Over 1,000 flights cancelled as air travel disrupted across US Northeast; services resume: What caused the outage?

Flyers across the United States faced massive disruptions to their travel plans on Monday after equipment outages stopped flights at Newark Liberty International Airport in New Jersey and Philadelphia International Airport in Pennsylvania.

More than 1,000 flights were canceled nationwide and over 6,000 were delayed on Monday, with Newark and Philadelphia hit the hardest, ABC News reported.

What caused the disruption?

According to the Associated Press, the primary communications line at the Federal Aviation Administration (FAA) facility in Philadelphia had failed earlier Monday, while the backup fiber optic line was inadvertently severed by railroad construction workers.

The failures left controllers without radar or communications for hours.

"The issues began when a circuit blew at a Philadelphia air traffic control facility Monday morning," FAA Administrator Bryan Bedford said. He added, "When the backup system went into place, officials discovered the backup line in New Jersey had been cut."

To change the circuit in Philadelphia, Bedford was quoted by ABC News as saying that the airspace in the Northeast would temporarily close.

Ground stops were expanded to include New York City's John F. Kennedy International Airport and LaGuardia Airport as part of the work, though airspace at those locations has since reopened.

The outage brought commercial air traffic to a near-standstill at Newark for much of the day, with no commercial flights landing at the airport from about 9:40 am to just after 5:20 pm local time, according to flight-tracking site Flightradar24, the Associated Press reported.

As per the report, more than 200 flights into and out of Philadelphia International Airport were also canceled, while disruptions hit LaGuardia and Kennedy airports in New York. By Monday evening, flights were resuming at those airports.

Some airlines were allowing customers to change flights without charge. The FAA advised travelers to check the status of their flights before heading to the airport.

The outage occurred just as President Donald Trump and numerous foreign leaders were flying into New York City for the United Nations General Assembly.

What else do we know?

The technical problems stem from the same FAA facility in Philadelphia that caused numerous disruptions at the Newark airport last spring, AP reported.

Regional control centers like these have two lines that carry radar and communications data in and out: the primary and the backup.

The FAA said a circuit on the primary failed Monday morning, and when the system tried to switch to the backup, workers discovered that the fiber optic line had been cut.

Earlier in the day, Duffy said an Amtrak construction crew was to blame. Amtrak, however, pointed to a New Jersey Transit construction crew that was working in the area between Newark and New Brunswick, New Jersey.

FAA administrator Bryan Bedford told reporters earlier the agency had focused on fixing the primary connection as soon as possible by installing a new circuit on the primary line, while also working to repair the backup fiber optic line.

Verizon said at the time that its technicians were also working to restore service.

The outage caused a ripple effect across the Northeast and the US, diverting some flights and canceling and delaying others.

Repair done, flights resume

Hours after officials said the fix to replace the circuit in Philadelphia had not worked, leaving operations still halted at Philadelphia and Newark's airports, Transportation Secretary Sean Duffy updated Monday evening that the telecom lines have been repaired and "all airport operations in the Northeast are resuming," according to ABC News.

"It will take some time to clear all existing delays, so please check with your airline for the latest information," he said on X.

United Airlines, which has one of its hubs at Newark Airport, updated Monday evening that it was no longer canceling domestic flights at the airport for the remainder of the day.

"We know this outage is a significant disruption for our customers," United said in a statement. "Once it is resolved, we will work to safely get them to their destinations, and we have issued a travel waiver to give them the flexibility to manage their travel plans."