US airspace reopened at 8:15 p.m. Eastern Time after a nationwide ground stop that halted flights across the country, but Transportation Secretary Sean Duffy warned that departures would resume gradually and that airlines would need time to restore normal operations. More than 6,600 flights were canceled and 8,600 delayed, with another 1,500 already canceled for Sunday, according to FlightRadar24, and aviation analysts said the four-hour grounding could disrupt airline schedules for days. Duffy said tens of thousands of passengers were stranded at airports around the country.

The Federal Aviation Administration said the ground stop was imposed due to extraordinary circumstances and for reasons of safety, and planes already in the air were allowed to land while those on the ground could not take off. Duffy said the trouble began at 3:36 p.m., when air traffic control centers experienced instability in radar and communications systems. The communications issue affected 22 regional air traffic control centres that direct planes en route between airports. He said routers and switches became overloaded with data, producing inconsistencies that led officials to shut the system down, though he stressed that at no stage were radar or communications completely lost and thousands of planes were safely landed.S1S2

Homeland Security Secretary Markwayne Mullin said there was "zero evidence" so far that the outage resulted from a cyberattack or a rogue AI system, though investigators had not ruled out either possibility, and the National Security Agency was working with the Department of Homeland Security on the investigation. Mullin said investigators would examine whether there was any link to the FAA's nationwide rollout of a new artificial intelligence system to help identify problems and reroute planes. The outage appeared to affect a telecommunications network overseen by L3Harris Technologies, according to two airline officials, a network that carries the radio frequencies controllers use to talk with pilots. L3Harris did not respond to a request for comment. The FAA said there was no connection to the Microsoft outage that disrupted millions of users on Friday.S1S2

The episode is the latest sign of strain on a US air traffic control system beset by aging technology, equipment and communications problems and a shortage of several thousand controllers. Congress approved US$12.5 billion for modernization last year, and miles of old copper wiring have been replaced with fibre optic lines while old radio systems are gradually being swapped out at airports. Duffy said another US$30 billion will be needed to complete the work and called for $50 billion overall to modernize the system, blaming Congress for failing to fund a new system. Before the outage, the White House and Department of Transportation had urged Congress to approve $30.1 billion to upgrade airports and air traffic control infrastructure. It was the first nationwide ground stop since January 2023, when a key FAA computer system malfunctioned, and the first since the September 11 attacks before that. Aviation safety expert Steve Arroyo said the problem resembled a regional communications outage in September that snarled hundreds of flights across the northeast, but on a nationwide scale.S1S2

Duffy said the immediate problem had been fixed and that flying was safe, but he warned that a broader overhaul of the aging system was needed to prevent similar failures. Though the ground stop was lifted, the disruption for passengers was far from over, with delays and cancellations expected to ripple through schedules for days as airlines work to restore normal operations.S1S2