US air travel is not expected to return to normal schedules until late on Sunday, the head of the Federal Aviation Administration (FAA) said, as the aviation system gradually recovers from a nationwide flight shutdown that officials blamed on routine maintenance gone awry.

As of 9:35 pm Saturday ET (7:35 am Sunday PKT), airlines had cancelled more than 6,400 US flights and delayed more than 8,000, according to FlightAware, an aviation website. That followed about 9,200 flights cancelled and 7,500 delayed on Friday after the FAA ordered a nationwide ground stop because of disruptions to air traffic communications. FlightAware said 875 flights scheduled for Sunday were already cancelled.

FAA head Bryan Bedford said on Saturday that he expected airline operations to largely return to normal by late Sunday. He also apologised for the disruption, adding that the system remains stable.

The FAA issued a nationwide ground stop on Friday that halted nearly all departing flights for almost five hours. It was the longest nationwide halt to air travel since the attacks of Sept 11, 2001, and one of the single worst US travel days ever.

The issue prompted the White House to convene an emergency meeting in the Situation Room on Friday and federal agencies to scramble to determine if it was a cyberattack. But the problem was traced to FAA contractor L3Harris’ installation of at least one new router at a remote facility, Transportation Secretary Sean Duffy said on Saturday.

“We’re working to determine whether we had incompetence, meaning was it user error or installer error from L3Harris, or did they install a faulty router? We haven’t determined what the fault was,” Duffy said at a press conference.

The Professional Aviation Safety Specialists, a union representing FAA safety employees, said the issue was isolated to two routers that were updated on Friday. An official told Reuters it was believed the routers were in Arkansas and Texas.

An FAA official said the agency conducted a safety assessment before the work. The union said PASS-represented technicians were not involved in the upgrades that led to the ground stop.

L3Harris said it took the issue seriously and is supporting the FAA in its investigation. Its shares last traded down about four per cent after hours on Friday.

The Trump administration previously blamed the mess on a botched replacement of old copper wire communication lines.

Though the FAA lifted the ground stop — meaning no planes can take off — on Friday evening, the disruption for passengers was far from over, with tens of thousands of people stranded in airports. Airlines were trying to get flight crews and other personnel in position to resume flights.

It sparked widespread chaos and left hundreds of thousands of passengers in limbo, with some pulled off planes moments before they were due to depart by airline staff who appeared in the dark about unfolding events.

“Tired. Exhausted. Frustrated. Yeah, you know, it’s just a mess, right?” Lisa Beraid, 48, who works for an insurance company, told AFP as she waited for a flight on Saturday morning at New York’s La Guardia airport.

Speaking at a news conference in Washington, Bedford offered a “sincere apology to the American travelling public.” He predicted a full resumption of flight schedules by “close of business on Sunday.”

Transportation Secretary Sean Duffy said on Friday’s four-hour ground stop was declared after maintenance work to upgrade an old copper wire to a high-speed fiber cable had caused “issues” with communication with airplanes and radar feeds.

“Either through contractor error or a faulty piece of equipment… it started flooding other routers in our air traffic control system,” Duffy told reporters.

In another embarrassment, Saturday’s news conference — where administration officials also appealed for more funds to upgrade outdated systems — was delayed due to technical problems with the audio feed.

Overloaded communications centres

Duffy said the trouble began on Friday afternoon, when air traffic control centres experienced instability in radar and communications systems. The router issue led to data overloading other communications centres, officials said.

“We had over 1,000 connection attempts every four to five seconds, and what that did was [it] gave us issues with our communication with airplanes, and it also caused some disruption with our radar,” Duffy added.

The issue affected communications at 17 of the FAA’s 22 en route centres, which control aircraft during the cruise portion of a flight. Bedford said the FAA ordered the ground stop in case a centre lost all communications. “No facilities went dark. Nothing was fully lost,” Bedford added.

The FAA said it was doing additional testing with L3Harris to see if there are any additional risks.

Delta Air Lines, United Airlines and American Airlines all canceled about 25pc of their flights on Saturday, while JetBlue canceled nearly 45pc. Airlines have issued travel waivers allowing passengers to rebook flights for later dates without cost.

Funds sought to fix system strain

Friday’s disruption is the latest and most dramatic sign of strain on a US air traffic control system beset by ageing technology, equipment and communications problems and a shortage of several thousand controllers.

Duffy called on Friday for $50 billion in funding to upgrade air traffic control systems, up from his earlier request of $30bn after Congress approved $12.5bn last year.

House Speaker Mike Johnson told Fox News on Saturday that he supports approving more funding for air traffic this year. Johnson’s Republicans control Congress until next year, when Democrats will take over because of their gains in the Nov 3 midterm elections.

US Representatives Rick Larsen and Andre Carson, Democrats slated next year to chair the House Transportation and Infrastructure Committee and aviation subcommittee, respectively, said in a joint statement Saturday that their panels will “hold the administration accountable for what happened to make sure this chaos does not happen again.”

Since receiving $12.5bn last year, the Transportation Department has awarded contracts worth $4.3bn but only spent $1bn, Larsen and Carson said.

Responding to Duffy’s appeal for funds, Illinois state governor and prominent Democrat, JB Pritzker, retorted, “Cut the crap”.

He wrote on X, “Trump and Republicans have controlled Congress and could have funded the system.”

New York Governor Kathy Hochul called on Duffy to resign or be fired by Trump, writing on X: “We’re fortunate no lives were lost.”

US lawmakers called for investigations into what caused the massive disruption and whether the country is adequately secure from cyber criminality.

“If today proves anything, it’s that we need to update our air traffic control system and rebuild our airport infrastructure,” Republican Senator Eric Schmitt said on X.

President Donald Trump was uncharacteristically quiet about the outage that had knock-on effects for international flights to and from the United States, making no public statement about the ground stop as of Saturday night.