Most often, pilots feel airframe buffeting caused by turbulent air hitting the tail. As an airplane decelerates in level flight, the wing's AOA increases and the air flowing off the trailing edge becomes turbulent. If the horizontal tail is immersed in that disrupted air, you can usually feel both the airframe buffeting and the yoke pulsing as the elevator responds to the turbulence.
Depending on the physical relationship between the wing and the tail, natural buffeting may not occur. For example, a horizontal T-tail might remain above the disturbed air of the low-wing airplane, which means the pilot's only stall warning might come from the artificial system such as a light or horn. In discussing stall warning devices, FAR 23.207 says "The stall warning may be furnished either through the inherent aerodynamic qualities of the airplane or by a device that will give clearly distinguishable indications under expected conditions of flight. However, a visual stall warning device that requires the attention of the crew within the cockpit is not acceptable by itself."
A natural stall warning has a few advantages. As the airplane gets closer to the stall, the buffeting usually intensifies because more flow disruption occurs as AOA increases. This variable intensity cue lets the pilot know just how close to the stall he (or she) is.. A horn or light usually comes on at one intensity and remains there from the initial warning right through the stall.
Another advantage is that a natural stall warning can't fail because of a broken wire, electrical failure, or bent stall-sensing vane. However, a pilot can ignore a natural stall warning because it's not "in his face" like a horn is.
All types of stall warnings have the same purpose - to alert pilots to the approach of a potentially dangerous condition. To accomplish this effectively, the warning must be clear and unambiguous. A meek cabin speaker may be difficult to hear if the pilot is wearing a headset, while talking on the radio, or with the engine at high power. A good stall warning must be capable of alerting the pilot in every possible flight condition.
The warning must also be useful. A warning that occurs too close to the stall may not allow enough time to avoid the stall. FAR 23.207 requires the stall warning to begin at least five knots above the stall speed. A stall warning that begins too early can become a nuisance, with the possibility that pilots eventually will ignore the "false" alarms. Recently, the FAA revised Part 23 and removed the restriction on how much in advance of the stall the warning may begin.
Pilots should watch for other natural stall warning clues as well. These include an unusually high pitch attitude, unusually high pull force on the yoke, sluggish response to flight control inputs, unexplained small wing drops or the nose wandering in yaw, and changes in wind and engine noise. These can all be indications of a changing flight situation. Natural and artificial stall warnings are best as back-up mechanisms and not as license to disregard situational awareness.
Related Articles
