Angle-of-Attack (AoA) Measurement: Sensors, Heating, and Failure Modes
The Angle of Attack (AoA) or alpha ($\alpha$) is the angle between the wing's chord line and the relative airflow. It is a critical parameter for stall warning and flight envelope protection.
AoA Sensors
There are two primary types of sensors used to measure AoA:
- Vane Detector: An external vane (like a wind vane) that aligns itself with the relative airflow. Potentiometers measure its angle relative to the fuselage.
- Slotted Probe (Conical): A probe with two slots (upper and lower). Differential pressure between the slots causes internal paddles to rotate the probe until pressures equalize, aligning it with the airflow.
Anti-Icing: AoA probes are generally electrically heated to prevent ice formation which could freeze the vane/probe in a fixed position.
Failure Modes
If an AoA sensor becomes frozen/stuck:
- It will fail to detect changes in AoA.
- The Stall Warning System may fail to activate even if the aircraft approaches a stall.
- Flight Envelope protections based on alpha limits will be lost.
System Outputs
AoA data is sent to the:
- Air Data Computer (ADC)
- Stall Warning System (Stick shaker/pusher)
- Flight Envelope Protection System (e.g., Alpha Floor protection)
- AoA Indicator (if fitted)