How the Aircraft Stall Warning System Works
An aircraft stall is a condition in which the angle of attack has reached a point where lift begins to decrease. Such instances are quite dangerous, requiring a pilot to quickly fix their angle before they begin to drop. In order to prevent such hazardous incidents from occurring, many aircraft utilize the Stall Warning System which is capable of giving the pilot advance notice of any potential stall conditions. As stalls are very dangerous, understanding the functionality of the Stall Warning System and how they may be used is crucial for safe flight operations.
As is with most aircraft equipment that is used for safety, the Stall Warning System falls under aviation regulatory requirements and thus must be a part of any given aircraft. Depending on the weight, capacity, and role of the aircraft, the operational parameters of the Stall Warning System may vary. When the particular model is not included within the transport category of aircraft, there tends to be less robust regulations for stall warnings. Generally, such aircraft must have an alarm stall warning that is clear and distinct, and it should be given in both straight and turning maneuvers. The stall warning may also take advantage of either inherent aerodynamic qualities to detect a stall or may use a separate device which furnishes clear indications. While never acceptable by itself, there should also be a warning indicator that is visual as well. When the wings of the aircraft are decelerating and level, the stall warning is required to begin at a speed that is 5 knots greater than the stalling speed. Meanwhile, the stall warning should begin in advance during a turn or turning accelerated stall.
The regulations centered around transport aircraft stall warning systems are more rigorous, often being composed of increased rules to ensure safety. One example of increased rulings is in regard to when the pilot reduces the aircraft speed at a rate that does not exceed one knot per second. In such instances, the stall warning is required to begin when the speed exceeds the stall speed by either more than five knots or five percent of the calibrated air speed. Once the stall begins, it will continue until the aircraft’s angle of attack is reduced back to where the warning began. Stall warnings must also be designed to have sufficient margins during decelerated turns that feature airspeed reductions of two knots per second. With an increased warning margin, the pilot will be able to prevent the stalling by initiating a recovery upon the onset of the warning. Alongside such examples, many other rigorous rulings are also placed on the Stall Warning System of transport aircraft.
Depending on the aircraft and its regulations, there are a few common warning system types that may be used by pilots. The pre-stall buffet warning system is one in which the impending stall is announced through an aerodynamic buffet. During typical flight when the aircraft reaches a stall condition, the airflow across the upper cambered surface of the wing will begin to become turbulent. As the turbulence passes across the horizontal stabilizer, a buffet will occur to warn the pilot of stalling conditions.
Audible warning systems are those that utilize an electronic or mechanical device to emit an audible noise for stall indications. The most simplistic form of such systems is a stall warning horn that is mounted onto the airframe of the aircraft, and it will emit a noise when airflow passes through it at a specific angle. In more robust variations of the system, the device may also utilize a pressure sensor or movable metal tab for increased capability. In the most robust computer stall warning systems, there may even be a synthetic voice which can make the warning more clear for pilots.
Beyond such examples, other common devices include stick shakers and angle of attack sensors, both of which function through various means to ensure that the pilot has ample warning of an unsafe condition. With any aircraft Stall Warning System, it is crucial that all components are kept clean from contamination and are protected from icing. If the components of the Stall Warning System become dirty, stuck, or damaged, pilots will not be warned when a stall condition is approaching.
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