More than 1200 flights were cancelled across the US on Wednesday and 8000 delayed after a monumental computer glitch.
For two hours during the morning peak not one domestic flight was allowed to take off from any airport across the country.
Flights have now resumed but the mass grounding has heaped enormous pressure on the US airline network that transports three million passengers a day.
The failure of a national safety system between 7am and 9am New York time (11pm Wednesday to 1am Thursday AEDT) saw hundreds of thousands of passengers stranded including at airports overseas on US bound flights.
The Federal Aviation Administration (FAA) said early on Wednesday, US time, that the NOTAMS (Notice to Air Missions) system had gone down. That followed a similar issue late the night before.
NOTAMS gives pilots vital safety information before taking off and during the flight.
At around 7am New York (11pm AEDT) time the FAA announced it was looking to “fully restore” NOTAMS and aircraft movements were limited. However, by around 7.30am it said it had “ordered” all domestic flights to “pause” while it sorted out the issue.
Other crucial safety systems such as radar and air traffic control remained online.
The White House said there was “no evidence” of a cyberattack but an investigation was underway.
International and domestic flights were also affected, with passengers in Paris, London and Madrid complaining of their journeys to the US being cancelled. Travellers on a US bound plane in London were in the aircraft for three hours as it sat on the tarmac.
Images from flight tracking websites showed substantially fewer aircraft in the skies around major US cities. Across New York, one of the nation’s busiest air spaces with three major airports, there were barely any flights in the sky before 9am.
The websites also showed many flights being diverted. For instance several United planes headed to New York were being diverted to Washington DC instead
“The FAA is working to restore its Notice to Air Missions System,” the organisation said at the height of the US peak hour.
“We are performing final validation checks and repopulating the system now. Operations across the National Airspace System are affected.”
It then updated that statement by grounding flights.
“The FAA has ordered airlines to pause all domestic departures until 9am Eastern Time (1am Thursday AEDT) to allow the agency to validate the integrity of flight and safety information.”
Several passengers complained of being left stranded as chaos broke out at airports around the world.
What are NOTAMS?
NOTAMS are alerts given to pilots about potential hazards during the course of their flights.
They could include information about flight restrictions en route, military exercises, obstacles near air fields such as cranes, any faults at airports such as closed runways or taxiways, hazardous conditions and even large flocks of birds which could cause a danger to aeroplanes.
US President Joe Biden said he wanted a report on the outage.
“They don’t know what the cause of it is. They expect in a couple of hours they’ll have a good sense of what caused it and will respond at that time,” he added.
Asked if a cyberattack was to blame, Mr Biden said it was part of the investigation.
US transportation secretary Pete Buttigieg said he had been in touch with the FAA on the safety issue.
“The FAA is working to resolve this issue swiftly and safely so that air traffic can resume normal operations, and will continue to provide updates.”
Even before the FAA grounded flights many major airlines had decided to voluntarily ground planes due to the lack of NOTAMS.
United Airlines, one of the US’ biggest, said all its aircraft would remain on the ground until at least 10am New York time.
By 7am 3700 flights had been delayed in the U but as many as 20,000 take off and land in the morning peak. The FAA handles around 45,000 flights a day in the US with 5400 in the sky at any one time. Every day almost three million people are on a flight in the US.