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

022-12-04Pablo Asensio Martínez2026-04-011 min

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.