Another potential factor that led to the crash of the 737 aircraft was the power control unit ( PCU ) that controlled the aircraft 's rudder . The cause of some 737 crashes , such as USAir Flight 427 , had been attributed to the 737 's rudder issues . Although the NTSB and PCU manufacturer Parker Hannifin had already determined that the PCU was properly working , and thus not the cause of the crash , a private investigation into the crash for a civil lawsuit tried by jury in a state court in Los Angeles , which was not allowed to hear or consider the NTSB 's and Parker Hannifin 's conclusions , decided that the crash was caused by a defective servo valve inside the PCU , based on forensic findings from an electron microscope which determined that minute defects within the PCU had caused the rudder hard-over and a subsequent uncontrollable flight and crash . The manufacturer of the aircraft 's rudder controls and the families later reached an out-of-court settlement
USAir Flight 427
USAir Flight 427
SilkAir Flight 185