Stall Protection Systems (SPS): Stick Pusher and Active Prevention

Overview

While the Stall Warning System (SWS) provides an alert, the Stall Protection System (SPS) is an active safety mechanism designed to physically prevent the aircraft from entering a stall. It acts as a final barrier when the pilot fails to react to the initial warning.

Stick Pusher

The primary component of an SPS is the Stick Pusher.

• Function: It mechanically applies a forward force to the control column (elevator nose-down input) to forcefully reduce the Angle of Attack (AoA).
• Activation Point: It activates after the stall warning (Stick Shaker) but before the aircraft exceeds the Critical Angle of Attack ($AoA_{crit}$).
• Requirement: It is typically mandatory on large transport aircraft that exhibit dangerous stall characteristics (e.g., "Deep Stall" or "Super Stall" often associated with T-tail configurations) where recovery might be difficult or impossible for the pilot alone.

Other Protection Methods

Depending on the aircraft type, an SPS may also:

• Extend Flats/Slats: Automatically deploying leading-edge slats to increase the $AoA_{crit}$.
• Increase Thrust: Commanding the Autothrottle to apply power (e.g., Alpha Floor protection on Airbus).

System Logic & Diagnostics

Understanding the distinction between Warning and Protection is crucial for analyzing failure scenarios:

• Scenario: The aircraft approaches a stall. The Stick Pusher activates (nose drops), but there was no prior audible alarm or vibration.
• Analysis: The Stall Protection System is functioning correctly (it prevented the stall). The Stall Warning System (Stick Shaker/Audio) is inoperative.

System	Component	Purpose	Action
SWS	Stick Shaker / Audio	Alert	Vibrates column, sounds alarm.
SPS	Stick Pusher	Prevent	Pushes nose down.