|Stick Pusher vs. Stick Shaker On Aircraft|
A stick pusher is a device installed in some fixed-wing aircraft to prevent the aircraft from entering an aerodynamic stall. Some large fixed-wing aircraft display poor post-stall handling characteristics or are vulnerable to deep stall. To prevent such an aircraft approaching the stall the aircraft designer may install a hydraulic or electro-mechanical device that pushes forward on the elevator control system whenever the aircraft’s angle of attack reaches the pre-determined value, and then ceases to push when the angle of attack falls sufficiently.
A system for this purpose is known as a stick pusher. The safety requirements applicable to fixed-wing aircraft in the transport category, and also to many military aircraft, are very demanding in the area of pre-stall handling qualities and stall recovery.
Some of these aircraft are unable to comply with these safety requirements relying solely on the natural aerodynamic qualities of the aircraft. In order to comply with the requirements aircraft designers may install a system that will constantly monitor the critical parameters and will automatically activate to reduce the angle of attack when necessary to avoid a stall.
The critical parameters include the angle of attack, airspeed, wing flap setting and load factor. Action by the pilot is not required to recognize the problem or react to it. Aircraft designers who install stick pushers recognize that there is the risk that a stick pusher may activate erroneously when not required to do so. The designer must make provision for the flight crew to deal with unwanted activation of a stick pusher. In some aircraft equipped with stick pushers, the stick pusher can be overpowered by the pilot. In other aircraft, the stick pusher system can be manually disabled by the pilot.
Stick pushers should not be confused with stick shakers. A stick shaker is a stall warning device whereas a stick pusher is a stall avoidance device.
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 indicate 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.
|©AvStop Online Magazine Contact Us Return To News|
↑ Grab this Headline Animator