Background
United Airlines Flight 585 was a scheduled passenger service from Denver, Colorado, to Colorado Springs, Colorado. The aircraft was a Boeing 737 with registration N999UA. Visual meteorological conditions prevailed at the time of the accident, and the flight operated under an instrument flight rules flight plan.
Accident Sequence
At approximately 0944 mountain standard time, shortly after completing its turn onto the final approach course for runway 35 at Colorado Springs Municipal Airport, the airplane began a steady roll to the right and pitched nose down. Numerous witnesses observed the aircraft reaching a nearly vertical attitude before impacting the ground in an area known as Widefield Park. The airplane was destroyed in the impact. All 25 occupants—2 flight crewmembers, 3 flight attendants, and 20 passengers—were fatally injured.
Probable Cause
The official probable cause of the accident was determined to be 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.