American Airlines said that improperly installed clamps caused seats to pop loose on two planes during flights and an inspection turned up four others with the same problem.
The airline said Tuesday that it inspected and fixed the seats on 36 of its Boeing 757 jets and planned to check 11 other planes.
In the past week, rows of seats have come loose on three separate flights, two of which made emergency landings. Federal officials are looking into the matter, which safety advocates consider to be serious.
David L. Campbell, the airline's vice president of safety, said in an interview that the clamps might have been installed incorrectly during maintenance work by American crews and an outside contractor, Timco Aviation Services. He could not recall a similar problem with any other American planes.
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 seats were tightened again that day in Boston. The same plane had to make an emergency landing Monday when seats came loose shortly after takeoff on a New York-to-Miami flight.
Another plane flying from Boston to Miami on Saturday diverted to New York after seats loosened in mid-flight.
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, which the airline attributed to a faulty fan in an entertainment system.
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 bad publicity could lead passengers to avoid American.
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."
The planes in the Saturday and Monday incidents were serviced in the past two months and seats had been removed and reinstalled, American said.
Campbell, the safety executive, said new seats on some of American's 757s have a different fastening system — instead of four bolts that are wrench-tightened, they are held in place by two bolts in back that are tightened with wrenches and two in front that are hand-tightened. The seats must be positioned precisely so that they lock into place.
"It's a very temperamental job," he said.
American said work was done at an American Airlines base in Tulsa, Okla., and a Timco facility in North Carolina. In both cases, American employees were the last to touch the seats, said airline spokeswoman Andrea Huguely — 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."