Angle of Attack Indicator: Purpose and Use

What Is an Angle of Attack Indicator?

An angle of attack indicator (AoA indicator) provides real-time feedback on the angle between the wing's chord line and the relative airflow. Unlike airspeed, which can be misleading in certain configurations or flight conditions, AoA directly relates to lift and stall risk. Every aircraft has a specific critical angle of attack where a stall will occur, regardless of speed or altitude.

Types of AoA Sensors

• Vane-Type Sensors: These use a small vane mounted externally that aligns itself with the airflow. Its angular position is measured and sent to cockpit displays and flight computers.
• Pressure-Type (Slotted Probe) Sensors: These use a conical probe with pressure ports. As the angle of attack changes, the pressure difference across the probe rotates an internal vane, which is then measured and converted to an electrical signal.

AoA Displays

• Simple Light Arrays: Green, amber, and red lights indicate safe, caution, and dangerous (near-stall) angles, respectively.
• Numerical Gauges: Show the exact angle of attack value, often with markings for optimum approach or stall warning.

Uses and System Integration

• Stall Warning Systems: AoA is the primary input for stall warnings, alerting the pilot before reaching the critical angle.
• Flight Envelope Protection: Modern aircraft use AoA data in flight control computers to prevent exceeding safe flight limits.
• Air Data Computers: AoA information is integrated with other data (like flap and slat positions) for accurate flight management.

Implications of Incorrect AoA Indication

If an AoA sensor provides erroneous data (e.g., frozen in position), stall warnings and flight envelope protections may trigger incorrectly or fail to activate, potentially leading to loss of control. Pilots must recognize unreliable AoA indications and rely on other cues if necessary.