United Airlines Flight 585

The National Transportation Safety Board (NTSB) commenced an investigation, which lasted for 21 months.[4]

Although the flight data recorder (FDR) outer protective case was damaged, the data tape inside was intact and all of the data was recoverable.[1]^:38 Five parameters were recorded by the FDR: heading, altitude, airspeed, normal acceleration (G loads), and microphone keying. The FDR did not record rudder, aileron or spoiler deflection data, which could have aided the NTSB in reconstructing the plane's final moments.[4]^:100 The data available proved insufficient to establish why the plane suddenly went into the fatal dive.[4]^:102 The NTSB considered the possibilities of a malfunction of the rudder power control unit servo (which might have caused the rudder to reverse) and the effect that powerful rotor winds from the nearby Rocky Mountains may have had, but there was not enough evidence to prove either hypothesis.[4]^:102

The cockpit voice recorder (CVR) was also damaged, but the data tape inside was also intact. However, the data tape had creases in it, resulting in the playback quality being poor.[4]^:40 The CVR determined that the pilots made a verbal (and possible physical) response to the loss of control.[4]^:102

The following is an excerpt of the last two minutes Flight 585 CVR, starting two minutes before impact (the full CVR recording started before Flight 585 took off from Stapleton):

Thus, the first NTSB report (issued on December 8, 1992) did not conclude with the usual "probable cause". Instead, it stated:[4]^:102

The National Transportation Safety Board, after an exhaustive investigation effort, could not identify conclusive evidence to explain the loss of United Airlines flight 585.

This was only the fourth time in the NTSB's history that it published a final aircraft accident report with an undetermined probable cause.[5]

Following the failure to identify the cause of Flight 585's crash, another Boeing 737 crash occurred under very similar circumstances when USAir Flight 427 crashed while attempting to land in Pennsylvania in 1994.[6]

The NTSB reopened its investigation into Flight 585 in parallel with the Flight 427 investigation, due to the similar nature of the circumstances.[6]

During the NTSB's renewed investigation, it was determined that the crash of Flight 585 (and the later Flight 427 crash) was the result of a sudden malfunction of the aircraft's rudder power control unit. Another incident (non-fatal) that contributed to the conclusion was that of Eastwind Airlines Flight 517, which had a similar problem upon approach to Richmond on June 9, 1996.[7] On March 27, 2001, the NTSB issued a revised final report for Flight 585, which found that the pilots lost control of the airplane because of a mechanical malfunction. The renewed investigation concluded with a "probable cause" that stated:[1]^:114

The National Transportation Safety Board determines that the probable cause of the United Airlines flight 585 accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit. 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.