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|Category:||Loss of Control|
A Stick Pusher, sometimes referred to as a Stick Nudger, is a device installed in some fixed-wing aircraft to help prevent an aerodynamic stall.
The International Civil Aviation Organisation (ICAO) mandated safety requirements applicable to transport category, fixed-wing aircraft are very demanding both in pre-stall handling qualities and in stall recovery characteristics. Some aircraft are not able to comply with these safety requirements by relying solely on their inherent aerodynamic traits and must, therefore, achieve the equivalent level of safety by other means. There are also a number of aircraft designs which are vulnerable to the deep stall phenomenon which potentially can make the recovery from a stall impossible. To help prevent these aircraft from being exposed to the risks asssociated with entering a stalled condition, the designer may incorporate a stick pusher, which is a stall prevention device, into the aircraft design.
A stick pusher system incorporates both a passive monitoring component and an active intervention component that consists of an hydraulic or electro-mechanical pusher mechanism. The monitoring component constantly evaluates critical flight parameters such as airspeed, load factor and flap setting against the aircraft angle of attack. If the angle of attack reaches a predetermined value, the intervention component of the system is activated and pushes forward on the elevator control system until the angle of attack is reduced to an acceptable value. The system operates autonomously and without any required action by the pilots.
A stick pusher is normally incorporated in an aircraft in conjuction with a stall warning system such as a aural alert or a stick shaker. In most circumstances, the warning system would energise prior to the activation of the stick pusher. This would allow the pilots the opportunity to correct the high angle of attack before the intervention of the stall prevention system.
There are some segments of flight such as the takeoff rotation and the landing flare where activation of a stick pusher system would not be desirable. Most stall protection systems will automatically disable the stick pusher during all or a portion of the associated flight phases.
As with almost any system, there are potential risks caused by certain failures. In this case, a failure could result in activation of the stick pusher when the flight parameters do not require it. The designer must, therefore, make provision for the flight crew to override an unwanted activation. In some aircraft, the stick pusher can be overpowered by the pilot, while in others, the stick pusher system can be manually disabled by means of a switch which is usually located on the control column.
NTSB Safety Alerts on General Aviation risks