16 years ago, just after 7 pm local time, USAir flight 427 was preparing to land on runway 28R after a flight from Chicago-O'Hare International Airport to Pittsburgh International Airport when the Boeing 737-3B7 (N513AU, SN 23699 / LN 1452) began a hard roll to the left. 22 seconds after the onset of the event the aircraft impacted the ground nose first, killing all 127 passengers and five crew.
The investigation was lengthy and challenging; the final report was published over 4.5 years after the accident and determined the following 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 blowdown 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 overtravel of the primary slide.
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Karl L. Swartz.
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