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| Title: | In-flight upset, United Airlines Flight 585, Boeing 737-291, N999UA, Uncontrolled Collision with Terrain for Undetermined Reasons, 4 Miles South Of Colorado Springs Municipal Airport, Colorado Springs, Colorado, March 3, 1991 |
| Micro summary: | This Boeing 737-291 encountered an in-flight upset and subsequent violent collision with the terrain. |
| Event Time: | 1991-03-03 at 0944 MST |
| File Name: | 1991-03-03-US.pdf |
| Publishing Agency: | National Transportation Safety Board (NTSB) |
| Publishing Country: | USA |
| Report number: | NTSB-AAR-92-06 |
| Pages: | 170 |
| Site of event: | 3.47 nm south of south end of runway 35, .17 nm to east of CL of RWY 35 |
| Latitude/Longitude: | N38°44'09.4" W104°42'42.4" |
| Departure: | Denver Stapleton International Airport, Denver, Colorado, USA |
| Destination: | Colorado Springs Airport, Colorado Springs, Colorado, USA |
| Airplane Type(s): | Boeing 737-291 |
| Flight Phase: | Descent |
| Registration(s): | N999UA |
| Operator(s): | United Airlines |
| Type of flight: | Revenue |
| Occupants: | 25 |
| Fatalities: | 0 |
| Serious Injuries: | 0 |
| Minor/Non-Injured: | 0 |
| Other Injuries: | 0 |
| Executive Summary: | On March 3, 1991, a United Airlines Boeing 737, registration number N999UA, operating as flight 585, was on a scheduled passenger flight from Denver, Colorado, to Colorado Springs, Colorado. Visual meteorological conditions prevailed at the time, and the flight was on an instrument flight rules flight plan. Numerous witnesses reported that shortly after completing its turn onto the final approach course to runway 35 at Colorado Springs Municipal Airport, about 0944 Mountain Standard Time, the airplane rolled steadily to the right and pitched nose down until it reached a nearly vertical attitude before hitting the ground in an area known as Widefield Park. The airplane was destroyed, and, the 2 flight crewmembers, 3 flight attendants, and 20 passengers aboard were fatally injured. The National Transportation Safety Board, after an exhaustive investigation effort, could not identify conclusive evidence to explain the loss of United Airlines flight 585. The two most likely events that could have resulted in a sudden uncontrollable lateral upset are a malfunction of the airplane's lateral or directional control system or an encounter with an unusually severe atmospheric disturbance. Although anomalies were identified in the airplane's rudder control system, none would have produced a rudder movement that could not have been easily countered by the airplane's lateral controls. The most likely atmospheric disturbance to produce an uncontrollable rolling moment was a rotor (a horizontal axis vortex) produced by a combination of high winds aloft and the mountainous terrain. Conditions were conducive to the formation of a rotor, and some witness observations support the existence of a rotor at or near the time and place of the accident. However, too little is known about the characteristics of such rotors to conclude decisively whether they were a factor in this accident. The issues in this investigation focused on the following: 1 Potential meteorological hazards to airplanes in the area of Colorado Springs, Colorado, especially on the approach and departure paths associated with Colorado Springs Municipal Airport. 2. Potential airplane or systems anomalies that could have precipitated a loss of control. 3. The design of the mach rudder power control unit servo valve that could present significant flight control difficulties under certain circumstances. Recommendations concerning these issues were addressed to the Federal Aviation Administration. |
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