USAir Flight 427 Accident Investigation
The NTSB final report on the 8 September 1994 accident involving a USAir 737 included conclusions and recommendations that also applied to a fatal 1991 accident involving a United Airlines 737 and an incident involving an Eastwinds 737.
The National Transportation Safety Board determined that the probable cause of the accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit, and that the rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a malfunction of the main rudder power control unit. More information is available from the NTSB accident report
http://airsafe.com/events/flt427.htm -- Revised: 4 October 2014