Flights have resumed across the United States after a computer outage at the Federal Aviation Administration (FAA) resulted in thousands of planes being grounded, forcing cancellations and delays across the country.
At 9am ET on Wednesday (1am Thursday AEDT) the FAA finally lifted the ground stop that had resulted in chaos for many thousands of travellers and a ripple effect expected to impact airports nationwide for hours.
"Normal air traffic operations are resuming gradually across the US following an overnight outage to the Notice to Air Missions (NOTAMs) system that provides safety info to flight crews," the FAA said in a statement on Twitter.
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The order to ground flights had been made almost an hour and a half earlier, about 7.20am ET (11.20pm AEDT).
By 8.50am ET (12.50am Thursday AEDT), over 4000 flights were stuck on the ground, which is more than all of the delayed flights for the entirety of the previous day, according to the flight tracking website FlightAware.
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As flights resumed, there were over 7100 flights delayed by 12pm ET (4am Thursday AEDT) and more than 1100 flights cancelled.
Those numbers are expected to grow as the chaos flows on throughout the day across both cargo and passenger flights.
Some medical flights were given clearance during the outage, which also did not impact military operations.
Due to heavy congestion, the FAA cleared flights to depart at Newark Liberty and Atlanta Hartsfield-Jackson airports first, before operations gradually resumed across the country.
The FAA's website was still showing a ground delay at New York's LaGuardia Airport as of 10.50am ET (2.50am Thursday AEDT). The site also showed a ground delay at Charlotte Douglas International Airport in North Carolina, one of American Airlines' largest hubs.
The Chicago Department of Aviation said ground stops at O'Hare and Midway had been lifted but "residual delays or cancellations" are likely.
Airlines have continued to delay or cancel flights because of ongoing congestion. An airline source familiar with the situation said airlines may implement ground delay programs, which could potentially lead to further timetable issues.
More than 21,000 flights were scheduled to take off in the US across Wednesday – most of these domestic trips along with about 1840 international flights expected to fly to the US, according to aviation data firm Cirium.
Airlines for America, an association representing US airlines, earlier said the outage was "causing significant operational delays".
Major US carriers including United Airlines, Delta and American Airlines all had flights grounded.
Longtime aviation insiders could not recall an outage of such magnitude caused by a technology breakdown. Some compared it to the nationwide shutdown of airspace after the terror attacks of September 2001.
"Periodically there have been local issues here or there, but this is pretty significant historically," said Tim Campbell, a former senior vice president of air operations at American Airlines and now a consultant in Minneapolis.
Passengers were first told of the FAA outage around 6.30am (10.30pm AEDT), with the agency providing updates approximately every half hour.
The NOTAMS system that experienced the outage provides "critical flight safety operation information", the Airlines for America said.
The FAA stipulates NOTAMS are not to be relied on as a sole source of information, and so some flights may be able to satisfy safety requirements by using other data.
'We don't know' if cyberattack: Biden
While the White House initially said that there is no evidence of a cyberattack, President Joe Biden said "we don't know" and told reporters he had directed the Department of Transportation to investigate the cause of the disruption.
The president addressed the FAA issue before leaving the White House on Wednesday morning to accompany his wife to a medical procedure at Walter Reed National Military Medical Center outside of Washington.
He said he had just been briefed by Transportation Secretary Pete Buttigieg, who told him they still had not identified what went wrong.
"I just spoke to Buttigieg. They don't know what the cause is. But I was on the phone with him about 10 minutes," Biden said.
"I told him to report directly to me when they find out. Air traffic can still land safely, just not take off right now. We don't know what the cause of it is."
Buttigieg said in a tweet that he is in touch with the FAA and monitoring the situation.
The FBI did not immediately respond to a request seeking comment.
John Cox, a former airline pilot and aviation safety expert, said there has been talk in the aviation industry for years about trying to modernise the NOTAM system, but he did not know the age of the servers that the FAA uses.
He couldn't say whether a cyberattack was possible.
"I've been flying 53 years. I've never heard the system go down like this," Cox said. "So something unusual happened."
International flights bound for the US not impacted
The last flight out of Sydney International Airport to the US was a Hawaiian Airlines flight, which took off at 9.26pm on Wednesday.
The Qantas flight from Brisbane to Los Angeles appeared to be taking off as scheduled Wednesday night as well.
Julia Macpherson, who was travelling to the US from Tasmania learned of the flight delays while on a United Airlines flight from Sydney to Los Angeles.
"As I was up in the air I got news from my friend who was also traveling overseas that there was a power outage," Macpherson, who was returning to Florida from Hobart, said.
Once she lands in Los Angeles, she still has a connection in Denver on her flight to Jacksonville, Florida.
She said there have been no announcements on the flight about the FAA issue.
Macpherson said she had already experienced a delay in her travels because her original flight from Melbourne to San Francisco was cancelled and she rebooked a flight from Sydney to Los Angeles.
Meanwhile European flights into the US appeared largely unaffected by the outage, with planes continuing to take off from Amsterdam and Paris despite the situation.
A spokesperson from Schiphol Airport in the Netherlands told CNN that "a workaround had been issued" and flights were still departing from Amsterdam.
No flights had been cancelled from Paris' Charles de Gaulle Airport, but delays were expected, according to the airport's press office. Frankfurt Airport also told CNN it had not been impacted.
A London Heathrow Airport spokesperson told CNN that they were "not aware of cancelled flights and that flights to the US had left recently", however there were passenger reports of significant delays.
Shabnam Amini told CNN that she and other travellers had been sitting on board Americans Airlines flight 51 to Dallas for almost three hours at Heathrow because of the FAA outage.
She said they had been informed that there were delays but were still boarded onto the aircraft.
Irish carrier Aer Lingus said services to the US continue and Dublin Airport's website showed that its flights to Newark, Boston, Chicago and Los Angeles were running on schedule.
"Aer Lingus plan to operate all transatlantic flights as scheduled today," the carrier said in a prepared statement.
"We will continue to monitor but we do not anticipate any disruption to our services arising from the technical issue in the United States."
Inbound international flights into Miami International Airport continued to land, but all departures have been delayed since 6.30am (10.30pm AEDT), said airport spokesman Greg Chin.
