All flights across the US were grounded due to a glitch with the Federal Aviation Administration’s computer system this morning.
The FAA said it was working to restore its Notice to Air Missions (NOTAM) system, which alerts pilots of potential hazards along a flight route.
It said it had ordered airlines to pause all domestic departures until 9am Eastern Time (2pm GMT) “to allow the agency to validate the integrity of flight safety information”.
Some 1,230 flights within, into or out of the US have been delayed today, according to flight tracker FlightAware.com, while 108 have been cancelled.
Most delays were concentrated along the East Coast.
Sky News correspondent Mark Stone, who was at Ronald Reagan Airport in Virginia, said: “Well an almighty mess for the aviation industry in the United States.
“All flights as of this morning were grounded, although from the noise I just heard over my head and the sights I just saw they appear now to have started taking off again from here at DCA Airport Washington Reagan.
“As of this morning we were told that the NOTAM system had failed, which is part of the air traffic control system. This is critical for flights to be able to take off safely. So as a consequence the busiest airspace in the world, the airspace over the United States, didn’t open as it should have done this morning.
“Looking at the flight tracker websites it was very clear you could see flights clustered around many cities around the United States and none of them taking off. Chaos for passengers, as you might imagine.”
The FAA said: “The FAA is working to restore its Notice to Air Missions System.
“We are performing final validation checks and reloading the system now. Operations across the National Airspace System are affected.
“We will provide frequent updates as we make progress.”
In a later update, the agency said it was “still working to fully restore” the NOTAM system.
“While some functions are beginning to come back on line, National Airspace System operations remain limited.”
United Airlines said it had temporarily delayed all domestic flights.
Several people tweeted to say they had been stranded due to the outage, with one passenger at Paris’ Charles de Gaulle airport saying no flights were flying to the US.
A total of 21,464 flights were scheduled to depart airports in the US today, according to aviation analytics firm Cirium.
Nearly 2.9 million seats are available on those departures.
