US airports are slowly beginning to resume departures after a ground stop order was lifted.
The order was imposed as 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 with a pilot-alerting system, 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.
The outage occurred at a historically slow time for US travel after the December holiday travel season, but airlines have said demand remains strong as travel continues to recover to near pre-pandemic levels.
"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 Tweeted.
The number of flights impacted continued to steadily rise even after the ground stop was cancelled. One issue airlines are facing is trying to get planes in and out of crowded gates, which is causing further delays.
More than 5,400 flights had been delayed and 900 cancelled according to the FlightAware website 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, officials said.
The FAA is expected to implement a ground delay programme 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, Mr Biden told reporters at the White House, "We don't know."
Transportation Secretary Pete Buttigieg pledged a "process to determine root causes and recommend next steps".
Essential information
A NOTAM is a notice containing information essential to personnel concerned with flight operations but not known far enough in advance to be publicised by other means. A ground stop is an air traffic control measure that slows or halts aircraft at a given airport.
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.
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 today with a 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.
Updated systems are required
Chicago-based United said it has resumed operations, but warned customers might continue to see some delays and cancellations.
Shares of US carriers initially fell in today's pre-market trading, but most rallied after the market opened to positive territory as flights resumed.
A trade group representing the US travel industry, including airlines, called the FAA system failure "catastrophic".
"America's transportation network desperately needs significant upgrades," Geoff Freeman, president of the US Travel Association, said in a statement. "We call on federal policymakers to modernise our vital air travel infrastructure."
Senate Commerce Committee chair Maria Cantwell said the panel would investigate. "We will be looking into what caused this outage and how redundancy plays a role in preventing future outages," she said. "The public needs a resilient air transportation system."
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, criticised Southwest's failures and pressured the airline to compensate passengers. There is no legal requirement that the FAA must compensate passengers for flight delays caused by agency computer issues.
