Airline officials said late Thursday that they had come up with a fix for the seats, and they began pulling 48 Boeing 757s out of service to make repairs. All the planes should be back in service by Saturday.
The seat repairs, however, could inconvenience thousands of passengers. American said the work caused it to cancel 50 flights on Thursday and 44 on Friday. Each 757 that American operates in the U.S. has 188 seats.
It's the latest black eye for American parent AMR Corp., which is operating under bankruptcy protection and trying to fend off a takeover by US Airways Group Inc. Flight cancelations and delays surged in September, which American blamed on a slowdown by pilots who are unhappy that American canceled their labor contract.
Since last week, seats have come loose on three American Airlines flights involving two 757s that had been recently refurbished. The seats had been removed and reinstalled as part of the work.
The airline told CBS News that inspectors found a loose row of seats on six planes.
Federal officials said they are continuing a safety investigation into the events at the nation's third-biggest carrier.
American originally said the problem was due to a clamp that holds rows of seats to tracks on the aircraft floor.
But officials offered a new explanation Thursday, saying that a combination of wear, poor design and even soda spilled into the tracks caused pins to pop out of the grooves.
The airline has used the same seats for 20 years without incident until now, said David L. Campbell, American's vice president of safety.
"The fundamental design of this seat is not as robust as some of the latest designs," Campbell said in an interview.
A spokesman for the seat manufacturer, Weber Aircraft LLC, declined to comment beyond saying that his company is still investigating.
A spokesman for American said later that Campbell agreed that removing and reinstalling the seats might have hastened their failure to stay in place.
Campbell said the new fix will consist of installing an additional locking mechanism that was designed by American's engineers and approved by the Federal Aviation Administration.
The FAA issued a statement saying that it approved of American's decision to conduct more inspections. The agency said its safety investigation was continuing and it would "take additional action as appropriate." The agency is likely to examine whether American adequately inspected the seats after the cabin-refurbishment jobs.
Todd Curtis, a former safety engineer with Boeing and director of the Airsafe.com Foundation, said American made the right decision to inspect more planes once it discovered the extent of the problem.
No other airlines have reported loose seats. United Airlines doesn't use the same seat, and US Airways uses different hardware to mount seats, officials for those carriers said.