NEW YORK (CNN) — Following a brief outage that grounded all American Airlines flights during one of the busiest travel periods of the year, the company says it has resumed service Tuesday morning, albeit with substantial delays.

American Airlines said a “vendor technology issue” was responsible for delaying all of its flights in the United States. But the company began boarding flights again at around 8 a.m. ET, and flights have since resumed, airline spokeswoman Sarah Jantz told CNN.

• From WISHTV.com: FlightAware reports Indianapolis International Airport had more than 40 delays and one cancellation on Tuesday.
• Story: Holiday travel resumes after ground stop order affects IND airport

The Allied Pilots Association, which represents the 16,000 pilots at American Airlines, said American Airlines’ Flight Operations System briefly went down Tuesday morning, which led to the delays. Known as the FOS, the system handles the airline’s operations, key to getting airplanes released for departure — including passenger boarding. American Airlines has backup flight planning tools to prevent an outage from shutting down the entire airline for extended periods of time, according to the union, which said pilots and crew are trained to understand how to navigate a system outage.

American Airlines didn’t confirm the specific system that had an outage but acknowledged the issue prevented it from releasing flights from the gates.

The Federal Aviation Administration lifted its nationwide groundstop order for all American flights. The FAA in a statement said American requested a nationwide ground stop but it referred all questions to the airline for more information.

The outage came on a day when the airline is scheduled to have more than 3,300 domestic flights, according to the aviation analytics firm Cirium. The airline resumed service without a substantial number of cancelations. But only 37% of its flights took off on time Tuesday and just 36% of flights had an on-time arrival, according to Cirium. Still, 93% of American’s flights Tuesday took off within 2 hours of their scheduled departure time.

“It’s all hands on deck as our team is working diligently to get customers where they need to go as quickly as possible,” the airline said. “We apologize to our customers for the inconvenience.”

American said delayed customers should be on their way with minimal disruption.

“Expect some delays throughout the day but (we’re) working to mitigate those and avoid cancels,” Jantz said. “We have a smaller schedule today and have the staffing to support the quick recovery.”

Just 26 flights across all airlines were canceled nationwide, according to data tracker FlightAware. More than 1,000 flights into, within and out of the United States were delayed – a number that picked up shortly after the American outage, but it could also be caused by airport congestion and some winter weather in the Northeast.

‘Complaining doesn’t seem quite right’

David Myers, a 62-year-old disaster consultant traveling from from Salisbury, Maryland, to New Orleans with a layover in Charlotte said he was first alerted to the issue at 6 a.m. Tuesday. He and his wife are trying to spend Christmas with their children.

“It’s Christmas Eve, so complaining doesn’t seem quite right,” Myers told CNN. “And safety always comes first. But more information at the gate would be helpful.”

Customers on social media began complaining early Tuesday morning that some flights were returning to their gates and that delays are about 90 minutes.

“Captain says @AmericanAir software outage preventing weight & balance calculations ‘company-wide’ with no estimate on resolution. Flights unable to depart as a result. Not a good start to Christmas Eve travel!,” a person posted on X.

In a video posted from Fort Lauderdale airport in Florida, an agent is heard saying “our system is down” and they are keeping passengers at the gate.

American Airlines’ (AAL) stock initially fell nearly 3% in premarket trading, but rose 1% following the resolution.

Service outages

Holiday travel delays are nothing new, but glitches and outages can make annoying situations significantly worse.

In July, during the peak of summer travel season, a global tech outage led to travel chaos around the world. Most airlines recovered within a day, but Delta’s systems failed to recover for about a week.

The meltdown ensnared an estimated half a million people, ruined holidays and travel plans and prompted a federal investigation

In December 2022, a punishing winter storm that dumped multiple feet of snow across much of America led to widespread flight cancellations over the Christmas holiday. Although air travel was more or less back to normal a few days later, Southwest Airlines continued to face massive cancellations and delays as its outdated scheduling systems became overwhelmed.

That storm had hit Chicago and Denver hard, where Southwest has two of its biggest hubs, exposing the airline’s technology shortcomings. The airline faced massive fines and lawsuits, but it has since upgraded its tech.

This story has been updated with additional developments and context.