Normal air traffic operations are resuming gradually across the United States, following a technical failure of a key pilot notification system that resulted in the grounding of thousands of flights for several hours.
The Federal Aviation Administration announced earlier in the morning that it suffered an outage of its Notice to Air Missions or NOTAMs, which alerts pilots and other personnel about airborne issues.
The FAA ordered airlines to delay all domestic departures early Wednesday morning, but lifted the ground stop just before 9 am Eastern after several hours.
'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,' FAA said in a tweet.
According to FlightAware, a flight tracking company, more than 9,500 flights within, into or out of the United States were delayed and more than 1,300 were cancelled due to the system failure.
The number of cancellations and delays has continued to climb despite the agency lifting a ground stop.
More than 21,000 flights were scheduled to take off in the US on Wednesday, mostly domestic trips, and about 1,840 international flights expected to fly to the US, AP reported, citing aviation data firm Cirium.
Airports have been advising travellers to check with individuals airlines to see their latest flight status.
The FAA earlier said it is making progress in restoring its Notice to Air Missions system and departures are resuming at the Newark Liberty International Airport and Atlanta International Airport due to air traffic congestion in those areas.
'We expect departures to resume at other airports at 9 a.m. ET,' it said.
'All flights currently in the sky are safe to land. Pilots check the NOTAM system before they fly. A Notice to Air Missions alerts pilots about closed runways, equipment outages, and other potential hazards along a flight route or at a location that could affect the flight,' FAA said.
It earlier said that operations across the National Airspace System are affected.
President Joe Biden was briefed by Secretary of Transportation Pete Buttigieg on the FAA system outage.
Biden said that Buttigieg 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.'
White House Press Secretary Karine Jean-Pierre said in a tweet that there is no evidence of a cyberattack at this point, but the President 'directed DOT to conduct a full investigation into the causes. The FAA will provide regular updates.
Transportation Secretary Buttigieg said that he is in touch with the FAA and monitoring the situation.
'I have been in touch with FAA this morning about an outage affecting a key system for providing safety information to pilots. FAA is working to resolve this issue swiftly and safely so that air traffic can resume normal operations, and will continue to provide updates,' he said in a tweet.
Speaking to CNN, Buttigieg said there's been no direct evidence or indication of a cyberattack, but added that he also wouldn't rule it out.
He defended the FAA's decision to ground flights nationwide following the system outage, saying it was 'the right call'.
He, however, said that 'these kinds of disruptions should not happen'.
'And my primary interest, now that we've gotten through the immediate disruptions of the morning, is understanding exactly how this was possible and what steps are needed to make sure it doesn't happen again,' Buttigieg told CNN.
Buttigieg said federal officials will conduct a review of the aviation system failure that snarled air traffic.
'I have directed an after-action process to determine root causes and recommend next steps,' he said on Twitter.
United Airlines warned passengers that they may continue to see delays and cancellations.
The airline also said it would issue a travel waiver and offer refunds.
