A CFI explains to a student that the stall warning system uses a small heated metal vane mounted on the leading edge of the wing. What triggers this system to activate the aural or visual warning?
- AThe vane senses reduced engine manifold pressure associated with an impending stall condition
- BThe vane detects icing accumulation and warns of reduced lift before a stall occurs
- CThe vane measures airspeed directly and triggers the warning below a fixed calibrated airspeed
- DA pressure switch senses the airflow separating from the vane at a preset angle of attack, closing an electrical circuit
Show answer & explanationAnswer & explanation
Correct answer: D. A pressure switch senses the airflow separating from the vane at a preset angle of attack, closing an electrical circuit
The stall warning vane is deflected by changing airflow direction as angle of attack increases; at a preset angle just before the critical angle of attack, the airflow shifts enough to actuate a switch that completes an electrical circuit, triggering the horn or light. It is heated to prevent ice from immobilizing it, but detecting ice is not its function; it senses angle of attack, not airspeed directly.
Why the other options are wrong
- A. Manifold pressure has no relationship to the stall warning vane.
- B. The heating element prevents ice from jamming the vane; it does not detect icing itself.
- C. Stall warning is based on angle of attack, not a fixed airspeed value, since stall speed varies with weight and load factor.
Stall Warning Vane System
A heated, spring-loaded vane on the wing leading edge that senses changing airflow direction (angle of attack) and triggers a warning before the critical angle of attack is reached.
- Senses angle of attack via airflow direction, not airspeed directly
- Heated element prevents ice from jamming the vane
- Activates several knots above actual stall speed as a margin
Memory trick: Vane feels the flow shift before the wing gives up