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The Most Basic Parameter of Flight Angle of attack By Jim Sparks October 1999 Angle of Attack is the most basic parameter of an aircraft in flight. Depending on configuration of the aircraft, there are specific values of angle of...


Angle of Attack can be presented in units representing airflow direction relative to the fuselage where the AOA transmitter is located. The face of the indicator is graduated from zero to one, with zero representing zero lift and one being the point of stall. A green highlighted area begins at zero and goes up to .6 or sixty percent of stall. This is considered the area of normal aircraft operations. From .6 to .85, the face of the instrument is identified in amber. Operation here corresponds to the high angle of attack phases of flight such as initial climb or flight exercise. A red area beginning at .85 through 1 is considered a "Do Not Use In Normal Operation Zone," as the margin up to stall is lower than 15 percent.

In many indicators, a movable index can be set by the flight crew to establish a specific approach speed. This display is referred to as the V/VS Range. This relates to the stall speed of the aircraft relative to a safety margin. If the stall speed of an aircraft in a specific condition such as Full Flaps for landing is 100 knots and a 1.3 V/VS reference is set on the indexer, this means the aircraft will be flying at about 130 knots or about 30 percent better than stall speed if the pilot follows the speed reference. A movable index provides the flight crew with the ability to meet special conditions such as approach in turbulence or with an engine out where a higher stall margin is needed. A specific AOA can also be set to maintain an optimum cruise angle to enable the aircraft to achieve maximum range. This setting will change depending on aircraft type, but is frequently around .2 to .3. The indicator is an electromechanical device housed in a metal case with a glass face. Inappropriate cleaning techniques, such as dusting the face with a nylon-bristled brush can result in an electromagnetic charge that can have a significant effect on the display pointer. Mechanical shocks can also destroy the calibration of this type of instrument.

In addition to the indicator, several remote displays can be strategically located in the flight deck. These include a Fast - Slow scale on the Attitude Directional Indicator (ADI) or Flight Director Indicator (FDI). The ADI offers flight crew the ability to observe aircraft attitude, plus on many of these displays, other information can be viewed including Glide Slope and Localizer data used for an instrument landing. Having the Fast - Slow display enables the crew to constantly have an awareness regarding stall margin. Should the nose of the aircraft start coming up to where the stall margin is below what the pilot adjusted into the V/ VS reference, the Fast - Slow display pointer will start to show the pilot that the aircraft is a bit on the slow side. Likewise, if approach speed is to high, this display will instruct the crew to pull the nose up when they observe the pointer on the FAST side of the display. Many Fast - Slow displays will also work with selected airspeed compared to actual speed. This capability can be of great assistance when it comes to troubleshooting as a switch selection will provide a different information source without having to swap components.

An Angle of Attack Indexer is another remote display of the AOA indicator. A device of this type is generally installed on the top of the instrument panel or glare shield. This provides the crew with the ability to monitor stall margin while they are looking out the windshield. The Indexer consists of a Green Circle or "Doughnut," which is in the center of the display. There is a Chevron "V" installed above the Circle and another Chevron below the circle. The point of the "V" always aims to the circle. The lower chevron is amber while the upper one is red. As long as the aircraft is maintaining the preset V/VS on the AOA indicator, the Green Doughnut will be illuminated. Should the aircraft speed start to increase, the amber chevron, which points in an upward direction toward the doughnut, will illuminate, advising the crew to adjust the deck angle of the aircraft nose up. Should the red chevron illuminate, the flight crew should lower the nose as they have reduced their safety margin to stall below the preset point.

In many of the newer Electronic Flight Instrument Systems, AOA displays have been transformed into speed reference indicators. These Speed Queues typically appear next to the Airspeed display and can be automatically compensated when wing configuration is modified, or if the aircraft experiences an engine failure and best single engine rate of climb or decent needs to be maintained. Most of these later systems can also receive information regarding current aircraft gross weight from the Flight Management System (FMS) to provide a highly accurate reference to the point of stall. With newer technology systems, the air flow sensor is still an analog device that is either a potentiometer or a variable core transformer, and can generally be evaluated electrically by observing inputs from the sensor to a Data Acquisition Unit or air data computer. Many airframe manufacturers provide special tooling such as stall vane protractors that will enable precise troubleshooting as well as system calibrations.

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