Nearly 10,000 flights were canceled across the United States between Friday and Saturday after the Federal Aviation Administration ordered a nationwide ground stop lasting nearly five hours, the most severe air traffic disruption in years.

The FAA halted all departures at 3:36 p.m. Friday after instability hit radar and communications systems at air traffic control centers and the facilities that manage aircraft approaching and departing airports. The stop remained in force until 8:15 p.m., according to the agency. By Saturday morning, more than 4,000 additional flights had been canceled, flight tracking data from Flightradar24 and FlightAware showed.

Delta and United absorbed the heaviest losses, with each carrier canceling more than 2,000 flights — roughly 40% of the total cancellations between them.

What did Transportation Secretary Duffy say?

Transportation Secretary Sean Duffy warned Friday evening that the recovery would be gradual. In an update posted on X, he said delays and cancellations would continue but did not specify when operations would return to normal.

"Operations are slowly returning to normal this morning," Duffy said. "Planes are pushing back, taxiing, and taking off, and it is safe to fly. We are in close coordination with the airlines to support the ongoing recovery. You will continue to see some delays and cancellations as carriers reschedule flights and reposition aircraft/crews where they are needed most."

He added: "Our top priority right now is making sure travelers get home safely to their families as quickly as possible."

Duffy attributed the outage to aging hardware. He said Friday evening that old switches and routers became overloaded with data, producing inconsistencies in radar communications systems across the country. Newly installed equipment continued to function, he said, while older technologies failed.

The comment places the FAA's long-running modernization program under fresh scrutiny, given that legacy infrastructure buckled while recent upgrades held.

Which airports were hit hardest?

Detroit Metropolitan Wayne County Airport suffered the worst departure disruptions as of Saturday morning, with 38% of flights canceled and 55% delayed, according to tracking data.

Washington Ronald Reagan National Airport recorded the deepest arrival disruptions, with 71% of flights canceled and 21% delayed. When the ground stop first took effect, Reagan National ran short of tarmac space to park all the grounded aircraft.

The strain rippled beyond aviation. Travelers seeking alternatives overloaded Amtrak's booking website, leaving many unable to buy train tickets. The technical issue was resolved by Saturday morning, but Amtrak continued to face elevated demand as displaced flyers looked for rail options.

How did the government respond?

Homeland Security Secretary Markwayne Mullin told a Friday evening press conference that officials were working at speed across agencies to clear the backlog, while conceding the downstream effects of a nationwide stop.

"We're moving at an unprecedented pace as a whole of government, to make sure that we are doing the public justice by making sure everybody is safe," Mullin said. He appeared alongside Duffy at a news conference in the James Brady Briefing Room at the White House on Friday.

For airline executives, the episode crystallizes the operational risk of a single point of federal infrastructure failure. One overloaded router cluster grounded a national fleet of thousands of aircraft, forced carriers into mass rescheduling, and pushed passengers onto already-strained ground transportation — all inside 24 hours.

Airlines were still repositioning aircraft and crews as of Saturday, and no date for full normalization had been given.