Probable Cause
The NTSB reopened the UAL 585 case after the crash of another B-737, USAir Flight 427, which occurred three-and-a-half years later. It was eventually determined that both crashes were the result of a sudden malfunction of the rudder power control unit. The pilots lost control of the airplane because "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."
Read more about this topic: United Airlines Flight 585
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