Last Updated Apr 16, 2013 9:39 PM EDT
DALLAS A computer system used to run many daily operations at American Airlines failed Tuesday, forcing the nation's third-largest carrier to ground all flights across the United States for several hours and stranding thousands of frustrated passengers at airports and on planes.
Flights already in the air were allowed to continue to their destinations, but planes on the ground from coast to coast could not take off. And travelers could do little to get back in the air until the computer system was fixed.
American Airlines said that the system was fixed by 4:30 p.m. ET Tuesday. But the airline expects to see further delays throughout the rest of the day and more flight cancellations Wednesday.
"Despite the magnitude of today's disruption, we are pleased to report that we expect our operation to run normally with only a small number of flight cancellations expected tomorrow," said the company in a statement to CBS News. "We will add additional flights, as needed, tomorrow to ensure our customers are able to continue their travels."
The carrier also said that the computer problems were unrelated to yesterday's attack in Boston.
As of mid-afternoon Tuesday, American and its American Eagle offshoot canceled more than 700 flights, and another 765 flights were delayed, according to tracking service FlightAware.
Earlier Tuesday, a spokesman for the Federal Aviation Administration said American asked to halt its flights until 5 p.m. ET. Flights into American's five biggest cities -- New York, Dallas, Los Angeles, Chicago and Miami -- were stopped until 5:30 p.m. ET.
American Airlines flights out of Phoenix Sky Harbor International Airport now have resumed. Sky Harbor officials said five departing and four arriving American Airlines flights to and from Dallas and Chicago were canceled.
American said that customers traveling today who are at the airport can rebook their flight on another airline through the company or by going to another carrier's ticket counter. Travelers who aren't yet at the airport can change their flights through American's reservations system or through another airline, with American pledging to honor any difference in fares.
"If a customer chooses not to travel today, there will be no charge for reservation changes or a full refund will be provided," American said in a statement.
Passengers described long airport lines and 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 (LCC). 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.