Plane & Pilot
Tuesday, October 25, 2011

Angle Of The Wing

Angle-of-attack indicators, coming to a glass panel near you

An angle-of-attack indicator simply (or not so simply) monitors the wing's pitch with reference to the relative wind and reports that angle to the pilot, often in the form of a graphical, color readout on a horizontal or vertical instrument in the cockpit. The AOA can therefore accurately predict when the aircraft is approaching stall, regardless of airspeed, attitude or weight. (There's one instance where the AOA range of a given wing will change, and that's with the application of flaps. Lowering flaps modifies the configuration of the airfoil and produces a corresponding change in the chord line.)

Fortunately, the instrument can do much more than predict stall for those willing to study its capabilities. Properly used, an AOA can identify the exact pitch attitude for best angle or rate of climb. It also can indicate the optimum long-range cruise pitch attitude. It can serve as an instantaneous wind-shear detector, immediately suggesting a solution for any dramatic increase or decrease in wind velocity or direction. An angle-of-attack instrument will indicate the proper approach speed under all conditions of weight, CG, flap position, air density, turbulence or angle of bank.

Everyone can benefit from use of AOA rather than airspeed. The military adopted AOA early on to compensate for the demands of constantly changing weight. Military jets not only burn huge amounts of fuel, many of them depart on their missions with heavy weights of ordinance, then drop their loads and return at half their original weight, introducing major variations in stall speed and CG.

Navy and Marine pilots coming aboard a carrier use AOA religiously to maintain the proper attitude so they can snag one of the ship's four arresting cables. In fact, the Navy found the switch to using angle of attack rather than airspeed saved lives. When Navy aircraft were fitted with AOA indicators in 1957 and pilots were taught how to use the gauge, the fatality rate plummeted 50% in one year. Today, all military fighter aircraft use AOA in one form or another.

Similarly, bush pilots often fly on the ragged edge of stall during approaches to abbreviated runways. Without an AOA on the panel, the only method of monitoring approach speed is exactly that, watching airspeed and maintaining the proper number. Nothing wrong with that, but it's a little imprecise.

Imprecise can work well in the right hands. Bush pilots often fly their aircraft strictly by feel, sensing when the wing is on the ragged edge of stall and adjusting power to keep the airplane flying to the threshold. (Back in the '80s when I used to deliver Maules to the West Coast, the late Dan Spader of Maule Aircraft in Moultrie, Ga., demonstrated this technique, flying the Maule in a constant nose-high attitude actually in the stall, and adding a blast of power to cushion the touchdown. We stopped in less than 100 feet.)


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