United Airlines Flight 585 - Investigations

Investigations

The subsequent investigation by the NTSB lasted one year and nine months.

Although the flight data recorder (FDR) outer protective case was damaged, the foil tape inside was intact and all the data were extractable. The FDR only recorded five parameters: heading; altitude; airspeed; normal acceleration (G loads); and microphone keying. The data proved insufficient to establish why the plane suddenly went into the fatal dive. The NTSB considered the possibilities of a malfunction of the rudder power control unit (PCU) servo (which might have caused the rudder to reverse) and the effect that powerful rotor winds coming off of the nearby Rocky Mountains might have had, but there was not enough evidence to prove either hypothesis.

Thus, the first NTSB report (issued on 8 December 1992) did not conclude with the usual "probable cause." Instead, it said "The National Transportation Safety Board, after an exhaustive investigation effort, could not identify conclusive evidence to explain the loss of United Airlines flight 585."