USAir Flight 427, a Boeing 737-300, crashed following a loss of control during the approach to Pittsburgh International Airport, Pennsylvania, USA. All 132 on board were killed.PROBABLE CAUSE :" The National Transportation Safety Board determines that the probable cause of the USAir flight 427 accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blow down limit. The rudder surface most likely deflected in a direction opposite to that commanded,by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide to the servo valve housing offset from its neutral position and over travel of the primary slide."