United Airlines Flight 585

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United Airlines Flight 585 was a scheduled domestic passenger airline flight from the now-decommissioned Stapleton International Airport in Denver to Colorado Springs Municipal Airport in Colorado Springs, Colorado. On 3 March 1991, the Boeing 737-200, registered, carrying 20 passengers plus a flight crew of 5 (Captain Harold Green, 52, First Officer Patricia Eidson, 42, and three flight attendants) crashed while on final approach to runway 35 at the Colorado Springs airport. There were no survivors. Flight 585 suddenly rolled to the right and began to pitch downward, nose first. Attempts to initiate a go-around using a thrust increase and 15-degree flaps were unsuccessful. As the altitude decreased, acceleration increased to 4g. The 737 crashed into nearby Widefield Park, less than four miles from the runway threshold. Patricia Eidson was the first female pilot to die in an accident involving a United States jet airliner.

Investigations
The subsequent investigation by the NTSB lasted one year and 9 months. Although the flight data recorder (FDR) outer protective case was damaged, the foil tape inside was intact and all the data was 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 simply wasn't enough evidence to prove either scenario. 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."

Probable cause
The NTSB reopened the UAL 585 case after the crash of another B-737, USAir Flight 427, which occurred three-and-a-half years later. It was eventually determined that both crashes were the result of a sudden malfunction of the rudder power control unit. The pilots lost control of the airplane because "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."