"We're not sure where they were improperly installed," said spokeswoman Mary Frances Fagan. "A lot of people touch those airplanes."
American had planned to inspect eight of its 102 Boeing 757 jets. But by Tuesday afternoon it had inspected 36 planes and planned to check 11 more that have the same type of seats in the main cabin, the airline said.
The first sign of trouble showed up last Wednesday, when crews noticed loose seats on a plane that had flown from Dallas-Fort Worth International Airport to Vail, Colo. The same plane had to make an emergency landing Monday when seats came loose shortly after takeoff on a New York-to-Miami flight, and a Boston-to-Miami flight Saturday diverted to New York after seats loosened in mid-flight, according to the airline.
Separately, an American flight on Tuesday from Chicago to London was diverted to Shannon Airport in Ireland after a report of smoke in the cabin. An airline spokesman said it turned out to be a faulty cooling fan in an entertainment system, and the plane was expected to continue on to London Tuesday night.
The reports of smoky cabins and seats coming loose during flights raised questions about safety on the nation's third biggest airline. Aviation industry experts said publicity about the problem could make passengers stay away from American and fly on other airlines instead.
Matt Ziemkiewicz, president of the safety-advocacy group National Air Disaster Alliance, said passengers could be hurt or killed in an otherwise survivable crash if seats break loose from their moorings.
"What if it's a little kid or an old person in the row behind them?" he said. "That seat becomes a projectile with people on it."
Ziemkiewicz said he had never heard of seat rows coming loose. Asked if it had ever happened to American plane before last week, airline spokeswoman Andrea Huguely replied, "Not that I'm aware of."
The problem planes were worked on by several crews in different cities. After seats came loose the first time, a crew in Vail tightened them and the plane made a return flight to Dallas. It flew to Boston later that day, where the seats were tightened again, according to American.
No further problems were noticed until a flight Monday from New York to Miami, which returned to Kennedy Airport. Another plane making a Boston-to-Miami trip on Saturday made an emergency landing in New York after a row of seats came loose in flight.
The seats on both planes had been removed and reinstalled during recent maintenance at an American Airlines base in Tulsa, Okla., and a Timco Aviation Services facility in North Carolina.
In both cases American employees were the last to touch the seats, Huguely said a comment that drew a fierce response from the Transport Workers Union, which represents American's maintenance employees.
"Our workers were the last to touch the seats only in the sense that after the seats came loose we were dispatched" to fix them, said union official John Hewitt. The union blamed Timco and criticized management of American parent AMR Corp. for cutting costs by outsourcing maintenance work.
A Timco spokesman declined to comment beyond saying that the company is still investigating.
The Federal Aviation Administration said it is looking into the incidents as well.
American officials said the incidents were not related to its difficulties with union workers, who are unhappy about pending layoffs and cuts in pay and benefits that American has imposed since filing for bankruptcy protection in November. American accuses some pilots of conducting an illegal work slowdown that caused a jump in canceled and delayed flights in September.
Robert Mann, an aviation consultant who once worked at American, said delays, cancelations and bad publicity about broken seats could create an opening for rivals United Airlines and Delta Air Lines to poach American customers in competitive markets like Chicago and New York.
"I'm struck by how close this company is to losing its way," he said.
Spencer Nam, a stock analyst from Boston who was flying to Dallas on American for business, said his Wednesday evening flight was delayed after passengers boarding the planes noticed that the seats in Row 12 were leaning toward Row 13. Although the problem was fixed and his plane got to Dallas on time, he said he might book another airline the next time.
"When it comes to flying, I don't like unexpected events," Nam said. "I'm 42, I've been flying more than 20 years, and I've never seen where seats weren't screwed down."