Flights in the United States were resuming on Saturday after being grounded for several hours due to a major communications outage, though authorities said returning to normal operations would take time.
The nationwide stop on Friday sparked widespread chaos and left hundreds of thousands of passengers in limbo, with some pulled off planes moments before departure by airline staff who appeared in the dark about unfolding events.
The incident marked only the second time such an alert had been issued in US airspace -- which is among the busiest in the world -- since the 9/11 attacks.
The ground stop lasted around four hours and was lifted at 8:15 pm Eastern Time (0115 GMT Saturday).
Federal Aviation Administration officials reopened the airspace on Friday night "after restoring full functionality" at air traffic control facilities, according to Secretary of Transportation Sean Duffy.
"It will take us time (to) resume normal operations... Our top priority is the safety of you and your family when you fly," Duffy said.
Administration officials blamed the incident on telecommunications issues with decades-old equipment, adding there was no evidence of foul play.
"This is a legacy system problem," Duffy said. "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 had formally asked Congress on Thursday for $30.1 billion in funds to upgrade and modernize the air traffic control System.
As the ground stop 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.
There were almost 6,000 flights canceled, including some international routes, according to FlightAware, a tracking platform.
Footage from airports around the country showed gates full, with planes waiting on taxiways.
Passengers thronged concourses, desperate for information about an outage that had thrown their 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.
A nationwide ground stop 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 in September 2001, 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 cyberattack or (AI) attack," he told a news 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.
-AFP