United Airlines  Boeing 737 On Final Approach Rolled To The Right Then Crashes Killing 25
 

Mar-03-91 at Colorado Spgs, CO a United Airlines scheduled 14 cfr 121  Boeing 737-291, registration: n999ua  crashes killing 25. Numerous witnesses reported that shortly after completing its turn onto the final  approach course to runway 35 at Colorado Springs municipal airport, the airplane rolled steadily to the right and pitched nose down until it reached a nearly vertical attitude before hitting the ground. None of the anomalies found with the hydraulic and flight control systems would explain an uncommanded rolling motion or initial loss of control of the airplane.

Galling found on the input shaft and bearing from the standby rudder actuator power control unit could not cause sufficient rudder deflection to render the airplane uncontrollable. The airplane encountered a number of geographically induced atmospheric phenomena including updrafts and downdrafts, gusts, and vertical and horizontal axis vortices. However, the fdr does not conclusively support an encounter of a vortex of the strength necessary to cause an Uncontrollable roll of the airplane. Probable cause: loss of control for undetermined reasons. 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 sysytem, 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 a rudder movement 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.