Stall Warning Systems (SWS): Detection Margins, Inputs, and Alerts

022-12-03Pablo Asensio Martínez2026-04-012 min

Overview

The Stall Warning System (SWS) is a critical safety system designed to alert the flight crew of an impending stall condition. Its primary function is to provide a clear and distinctive warning with sufficient margin to allow the pilot to take preventative action before the stall actually occurs.

Regulations (CS-25)

For large transport aeroplanes, regulations (CS-25.207) dictate specific performance criteria for the SWS:

  • Margin: The warning must begin at a speed exceeding the stall speed by not less than 5 knots or 5% CAS (whichever is greater).
  • Conditions: The warning must be effective in both straight and turning flight, and with landing gear and flaps in any normal position.
  • Duration: The warning must continue until the angle of attack is reduced to approximately that at which the warning was initiated.

SWS Operation

Light Aircraft

  • Sensor: Typically a Moving Vane (Flapper Switch) located on the wing leading edge.
  • Mechanism: As the Angle of Attack (AoA) increases, the stagnation point moves downwards, causing airflow to lift the vane. This closes a micro-switch.
  • Alert: Aural buzzer or flashing light in the cockpit.
  • Icing: The vane is often electrically heated to prevent ice accumulation.

Large Transport Aircraft

Modern aircraft use a more sophisticated system driven by digital computers.

Inputs

To calculate the correct stall threshold, the SWS computers require several inputs:

  1. Angle of Attack (Alpha): The primary input, measured by Alpha Probes on the fuselage.
  2. Configuration: Flap and Slat position (high lift devices change the stalling AoA).
  3. Airspeed (CAS): To ensure the 5% / 5 kt margin is met.
  4. Air/Ground Logic: To inhibit the system while on the ground.
  5. Thrust: (On some systems).

Warning Types

  • Tactile (Haptic): Stick Shaker. An eccentric weight motor vibrates the control column to simulate pre-stall aerodynamic buffet.
  • Aural: Distinctive sounds (e.g., "Cricket" sound, continuous horn) or synthetic voice ("STALL, STALL").
  • Visual: Red Master Warning light or specific PFD indications.

Failure Scenarios

  • Frozen Alpha Probe: If the AoA sensor becomes frozen (e.g., due to icing) at a specific value, the computer may fail to detect an increasing angle of attack, resulting in no stall warning being triggered.

Distinction from Protection

It is important to distinguish between Warning and Protection:

  • Stall Warning (Stick Shaker): Alerts the pilot before the stall.
  • Stall Protection (Stick Pusher): Physically pushes the control column forward to prevent the stall if the pilot ignores the warning (discussed in the next section).