United Airlines Flight 585
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| Summary | |
|---|---|
| Date | March 3, 1991 |
| Type | Mechanical Malfunction |
| Site | Colorado Springs, Colorado, USA |
| Fatalities | 25 |
| Injuries | 0 |
| Aircraft | |
| Aircraft type | Boeing 737-291 |
| Operator | United Airlines |
| Tail number | N999UA [22742] (f/n 1299) |
| Passengers | 20 |
| Crew | 5 |
| Survivors | 0 |
United Airlines Flight 585 was a scheduled domestic passenger airline flight from the now-decommissioned Stapleton International Airport in Denver to Colorado Springs Airport in Colorado Springs, Colorado.
On March 3, 1991, the Boeing 737-200, registered N999UA, carrying 20 passengers and 5 crew under the command of Capt. Harold Green and First Officer Patricia Eidson, crashed as it was approaching the runway for landing, killing all 25 people on board.
While on final approach to Runway 35 at the Colorado Springs Airport, 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 4 times the acceleration due to gravity. The 737 crashed into nearby Widefield Park, exploding on impact. Patricia Eidson was the second female pilot to die in an accident involving a United States commercial airliner. The first female pilot fatality, was that of First Officer Zilda A. Spadaro-Wolan, in the Henson Airlines flight 1517 crash of September 23, 1985 near Groettoes, Virginia.
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[edit] Investigation
What had investigators baffled was the lack of any apparent pre-impact failure or malfunction of the aircraft's mechanical and electrical systems, and that both engines were producing power prior to impact.
The DFDR was extensively damaged in the impact; and the tape medium was found to be broken and crumpled. Investigators were able to extract data from the tape, however the recording only contained five channels even though the recorder was capable of recording many more parameters.
Many possible scenarios were considered; rotor winds from Pike's Peak, and wake turbulence from other aircraft were considered early on in the investigation but they turned out to be inconculsive.
[edit] Probable Cause
The subsequent investigation by the NTSB lasted one year and 9 months, but could not pinpoint the cause. However, after the crash of another B-737, USAir Flight 427, the investigation was re-opened in the UAL 585 case, and it was finally determined that both crashes were the result of a sudden malfunction of the rudder power control unit (PCU). 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." (NTSB revised report)
