FAA lifts ground stop on flights across U.S. following computer ...

A computer outage at the U.S. Federal Aviation Administration brought flights to a standstill across the U.S. and on Wednesday, with hundreds of delays quickly cascading through the system, including at Canadian airports.

The FAA is working to restore what is known as the Notice to Air Missions System.

Before commencing a flight, pilots are required to consult NOTAMs, or Notices to Air Missions, which list potential adverse impacts on flights, from runway construction to the potential for icing. The system used to be telephone-based, with pilots calling dedicated flight service stations for the information, but has now moved online.

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 that he’s directed the Department of Transportation to investigate the cause of the disruption.

The FBI did not immediately respond to a request seeking comment.

The stop order was lifted just before 9 a.m., but delays and cancellations are expected to snowball. Departure gates at major airports are filled with aircraft that had been ordered grounded for hours. More than 21,000 flights were scheduled to take off in the U.S. today, mostly domestic trips, and about 1,840 international flights expected to fly to the U.S., according to aviation data firm Cirium.

Nearly 5,000 flights were delayed and almost 900 were cancelled by around 10 a.m.

At Canadian airports on Wednesday morning, all U.S. flights were delayed, including least 35 departing Toronto Pearson International Airport and 26 at Montreal-Trudeau International Airport. The U.S. problems could disrupt almost 800 Canadian flights to and from the U.S. on Wednesday, said Mike Arnot, a spokesman for Cirium, an aviation analytics company.

The stop order by the FAA impacted almost all flights of shippers and passenger airlines.

Some medical flights could get clearance and the outage did not impact any military operations or the mobility of U.S. defence forces.

Biden addressed the FAA issue Wednesday before leaving the White House 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.

Delays and cancellations that began on East Coast quickly spread west, with disruptions at almost all major U.S. airports.

The FAA said it was working on restoring its Notice to Air Missions System.

“We are performing final validation checks and reloading the system now,” the FAA said. “Operations across the National Airspace System are affected.”

Julia Macpherson was on a United Airlines flight from Sydney to Los Angeles on Wednesday when she learned of possible delays.

“As I was up in the air I got news from my friend who was also travelling overseas that there was a power outage,” said Macpherson, who was returning to Florida from Hobart, Tasmania. 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.

Breakdowns in the NOTAM system appear to be rare.

John Cox, a former airline pilot and aviation safety expert, said there has been talk in the aviation industry for years about trying to modernize 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.”

According FAA advisories, the NOTAM system failed at 8:28 p.m. Eastern on Tuesday preventing new or amended notices from being distributed to pilots. The FAA resorted to a telephone hotline in an effort to keep departures flying overnight, but as daytime traffic picked up it overwhelmed the telephone backup system.

Air Force spokeswoman Ann Stefanek said the U.S. military flights were not impacted because the military has its own NOTAMS system separate from the FAA system and the military’s system was not affected by the outage.

European flights into the U.S. appeared to be largely unaffected.

Irish carrier Aer Lingus said services to the U.S. 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.”

This is just the latest headache for travellers in the U.S. who faced flight cancellations over the holidays amid winter storms and a breakdown with staffing technology at Southwest Airlines. They also ran into long lines, lost baggage, and cancellations and delays over the summer as travel demand roared back from the COVID-19 pandemic and ran into staffing cutbacks at airports and airlines in the U.S. and Europe.

The FAA said that it would provide frequent updates as it made progress.

With files from The Globe’s Eric Atkins.