American's system is hosted by Sabre Holdings, a onetime division of American that was spun off into a separate travel-reservations technology company. American said the outage wasn't Sabre's fault, and other airlines that use Sabre did not experience problems.
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.
Theoretically, an airline could do the same work as the reservation system manually for any one flight. But doing it for hundreds of flights isn't practical. American and American Eagle operate about 3,300 flights a day.
Brent Bowen, a professor of aviation technology at Purdue University, said massive system failures are inevitable as airlines grow increasingly reliant on technology.
"As those systems get bigger and more complex, at some point you're going to have a systemic failure," Bowen said.
Financially strapped airlines may have underinvested in technology during the past decade, making computer systems more vulnerable, he added.
AMR has lost more than $10 billion since 2001 and filed for bankruptcy protection in late 2011.
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 his airline had frustrated customers and would need to work to win them back.
Associated Press Airlines Writers Scott Mayerowitz in New York and Joshua Freed in Minneapolis contributed to this report.
David Koenig can be reached at http://www.twitter.com/airlinewriter .