The US Federal Aviation Administration (FAA) canceled a ground stop on all national flights hours after stopping departures at US airports on Friday.

"We have old switches, old routers, we use 1980s old copper lines. We use an old system," US Secretary of Transportation Sean Duffy said. "It was a legacy network problem."

Even with the order being lifted, airlines will not be able to immediately return to normal operations as many of their planes and crews are out of position.

"It's going to be slow," Duffy told reporters at the White House.

It was the first nationwide ground stop since January 2023, when the FAA dealt with a system outage. Prior to that, the last ground stop was after the September 11, 2001 terror attacks.  

US Homeland Security Secretary Markwayne Mullin said there was no indication the communications breakdown resulted from a cyberattack or artificial intelligence and that the ⁠public ​was ​never ​in danger.

Over 4,500 flights were canceled Friday and about 700 for Saturday and over 10,000 flights within, into, or out of the United States were delayed, according to the FlightAware aviation tracker. 

What do we know so far? 

"All departing traffic should remain groundstopped until further notice," the FAA had said earlier Friday. 

The agency said the ground stop was "due to extraordinary circumstances and reasons for safety." The FAA cited a temporary telecommunication outage and said, "Every flight has been accounted for and international flights will be handled on a case by case basis." 

The agency said flights were halted "out of abundance of caution."  

In a post on X, Duffy said the stop was due to a communication issue impacting all Air Route Traffic Control Centers. 

"FAA technicians are investigating the issue. We will provide immediate updates as we have them," he wrote.

The FAA ground stop order came a day after Duffy asked Congress for $30 billion to upgrade and modernize the US Air Traffic Control System.

"This investment is critical to complete the modernization of our national airspace — delivering a resilient telecommunications backbone for air traffic controllers, modern hardware and software, state-of-the-art control towers, and world-class airport infrastructure," Duffy and Office of Management and Budget Director Russell Vought wrote in a letter to lawmakers.

Canada implementing 'traffic management' measures 

The European Organisation for the Safety of Air Navigation, Eurocontrol, said all flights departing for US airports from its 42 member states were suspended following a request from the FAA to deal with technical issues. 

Canada's air navigation service provider, NAV Canada, said it is closely monitoring the US ground stop and later said, "It was implementing traffic management measures at Toronto Pearson, Montreal-Trudeau, Calgary and Vancouver to safely manage traffic volumes." 

"Travelers should expect delays and check with their airline. Our systems are operating normally," NAV Canada added.