A stick shaker is a mechanical device to rapidly and noisily vibrate the control yoke (the "stick") of an aircraft to warn the pilot of an imminent stall. It is connected to the control column of most business jets, airliners and military aircraft.
The stick shaker is a component of the aircraft's Stall Protection System, which is composed of wing-mounted angle of attack sensors that are connected to an avionics computer. The computer receives input from the AOA sensors and a variety of other flight systems. When the data indicates an imminent stall condition, the computer actuates both the stick shaker and an auditory alert.
The shaker itself is composed of an electric motor connected to a deliberately unbalanced flywheel. When actuated, the shaker induces a forceful, noisy and entirely unmistakable shaking of the control yoke. This shaking of the control yoke matches the frequency and amplitude of the stick shaking that occurs due to airflow separation in conventional aircraft as they approach the stall. The stick shaking is intended to act as a backup to the auditory stall alert, in cases where the flight crew may be distracted.
In larger aircraft (especially in T-tailed jets that might be vulnerable to deep stall), some Stall Protection Systems also include a stick pusher system to automatically push forward on the elevator control, thus reducing the aircraft's angle of attack and preventing the stall.
Both systems have to be tested and armed before takeoff and remain on during flight.
References
Patent for Boeing Stall Protection System (with description)
http://www.patentstorm.us/patents/5803408-claims.html
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