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. A stick shaker is connected to the control column of most civil jet aircraft and large military aircraft.
The stick shaker is a component of the aircraft's stall protection system, which is composed of fuselage or wing-mounted angle of attack (AOA) 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 indicate an imminent stall, 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 low-speed 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 armed during flight.
The vibration of the stick shaker is often heard on black box recordings of aircraft that entered a stall condition prior to a crash.
Following the crash of American Airlines Flight 191 on May 25, 1979, the FAA issued an airworthiness directive making stick shakers mandatory for both sets of flight controls on DC-10, -10F, -30, -30F and -40 series aircraft.
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