What happens when angle of attack exceeds the critical angle of attack in level flight?

• A. Lift decreases with AoA
• B. Stall is unrelated to AoA
• C. Lift increases with AoA until critical AoA, beyond which flow separation occurs and lift falls away
• D. Stalling occurs at zero AoA

Answer: (C)

The key idea is that lift rises with increasing angle of attack up to a point, and then the airflow can no longer stay attached to the wing surface. When the angle reaches the critical value, the boundary layer separates from the upper surface, the pressure distribution becomes less favorable, and lift falls off dramatically while drag increases. In level flight this stall means you must reduce the angle of attack (and typically increase airspeed) to reattach the flow and regain lift.

Other statements aren’t correct because lift does not decrease with increasing angle of attack before the critical point, stall is directly related to angle of attack, and stalling does not occur at zero AoA.