The chief executives of major U.S. passenger and cargo carriers on Monday warned of an impending “catastrophic” aviation crisis on Wednesday when AT&T and Verizon are set to deploy new 5G service.

C-Band 5G could render large numbers of widebody planes unusable, and it “could potentially leave tens to thousands of Americans abroad without a C-Band 5G” service.

Chief executives at American Airlines, Delta Air Lines and Southwest Airlines stated that “unless our major hubs can fly, the vast majority the traveling and shipping publics will effectively be grounded.”

FAA warns that interference can affect the sensitive instruments of airplanes such as altimeters, and could impact low-visibility operations.

It warned that “On a day such as yesterday, over 1,100 flights would be cancelled, diverted or delayed”

They added that urgent action was required in the letter signed also by UPS Airlines and FedEx Express. Let’s just say that commerce in the country will come to an abrupt halt.

The letter, seen by Reuters was sent to Brian Deese from the White House National Economic Council and Pete Buttigieg (Transportation Secretary), Steve Dickson (Administrator of Federal Aviation Administration) and Jessica Rosenworcel (Federal Communications Commission) Chairperson.

Airlines for America (the group behind the letter) declined to comment. The agencies of the government did not comment immediately.

AT&T and Verizon, which won nearly all of the C-Band spectrum in an $80 billion auction last year, on Jan. 3 agreed to buffer zones around 50 airports to reduce interference risks and take other steps to cut potential interference for six months. In an agreement, they agreed to temporarily defer deployment from Wednesday through Thursday in order to prevent an aviation safety standoff.

According to officials, the CEOs of large airlines and Boeing Chief Executive Dave Calhoun called Buttigieg on Sunday in order to warn about the looming crisis.

Some key airports ask that 5G is implemented in all parts of the country, except for the 2 mile (3.2km) distance between airport runways.

To avoid major operational disruptions to passengers and shippers as well as supply chains, delivery, and air travel, it is imperative to intervene immediately.

They also stated that restrictions on flight will not apply to weather-related problems. Multiple modern safety systems will be considered unusable, causing an even bigger problem than we thought. According to aircraft makers, large areas of the fleet could need to remain grounded for indefinitely.

Carriers added that they demand action from the FAA to make sure “5G” is not deployed when towers are in too close proximity to runways.

According to the FAA, 45% of U.S. commercial aircraft fleet has been cleared by Sunday for low-visibility landings at airports in which 5G C band will begin on Wednesday. However, large airports were not included in the list of airlines that was released Monday.