Flights in the air were allowed to continue to their destinations, but planes on the ground could not take off.
The airline blamed its computerized reservation system, which is used for much more than booking flights. Airlines use such systems to track passengers and bags, monitor who has boarded a plane and to update flight schedules and gate assignments and file flight plans.
The failure caused cascading delays and cancelations nationwide.
As of mid-afternoon, American and its American Eagle offshoot canceled more than 700 flights and another 765 flights were delayed, according to tracking service FlightAware.
The outage began in midmorning stretched into the afternoon. The systems were fixed by 4:30 p.m., airline spokeswoman Stacey Frantz said.
But even as some flights took off, the airline expected delays and cancellations to continue for the rest of the day.
At airports, customers whose flights were canceled couldn't rebook on a later flight. Passengers already at the airport were stuck in long lines or killed time in gate areas. They described frustration at the lack of information from airline employees.
"Tensions are high. A lot of people are getting mad. I've seen several yelling at the American agents," said Julie Burch, a business-meeting speaker who was stuck at Dallas-Fort Worth International Airport waiting for a flight to Denver. "Nobody can tell us anything."
Terry Anzur, a TV news consultant from Los Angeles who was also stranded in Dallas, said American Airlines gate employees were doing everything the old-fashioned, manual way because their computers were useless.
"No one at the counter can do anything. They can't check people in," Anzur said. "The airline is at a dead halt."
American's problems on Tuesday were reminiscent of what United Airlines passengers endured for several days last year. After merging with Continental, United experienced computer glitches in the combined reservation system. On one day in August, 580 United flights were delayed, and its website was shut down for two hours. Another outage in November delayed 636 flights.
The problems prompted an apology from United Continental Holdings Inc. CEO Jeff Smisek, who acknowledged that the airline had frustrated customers and would need to work to win them back.
United's technology glitches arose after a merger, but American's headache occurred as parent company AMR Corp. seeks government approval to merge with US Airways Group Inc. A merger would let American leapfrog United and become the world's biggest airline.
The combined American-US Airways plans to use the American system that broke down on Tuesday.
Passengers used social media to flood the airline with complaints. The airline tweeted back that it was working to fix the problem, and it apologized for the inconvenience.
To make amends, American offered to book people who needed to travel Tuesday on another airline and pay for the fare difference. For those who wanted to delay their trips, American offered refunds or waivers from the usual fee for changing a reservation.
But for several hours, the airline wasn't able to process those changes and refunds because the reservations system was down.
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