For more than four hours on Friday, November 13, almost nothing took off in the United States. The Federal Aviation Administration issued a rare nationwide ground stop after communication problems hit multiple air traffic control centers, leaving tens of thousands of travelers stuck in terminals and setting off delays that airlines warned could ripple through the weekend.

By Friday night the stop had been lifted and officials were pointing to a familiar culprit: old equipment. Here is what we know so far.

What happened

In a short preliminary statement, the FAA said it issued a nationwide ground stop for all U.S. airports “following communication issues impacting multiple Air Route Traffic Control Centers,” and that technicians were investigating.

According to AVweb, the restriction began around 3:54 p.m. Eastern time and was lifted at 8:15 p.m. ET, allowing departures to resume. It was the first nationwide ground stop since the FAA’s NOTAM system outage in January 2023.

Transportation Secretary Sean Duffy said more than 3,500 aircraft were in the air when the decision was made. Planes already flying were brought down safely, and exceptions were made for medical flights, critical cargo and law enforcement operations.

What caused it

At a Friday evening briefing at the White House, Duffy said switches and routers on the FAA’s analog air traffic control communications system became overloaded with data, causing inconsistencies in radar data and in communication with aircraft.

“We have old, old switches, old routers, we use 1980s little copper lines,” Duffy said, as reported by the Los Angeles Times. “It’s an old system that went down and went down everywhere.”

He identified the weak point as TDM, or time-division multiplexing, a dated and still mostly copper-wire technology used to carry air traffic control voice and radar data. The breakdown caused intermittent communication and radar problems rather than a full blackout, but Duffy said not being able to immediately see and talk to aircraft was “a risk that we’re unwilling to tolerate.”

Homeland Security Secretary Markwayne Mullin added a twist: the failure happened while the FAA was installing upgrades. “That new equipment, new technology, it was overwhelming the older system,” he said. “The patchwork doesn’t work. We need one complete system.”

Was it a cyberattack?

Officials said there is no evidence of that. Mullin said investigators had seen “zero evidence” of a cyberattack, though the investigation is ongoing.

Duffy was also asked whether the FAA’s recently introduced SMART artificial intelligence system could have played a role. He said the tool only provides recommendations to controllers and is not connected to the core radar and en route systems. “There’s no possible way SMART would have impacted what happened,” he said.

The impact on travelers

The disruption hit on a Friday afternoon, one of the busiest times of the travel week. At least 827 flights had been diverted by 7:05 p.m. ET, according to Cirium data cited by the Associated Press and reported by AVweb. Mullin said FEMA was mobilized to help make sure stranded passengers did not miss lifesaving medication, and some college football teams had to scramble for other ways to reach this weekend’s games.

Duffy said air traffic control would try to move as many planes as possible overnight, but that recovery depends on where crews and aircraft ended up. Airlines warned that delays and cancellations would continue as they reposition planes and staff.

If you are flying this weekend, check your airline’s app before leaving home, sign up for flight alerts and allow extra time at the airport.

The bigger fight over modernization

The outage quickly turned into a debate about funding. Duffy said the FAA is already upgrading its system with $12.5 billion approved by Congress, and he called on lawmakers to approve an additional $50 billion to finish the job. Mullin described the aging system as a national security risk and said the president had declared the situation a national emergency.

Critics and supporters alike have warned for years that much of the country’s air traffic control backbone is decades old. Friday’s shutdown gave travelers a very visible example of what happens when that backbone fails.

What comes next

The FAA’s investigation into the exact cause is ongoing, and the agency described its early information as preliminary. Key questions include why the new equipment overloaded the older system, whether safeguards can prevent a repeat, and how quickly the remaining upgrades can be completed.

For now, the skies are open again. But with airlines still untangling Friday’s mess, the effects of a few hours on the ground are likely to be felt in terminals for a few more days.

Image: Free airport departure board image via Rawpixel, CC0 1.0