Travelers check the status of their flight as they arrive at the United Airlines Terminal 1 ahead of the Christmas holiday at O'Hare International Airport on Dec 22, 2022 in Chicago. (KAMIL KRZACZYNSKI / AFP)
WASHINGTON — US flights were slowly beginning to resume departures and a ground stop was lifted after the Federal Aviation Administration (FAA) scrambled to fix a system outage overnight that had forced a halt to all US departing flights.
The cause of the problem, which delayed thousands of flights in the United States, was unclear, but US officials said they had so far found no evidence of a cyberattack.
More than 4,300 flights had been delayed and 700 canceled as officials said it will take hours to recover from the halt to flights
"Normal air traffic operations are resuming gradually across the US.following an overnight outage to the Notice to Air Missions system that provides safety info to flight crews. The ground stop has been lifted. We continue to look into the cause of the initial problem" the FAA said in a Tweet.
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More than 4,300 flights had been delayed and 700 canceled as officials said it will take hours to recover from the halt to flights.
The FAA had earlier ordered airlines to pause all domestic departures after its pilot alerting system crashed and the agency had to perform a hard reset around 2 am, officials said.
The FAA said shortly before 8:30 am departures were resuming at Newark and Atlanta airports.
The FAA is expected to implement a ground delay program in order to address the backlog of flights halted for hours. Flights already in the air had been allowed to continue to their destinations during the ground stop.
US President Joe Biden ordered the Transportation Department to investigate the outage and said the cause of the failure was unknown at this time. Asked if a cyber attack was behind the outage, Biden told reporters at the White House, "We don't know."
Transportation Secretary Pete Buttigieg pledged "an after-action process to determine root causes and recommend next steps."
The FAA said it was working to restore the Notice to Air Missions system that alerts pilots to hazards and changes to airport facilities and procedures that had stopped processing updated information.
A total of 4,314 US flights were delayed as of 9:04 am ET, flight tracking website FlightAware showed. Another 737 were canceled.
United said it has resumed operations. The Chicago-based carrier, however, warned that customers might continue to see some delays and cancellations.
Shares of US carriers fell in Wednesday's premarket trading. Southwest Airlines was down 2.4 percent, while Delta Air Lines Inc, United Airlines and American Airlines were down about 1 percent.
"America’s transportation network desperately needs significant upgrades … We call on federal policymakers to modernize our vital air travel infrastructure.” said Geoff Freeman, President and CEO of the US Travel Association, a group representing US airlines, hotels, car rental companies, and theme parks.
FAA's system outage comes weeks after an operational meltdown at Southwest at the end of last year left thousands of passengers stranded.
A severe winter storm right before Christmas coupled with the Texas-based carrier's dated technology led to over 16,000 flight cancellations last month.
The DOT, FAA's parent agency, heavily criticized Southwest's failures and pressured the airline to compensate passengers for missed flights and other related costs. There is no legal requirement that the FAA must compensate passengers for flight delays caused by agency computer issues.
A NOTAM is a notice containing information essential to personnel concerned with flight operations, but not known far enough in advance to be publicized by other means.
Information can go up to 200 pages for long-haul international flights and may include items such as runway closures, bird hazard warnings and construction obstacles.
United Airlines said it had temporarily delayed all domestic flights and would issue an update when it learned more from the FAA.
Germany's Lufthansa and Air France both said they were continuing to operate flights to and from the United States, while the French airline said it was monitoring the situation.
The operator of Paris international airports – Paris Charles de Gaulle airport and Orly airport – said it expects delays to flights.
Austin-Bergstrom International Airport said on Twitter that ground stops across the country were causing delays. A ground stop is an air traffic control measure that slows or halts aircraft at a given airport.
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In an earlier advisory on its website, the FAA said its NOTAM system had "failed", although NOTAMs issued before the outage were still viewable. Earlier this month, a problem with a different airline computer control system delayed dozens of flights in Florida.
A total of 21,464 flights are scheduled to depart airports in the United States on Wednesday with a carrying capacity of nearly 2.9 million passengers, data from Cirium shows.
American Airlines has the most departures from US airports with 4,819 flights scheduled, followed by Delta and Southwest, Cirium data showed.