US flights have resumed after thousands were delayed or cancelled due to a system failure which caused an incredibly rare nationwide “ground stop” alert on Friday.

Tens of thousands of passengers were stranded when the Federal Aviation Administration (FAA) grounded flights because of “extraordinary circumstances and safety reasons” after the agency reported serious air traffic control communications issues.

Flights are slowly resuming, but disruption from the over four-hour ground stop, only the second since 9/11, is set to disrupt airline schedules for days.

Sean Duffy, the US Transportation Secretary, said upgrades to aviation radio communication systems led to routers being “overloaded with data”.

Markwayne Mullin, the US Homeland Security Secretary, said the grounding was not believed to be caused by a cyber attack, but added: “There’s nothing certain right now”.

Although the ground stop alert has lifted, the UK Civil Aviation Authority said delays could remains for travellers.

In a statement on Saturday, Duffy said: “You will continue to see some delays and cancellations as carriers reschedule flights and reposition aircraft/crews where they are needed most.”

Here’s what we know so far.

Communication glitch hits air traffic control

The FAA initially halted traffic in Chicago ​due ⁠to air traffic ‌control communications issues on Friday. The agency said: “All departing traffic should remain ground stopped until further ‌notice.”

International ‌flights were set to be managed on a case-by-case basis.

Duffy confirmed that the stop was due to a communication issue affecting all of the country’s air traffic control centres.

US transportation secretary Sean Duffy confirmed on Saturday: “Our switches, our routers, actually got overloaded with data and they started to give us inconsistencies in the data from those routers and because of that we shut the system down.”

He added that a contractor called L3Harris was “doing routine maintenance” to switch telecommunication lines from old copper wire to fibre in an FAA location.

“It was such an old wire that it was being discontinued,” he says.

He added that at ​no stage did ⁠they completely lose radar or communications and they were able ‌to safely land thousands of planes.

US President Donald Trump blamed the grounding of flights on “some old broken stuff”.

Bryan Bedford, who heads the FAA, said: “We see zero evidence of anything nefarious, and zero evidence of anything AI related.”

He added there will be a “very transparent hearing on what actually happened”.

Both Bedford and Duffy asked Congress to confirm additional funding to upgrade the system as soon as possible.

Bedford added: “Let’s not kid ourselves, it will continue to break until we replace it.”

Thousands of flights delayed or cancelled

The flight tracking website Flightradar24 had 3,100 flights en route to US airports when the disruption began.

More than 10,000 flights were cancelled and 8,600 delayed, with another 1,500 already cancelled for Sunday, according to the website.

Duffy said the immediate problem had been fixed and that flying was safe, but warned that a broader overhaul of the ageing air traffic control system was needed to prevent similar failures.

Last year, Congress ​approved $12.5bn to fix the ‌FAA’s ageing communications system, ⁠and on Friday the FAA ⁠and White House asked Congress for another $30.1bn to ‌address ​other issues.

Addressing media after the issue was resolved, Duffy, the Transportation Secretary, said the “emergency” amount needed to overhaul ageing traffic control systems was closer to $50bn.

“What went down was old. It was old legacy equipment that failed,” he added.

Speaking at the same press conference, Mullin warned another, similar failure was possible unless Congress was willing to back a “complete overhaul” of the transport system.

The world’s busiest airspace has ‌experienced setbacks in recent years that have disrupted flights and ‌risked safety. In January 2025, a midair collision at Ronald Reagan Washington National Airport killed 67 people. 

Mullin said there was “zero evidence” so far that the outage was caused by a cyberattack or a rogue AI system, but investigators had not ruled it out.

“It appears this was mechanically ⁠driven, however, we’re still looking,” Mullin said, adding that the public “was never in danger”.

UK passengers hit by delays

On Friday British Airways had confirmed the cancellation of at least one service due to depart for New York, with several flights arriving in Heathrow from the US listed as delayed or cancelled on Saturday.

By Saturday morning the knock-on effect in the UK was limited to delays on some flights on US carriers.

But the UK Civil Aviation Authority (CAA) warned: “While US flights have resumed, there may be some follow-on disruption as airlines work to re-book affected passengers and get all services back to normal.

“Anyone due to travel on flights between the UK and USA should check with their airline for the latest information before heading to the airport.

“We know that UK airlines are doing all they can to minimise inconvenience and to make sure their customers are properly looked after.”

British Airways said its flights were operating as normal having earlier apologised for possible delays and cancellations.

A spokesperson for Manchester Airport also confirmed three US-bound flights operated by Virgin Atlantic, all scheduled to depart on Saturday morning, were likely to be affected if the issue was not resolved quickly.

Virgin Atlantic is advising passengers to check for updates at virginatlantic.com.

Meanwhile, Ireland’s Department of Transport advised all travellers departing for the US to check with their airline for updates.

“We understand that some flights currently en route to the US are being permitted to land. The US authorities are handling international flights on a case by case basis,” it said.

“This is an evolving situation and the Department advises all passengers who are due to travel to the US on Saturday 14 November to check with their airline for information regarding the status of their flight.”

Delays expected to last for days

The FAA has indicated normal service is set to resume slowly, meaning many passengers in the US and overseas could still face delays over the coming hours.

Thousands of passengers were stranded overnight on Friday with Duffy telling reporters the ripple effects will last days.

Edward Kargbo, 50, from West Chester, Pennsylvania, had been scheduled to take a 9pm Ethiopian Airlines flight to Lomé, Togo. Instead, he planned to spend the night ‌at the airport before a rescheduled flight at 6am on Saturday.

“I have to squat the night here,” he said. “I’m stranded.”

Louise Maher, 47, ​was also caught up in the disruption at Newark after her British Airways flight to London was caught up in the aviation meltdown.

“I ‌might cry at some point but not yet,” she said.

Speaking on Friday, Mullin repeatedly told reporters that the situation “isn’t another 9/11”.

He said that President Donald Trump was briefed on the situation and ‌all flights were ​accounted for.

This is only the second time the US has grounded its departing flights since the 11 September attacks in 2001. A ground stop was issued for less than two hours in January 2023 when a pilot notification system went down.

The outage came amid heightened concern ​about aviation security worldwide after a wave of hoax bomb threats this month disrupted dozens ‌of passenger flights across Europe, forcing unscheduled landings ⁠and prompting fighter jets to escort or intercept aircraft.

There was no immediate indication the ⁠US communications failure was related to those incidents or caused by a cyberattack.

The US National Weather Service also ⁠said on Friday that it was experiencing ⁠nationwide ​internet ⁠and communications problems ⁠that could ​prevent ⁠users from ‌accessing products on its ‌websites or ‌cause outdated information ⁠to be displayed.

There was no immediate indication the ‌two incidents were connected.