# Why are the positions of an aircraft's angle-of-attack vanes dependent on airspeed?

According to this answer to an earlier question of mine, the positions of the angle-of-attack sensor vanes on the A330/A340 are dependent not only on AoA, but also on airspeed (and, according to @ymb1's comment, this is true for all aircraft, not just the A330/A340):

## 1. AoA vanes are speed sensitive

An AoA vane is in essence a weather vane, but due to various aerodynamic interactions it's not perfect. From flight testing they would have worked out how speed affects the position, and based on that the sensed position is corrected by speed input.

This doesn't make sense - the airflow over the wing is qualitatively the same for a given angle of attack, no matter what the airspeed (as long as you stay below the first critical mach number, at which point you get shockwave formation and all sorts of other goodness), so how can the airspeed affect the readings from the AoA sensors? Shouldn't the airspeed merely affect how long it takes for them to weathervane in the direction of the local airflow?

• I don't have the aerodynamic answer you're looking for, but it's not Airbus A330/340 specific, it's all AoA vanes. See this Boeing Aero article (scroll down to 3. AOA MEASUREMENT). I'd recommend generalizing it (not just A330/340).
– user14897
Commented Feb 1, 2019 at 4:48

## 1 Answer

It's because the vane isn't weathervaning into the freestream, it's weathervaning into the local flow a few inches away from the skin, and because of the curvature of the nose, this flow changes with body AOA (and therefore speed) with some variation relative to the change in freestream AOA. The Stall Protection Computer software will have calibration tables derived from development flight testing to compensate for most of this.

• Assume straight-and-level flight for simplicity. You are saying that for the exact same angle-of-attack, if the aircraft weighs A pounds and is flying at X miles per hour, the direction of the local flow a few inches away from the skin will be different than if the aircraft weighs B pounds and is flying at Y miles per hour? Interesting. Commented May 5, 2019 at 1:40
• No you're inferring more from the "speed" bit than intended. The calibration tables are just {Vane angle X = True AOA Y} based on the difference between the actual nose vane angles and the test boom vane that is reading true AOA for the range of angles. To the extent that the deviation between between measured and true AOA is speed sensitive, it's that the deviation isn't constant but varies with AOA, which varies with speed for a given configuration. The calibration itself doesn't vary with speed. Commented May 5, 2019 at 12:37