- Jet upset
Jet upset is a catastrophic condition in high-altitude jet aircraft operations that results in the loss of control of the aircraft and many times in the loss of the aircraft itself.
Overview
The condition first gained notoriety in the late 1960s with the
Oneil incident , in which a CanadianRAF Boeing 707 exceeded the speed of sound and was damaged beyond repair.Three elements are necessary for jet upset to occur:
*High altitude wave phenomenon
*Outdated technology -airfoils with abruptmach tuck characteristics and autopilots without built-in protections and primitive servos
*Crew inattention — as little as 15 seconds.In a jet upset, the aircraft enters rising high altitude wave conditions with autopilot on and full speed trim follow up trimming nose down. Shortly after encountering wave down action, autopilot reaches force limits and disconnects. The crew is behind the aircraft, and the aircraft exceeds
critical mach with mach tuck aggravating problem. All jets have altitude/pitch/speed combinations that are unrecoverable. Through the use of drag devices or pitch/power coupling, the aircraft may respond. Control forces become very high and even trimmable stabs may not have ability to increase pitch of aircraft.Though many civilian aircraft were lost to jet upset in the 1970s, advances in aerodynamics and autopilot design have all but eliminated the problem in the last ten years. One of the most significant factors was the change to
FAR 25 , resulting to limiting autopilots to one half trim speed compared to pilot operations. Airline and civilian crews are no longer trained in jet upset or may not even be aware of the phenomenon. This is a credit to modern aerodynamics and digital autopilots proving unlimited operating parameters.References
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