The United States grounded every departing flight in the country on Friday after a communications failure hit air traffic control centres, in only the second nationwide shutdown of its kind since the September 11, 2001 attacks.

The Federal Aviation Administration announced the nationwide ground stop at 3:54 p.m. Eastern Time, ordering all airports to hold departures "due to extraordinary circumstances" and "for reasons of safety". The order stayed in force for about four hours before being lifted at around 8:15 p.m., leaving thousands of travellers stranded and airlines scrambling to rebuild their schedules.

Chaos at airports

The shutdown caused chaos at airports across the country. Airlines cancelled more than 1,000 flights and delayed nearly 10,000, according to flight-tracking service FlightAware, and by early evening the tally had risen to more than 10,000 delays and over 3,000 cancellations. At the moment the order was issued, 4,042 flights were already airborne and bound for US airports, according to Flightradar24; those aircraft were allowed to continue to their destinations.

The FAA said the stop followed a "temporary" telecommunications outage affecting all of its Air Route Traffic Control Centres, the facilities that manage high-altitude traffic across the country. Transportation Secretary Sean Duffy said technicians were investigating the failure and later blamed outdated air traffic control technology. The agency also asked European authorities not to send more flights toward the United States while the stop was in effect.

No sign of a cyberattack

Officials moved quickly to rule out foul play. Homeland Security Secretary Markwayne Mullin told reporters there was no indication the outage was caused by a cyberattack. "There is no indication as of yet that this was a nefarious attack against us. It's just a telecommunication outage as of right now," he said. The Federal Bureau of Investigation also does not believe the incident was due to a cyberattack or criminality, according to a person familiar with the matter.

The only previous nationwide ground stop of all departures came on January 11, 2023, when a computer system failure knocked out the FAA's pilot-notification system. Friday's halt underlines a long-running concern: the country's air traffic control infrastructure is ageing, and old equipment regularly disrupts flights, as a Reuters analysis noted the same day.

With the stop lifted, airlines warned it would take time for operations to return to normal, with aircraft and crews out of position after hours on the ground. For passengers, the disruption was a reminder of how quickly a single point of failure in the system that guides America's skies can bring the world's busiest aviation network to a standstill.