US flights resume after longest nationwide ground stop since 9/11. Here’s what we know

The US Federal Aviation Administration (FAA) lifted a nationwide ground stop on Friday (November 13) night after a communications outage halted most departures for more than four hours, stranding thousands of passengers at airports across the country.

Homeland Security Secretary Markwayne Mullin said technology upgrades had “overwhelmed” the existing, antiquated system.

The ground stop began shortly before 4 pm Eastern Time and was lifted at 8.15 pm (6.45 am IST on Saturday). It was the first nationwide halt since January 2023, when a failure in the system that sends safety alerts to pilots paused departures for about 90 minutes.

It was the longest nationwide ground stop since the September 11, 2001 attacks, when hijackers flew two passenger planes into the twin towers of New York’s World Trade Center. US airspace reopened to commercial flights about two days later.

“This isn’t another 9/11,” Mullin said. “This is a telecommunication outage.”

Here is what we know.

What caused the US air traffic control outage?

At a press briefing after the FAA lifted the ground stop, Transportation Secretary Sean Duffy said radar communications at air traffic control centres became unstable at 3.36 pm ET. He said the fix pointed to a problem with an older network.

“We have old switches, old routers. We use 1980s little copper lines,” Duffy said. Homeland Security Secretary Markwayne Mullin said technology upgrades had overloaded existing equipment.

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CBS News, citing multiple sources, reported that an update to the private communications network linking air traffic control facilities prompted a reboot, after which the system crashed.

“By doing nothing, you inject way more risk into the system,” Transportation Secretary Sean Duffy said, defending the upgrades. “I didn’t anticipate that we would have an outage like this.”

Mullin said officials had found no evidence of a cyberattack so far. “We are still looking, and we’re not going to rule it out,” he said. He also said the outage was unrelated to the recent Microsoft disruption.

How do pilots talk to controllers, and why were flights grounded?

On a typical US commercial flight, the airport tower clears the plane for take-off and hands it over to a departure controller, who guides it as it climbs. Regional air traffic control centres then guide it along its route. Near its destination, an approach controller takes over before handing it to the airport tower for landing. At each handoff, the controller tells the pilot whom to contact and which radio frequency to use, unless that frequency is already known.

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A telecommunications network overseen by L3Harris Technologies appeared to be among the systems affected, The Wall Street Journal reported. The network supports radio communications between pilots and controllers.

Holding departures limited the number of aircraft controllers had to manage while they guided planes already in the air.

Did the FAA’s new AI tool play a role?

Three days before the outage, the FAA announced it had expanded SMART, an AI-assisted flight planning tool, to all 22 US air route traffic control centres. The tool uses flight schedules, weather and airport capacity to flag potential bottlenecks and recommend routes and departure times. Controllers remain responsible for keeping aircraft safely apart.

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Mullin did not identify SMART as the cause. Air Space Intelligence, the company that makes SMART, said its tool was not involved in the ground stop.

How long could US flight delays and cancellations last?

By 8 pm ET on Friday, more than 10,000 US flights had been delayed and 5,000 cancelled, NPR reported, citing flight-tracking service FlightAware. Duffy said departures would resume slowly and urged passengers to check with their airlines.

Vikrant Vaze, a Dartmouth professor who studies airline logistics, told The New York Times that it could take days for air travel to return to normal. Airlines need to get aircraft and crews back into position and rebook stranded passengers. They can bring in reserve crews or fly empty aircraft to airports where they are needed.