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| Aircraft Stall Warning Systems |
Warning Systems
As per the regulations, the stall warning system requirements can be satisfied by the inherent stall characteristics of the aircraft itself or by other appropriate means. Some of the most common stall warning systems are as follows:
Pre-Stall Buffet. In this case, the warning of the impending stall is provided solely by aerodynamic buffet. As the aircraft approaches the stall, the airflow across the upper cambered surface of the wing ceases to flow smoothly, it looses contact with the wing surface and it becomes turbulent. If the turbulent air then flows across the horizontal stabiliser, buffet results. In many aircraft, even some as large as the LOCKHEED AC-130 Spectre, this buffet provides the sole warning of the impending stall.
Audible Warning. Stall warning is provided by an electronic or mechanical device that sounds an audible warning as the stall speed is approached. The simplest such device is an airframe mounted stall warning horn which sounds when the airflow through it occurs at a specific angle. Slightly more sophisticated audible warning devices consist of either a pressure sensor or a moveable metal tab that actuates a switch as the stall is approached. The switch, in turn, activates an audible warning horn. In some installations, the audible warning is provided by a synthetic voice which helps to reduce warning ambiguity.
Stick Shaker. A stick shaker is a mechanical device that shakes the control column to warn of the onset of stall. A stick pusher may be installed in association with a stick shaker system in aircraft which are susceptible to the deep stall phenomenon. A deep stall affects certain aircraft designs, most notably those with a T-tail configuration, and results in a substantial reduction or, in some cases, complete loss of elevator authority making normal stall recovery actions ineffective; in many cases, a deep stall might be unrecoverable. The stick pusher is designed to prevent the pilot from allowing the aircraft to enter a stall. In all cases, the stick shaker will activate before the stick pusher.
**Special Note ( A stall warning "stick shaker" reference level signal is produced as a function of both flap position and total aircraft thrust. More specifically, a correction term Δα is combined with a standard stick shaker reference level signal which is scheduled as a function of aircraft flap position. The Δα signal is scheduled as a function of the total aircraft gross thrust coefficient. The resultant reference level signal produces advance stall warnings that are essentially invariant with engine power level.)
Angle of Attack. Stall warning systems often involve inputs from a broad range of sensors and systems and include a dedicated angle of attack sensor. At a predetermined angle of attack, calculated for each possible configuration, the angle of attack sensor triggers the activation of the stick shaker or the audible warning device as appropriate to the aircraft fitment. An angle of attack indicator may or may not be incorporated into the pilot's instrument panel. When installed, the indicator will give a visual indication of the aircraft proximity to the critical angle of attack.
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