Investigators later discovered that the recovered accident rudder power control unit was much more sensitive to bench-tests than other new such units . The exact mechanism of the failure involved the servo valve , which remains dormant and cold for much of the flight at high altitude , seizing after being injected with hot hydraulic fluid that has been in continuous action throughout the plane . This specific condition occurred in fewer than 1 % of the lab tests , but explained the rudder malfunction that caused Flight 427 to crash . The jam left no trace of evidence after it occurred and a Boeing engineer later found that a jam under this controlled condition could also lead to the slide moving in the opposite direction than that commanded . In light of this , Boeing felt that the test results were not real-world and not applicable due to the extremes under which the valve was tested . Boeing stated that the rudder reversal was more likely psychological , likening it to examples when a human panics and intends to step on the brake during an automotive accident , but accidentally presses on the gas pedal instead while under duress . The FAA 's official position was that there was not enough evidence for probable cause of rudder system failure . After the longest accident investigation in NTSB history — lasting more than four and a half years — the NTSB released its final report on March 24 , 1999 . The NTSB concluded that the accident was due to mechanical failure : The NTSB concluded that similar rudder problems had caused the previously mysterious March 3 , 1991 crash of United Airlines Flight 585 and the June 9 , 1996 incident involving Eastwind Airlines Flight 517 , both Boeing 737s . The final report also included detailed responses to Boeing ’s arguments about the causes of the three accidents . At the time of the crash , Flight 427 was the second-deadliest accident involving a Boeing 737 ( all series ) ; as of 2019 , it now ranks as the ninth deadliest . It was also the seventh-deadliest aviation disaster in the history of the United States , and the deadliest in the US involving a 737 ; as of 2018 , it ranks eleventh . The accident marked USAir 's fifth crash in the period from 1989 to 1994 . The Commonwealth of Pennsylvania spent approximately $ 500,000 in recovery and cleanup for the accident site
United Airlines Flight 585
United Airlines Flight 585
USAir Flight 427