The unprecedented nationwide halt left hundreds of thousands of passengers stranded, with bewildered travelers being pulled off aircraft just before takeoff by airline staff who were equally in the dark about the unfolding crisis.
The incident marked only the second time such an alert had been issued in US airspace since the 9/11 attacks.
Federal Aviation Authority officials lifted the groundstop after around four hours, with President Donald Trump's administration saying there was no evidence the outage was the result of foul play.
"This is a legacy system problem," said Secretary of Transportation Sean Duffy. "We have old, old switches, old routers.
"We use 1980s little copper lines. It's an old system that went down, and went down everywhere. It got overloaded."
Duffy on Thursday had formally asked Congress for $30.1 billion in funds to upgrade and modernize the US Air Traffic Control System.
As the groundstop was lifted he repeated his plea.
"I'm going to say it a million times: the Congress has to give the resources to the president so he can finish this bill," he said.
With almost 6,000 flights canceled including some international routes according to FlightAware, the recovery from the stoppage was expected to take some time.
Footage from airports around the country showed gates full, with planes waiting on taxiways.
Passengers thronged concorses, desperate for information about an outage that had thrown their travel plans into chaos.
"We're trying to debate whether or not we need to reschedule or drive or what, but if the whole country is stopped, that means the highways are going to be jammed too," said Joseph Anderson, 72, who was trying to fly from Las Vegas to Oakland, California.
US airspace is among the busiest in the world, and is ramping up to its most frenetic as the Thanksgiving holiday moves into view.
A nationwide groundstop is highly unusual; there was one such stop in January 2023 when a pilot notification system failed, but the only other national stop happened in the wake of the 9/11 attacks, when no aircraft took to the skies for days.
Homeland Security Secretary Markwayne Mullin stressed there was no evidence of any hostile action on Friday.
"We've seen zero evidence that this was a cyber attack or (AI) attack," he told a press conference.
He also ruled out any connection with the outage that affected Microsoft this week, knocking email systems offline for millions of users, public services and businesses worldwide.
MNA
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