The longest halt in flight departures to U.S. airports since 9/11 came after updates to a telecommunications system made by L3Harris Technologies, Transportation Secretary Sean Duffy said Saturday, as travel disruptions lingered for thousands of passengers nationwide.
There was "zero evidence" that the problem was caused by something "nefarious" or anything related to artificial intelligence, Bryan Bedford, the head of the Federal Aviation Administration, said at a press conference.
The updates were part of ongoing work of switching from a copper wire system to fiberoptic cable, Duffy said. When a router was switched to fiberoptic cable from copper, it had "flooded" other routers in the air traffic control system with over 1,000 connection attempts roughly every five seconds, Duffy said, creating problems with airplane communications and radar feeds.
Bedford said no FAA facilities went dark but that officials decided to ground flights because of "instability" in information feeds to air traffic control centers that handle en route aircraft at high altitudes and eventually to others closer to airports.
"We were unwilling to take any risk that this propagation could actually result in a center losing connectivity with aircraft," said Bedford.
L3Harris didn't immediately respond to requests for comment.
Duffy told the press conference that the FAA is investigating.
"What we're working to determine was whether we had incompetence, meaning was it user error, installator error from L3 Harris, or did they install a faulty router," he said.
Flight disruptions continued on Saturday. More than 3,700 U.S. flights were canceled Saturday and close to 2,000 were delayed as of 9:40 a.m. ET, according to flight-tracking site FlightAware. That's on top of about 9,000 U.S. flight cancellations and more than 7,000 delays on Friday, though some disruptions were due to bad weather.
Part of untangling from the ground stop includes getting aircraft where they need to be. The ground stop forced many planes to divert to other airports.
Airlines run one of the lightest schedules on Saturday and are deploying as many spare planes and crews as possible to get travelers and employees where they need to be as they're eager to start fresh for the upcoming Thanksgiving holiday period.
American Airlines, for example, is offering flight attendants extra pay to pick up trips, according to a person familiar with the airline's plans.
Delta Air Lines was offering crews double pay to pick up trips and gave them confirmed seats on flights to get them into position, according to internal company messages seen by CNBC. United Airlines was also offering pilots and flight attendants extra pay to pick up flights through Sunday, according to people familiar with the matter.
American and other airlines said they would change customers' tickets at no fee or fare difference if the outage affected their plans.
Around 3 million people were scheduled to fly on Saturday, according to aviation data firm Cirium.
The FAA had halted departing U.S. flights around 4 p.m. ET on Friday, a ground stop that lasted more than four hours, the longest such pause on departing flights in 25 years.
Duffy on Friday blamed outdated U.S. infrastructure for the problem and said Congress needs to approve more money to overhaul the country's air traffic control. Aging equipment and a shortage of air traffic controllers have angered airline executives for years, who have complained about the cost of resulting disruptions.
It was the second time the FAA has issued a nationwide ground stop since the Sept. 11, 2001, terror attacks. Trump administration officials have repeatedly said it did not appear to be a cyberattack.
The U.S. has the world's busiest airspace.
"Before clearing flights to resume, we thoroughly tested our systems to confirm they were stable and that operations remain reliable," the FAA said late Friday.
The disruption also upended travel for NBA, NHL and college teams, which struggled to reach their destinations in time for games this weekend.
Duffy said college football teams traveling on charter flights could receive priority.
"I would encourage those football teams to reach out to the FAA to schedule their flight times and make sure they have games going off tomorrow," he said during a Friday press briefing.
The Southeastern Conference announced on X that some games had been moved to Sunday due to the flight disruption.
"We are in close communication with our member institutions to assess potential disruptions and coordinate appropriate responses," the SEC said in a statement shared on X.
The National Football League said that it is putting in three backup referees for three games on Sunday because of the ground stop, "reassigning officials to games within driving distance of their hometowns."
