps flights from landing in poor visibility conditions, if they’re landing at an airport where there’s deemed to be enough interference that their altimeters aren’t reliable. At the time, it wasn’t clear which airports and planes would be affected by the rule, particularly as Verizon and AT&T have been continually negotiating with the airline industry over these potential disruptions.

Although there’s technically 220MHz of clearance between the spectrum used by the plane’s equipment and the new 5G spectrum, airlines are worried that the older equipment found in some planes might still be affected. Air safety concerns have already delayed the rollout of C-band from last December to this week. 

United is calling for 5G to be implemented “everywhere in the country on Jan. 19, except within approximately 2 miles of airport runways at affected airports.” It indicates the size of this buffer could be reduced after further analysis by the Federal Aviation Authority (FAA). On January 3rd, Verizon and AT&T previously agreed to take steps to mitigate the potential effect on aviation, including placing buffer zones around 50 airports for six months. They agreed to halt 5G rollouts at even more towers this week, after the airline industry’s warnings.

C-band spectrum has already been rolled out in nearly 40 countries without causing altimeter failures according to one tech trade group, and United says the right government policies could allow for a safe deployment in the US.

AT&T and Verizon have a lot riding on this new spectrum, which they paid $65 billion for in an auction last year. Both need the extra spectrum to handle the traffic demands of 5G, especially given AT&T’s plans to shut down its 3G network next month. 

Update January 19th, 1:39PM ET: Added that many airlines have canceled or changed out planes on at least some flights.