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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 Indicating systems have become very commonplace in high performance aircraft. Some aircraft manufacturers will use these instruments to provide the reference for activation of the aircraft's stall protection systems. Others will provide the flight crew with visual reference to deck angle that also displays how much margin exists before a stall occurs. These instruments are particularly useful during take off and approach to landing.

An AOA transmitter senses the direction of the airstream and sends the information to the indicator. Several types of transmitter are currently used and employ either a variable core transformer or a potentiometer.

In the case of the potentiometer, the wiper serves as the data transmitting element and is mechanically connected to the airstream-sensing probe. Two pressure-sensing slots are symmetrically located above and below the centerline of the probe leading edge and an equal pressure distribution across the slots keeps the sensor centered to the relative airflow.

When the aircraft makes a pitch change, the probe remains in constant alignment with the airstream, which results in a position change between the vane and the probe case.

The Angle of Attack transmitter is installed so that the probe extends through the fuselage, perpendicular to the airstream. Icing protection is accomplished by several internal electrically operated heaters. One heating element is incorporated in the case and is used to dissipate moisture that may accumulate under the probe. A second element is installed in the probe and is a high wattage device to eliminate any erroneous indications resulting from ice build-up during flight. Controls of the heating circuits are detailed by the airframe manufacturer or the facility accomplishing the AOA system installation. Frequently, the aircraft weight on wheels sensing system will control probe heat. Sometimes a temperature sensitive resistor is used to determine when heat is needed.

Probe attachment to the aircraft is accomplished by using a mount plate that is riveted to the airframe. When the mounting plate is installed at the specified position, the transmitter-mounting angle is established. and a final adjustment is made during a test flight. Thereafter, the transmitter may be removed and replaced without affecting the calibration of the AOA system.

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The most frequently reported problems with this type system are damage to the probe. A prime consideration when washing the aircraft should be to avoid getting water directly in the area where the probe passes through the fuselage. This can result in moisture ingress that damages the internal electrical components.

Another source of damage results when the aircraft paint is removed by chemical means. If paint stripper is allowed to act on the fine bearings on which the AOA probe rotates, mechanical binding and subsequent erroneous indications will result. Another area of concern is unwanted activation of the heating system. Understanding what triggers probe heat is essential prior to conducting maintenance such as aircraft jacking or the application of external power for extended periods of time.

An interface unit receives the electrical signals from the probe and an internal computer then makes the computations needed to relate AOA vane angle to aircraft true angle of attack.

Other inputs to this computer may include flap and slat position as well as airbrakes or spoilers. In some cases, even landing gear extension and weight off wheels conditions are monitored. Once the information is processed, it is delivered to the various flight deck displays. Among these are the Angle of Attack Indicator, Fast - Slow Display on the Attitude Indicator, or the AOA Indexer. Failure of a flap position sensor is another frequent cause of erroneous indications.

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