American Airlines got their first few Boeing 757 aircrafts in 1990’s. The check airmen got their toys, the flight simulator to play around with. During the play session on one of the full flight simulators, they discovered that when a failure of Inboard Flap Asymmetry was simulated by pushing a button on the instructor control panel, the resultant effect on the aerodynamics created an irrecoverable situation.

Boeing was queried immediately and they were very candidly told that what American Airlines was saying was absolutely true. What followed later was taken down with a pinch of salt.

Boeing said that the situation would never arise since there are …….. safety features which would prevent the situation from arising. American Airlines, decided to remove the exercise from the flight simulator training as a management decision.

Guess what?. It happened not once but twice on other air carriers. One in-flight and the other on ground. The presence of mind of the Captain saved the day. As soon as the asymmetry happened on selection of flap, and the crew concluding that the aircraft would was uncontrollable, the Captain retracted the flap and the flap retracted, thereby removing the asymmetry and restoring stability.

American Airlines introduced this as a procedure to overcome the problem. The residual risk was, what if the flap did not retract? In the later scenario, there is no way out.

Airworthiness directive (AD), applicable to certain Boeing Model 757 series airplanes, that currently requires repetitive inspections for excessive wear of the internal and external splines of the torque tube couplings of the trailing edge flaps, and replacement of the couplings, if necessary.