American first gave one explanation for the problem aboard its 757s, then gave another two days later, saying pins that hold the seats to the floor had become gummed up over time from dirt and by passengers spilling sodas and coffee. The seats had been removed and reinstalled, which American's vice president of safety, David Campbell, said could have made the loose-seat problem pop up sooner.
The reinforcing work will be done on 49 of American's 58 Boeing 767-300 aircraft. The other nine, along with 15 Boeing 767-200s, have different seats and were not part of Friday's announcement, according to the airline.
American is owned by AMR Corp., based in Fort Worth, Texas.
Follow David Koenig at http://www.twitter.com/airlinewriter