Investigators later conducted an observation with its Rudder Power Control Unit ( PCU ) . The Main Power Control Unit ( MPCU ) of the rudder has been a subject of intense debate and discussions and has undergone mandatory modifications as a result of some previous accident investigations . The PCU has responsible for several high-profile crashes , such as United Airlines Flight 585 , USAir Flight 427 , and Eastwind Airlines Flight 517 . Flight 7412 nose-dived before the crash , as well as two previous crashes , Flight 585 and Flight 427 . The rudder locked up in a particular direction , and reversal of rudder to that of the commanded position ( anomalous rudder movements such as un-commanded movements ) . These have been attributed to the secondary slide jamming with the primary slide due to contamination or adverse buildup of tolerances during manufacture . However , investigators later dismissed it and ruled out the failure of the PCU as the cause , and concluded that it was serviceable and was in good condition
USAir Flight 427
USAir Flight 427
Alliance Air Flight 7412