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Overview

The Stall Protection System is the last automated defence against an aerodynamic stall — the condition where the wing exceeds its critical angle of attack and lift collapses abruptly. On conventional aircraft, the system provides escalating warnings: a stick shaker that vibrates the control column, an aural stall warning, and on some designs a stick pusher that physically forces the nose down. On Fly-By-Wire aircraft, the Flight Control Computers enforce an alpha limit that physically prevents the pilot from commanding an angle of attack beyond the maximum safe value.

How It Works

Angle-of-attack vanes mounted on the forward fuselage measure the local airflow angle continuously. The stall protection computer compares this measured alpha against the computed stall alpha for the current configuration (flap setting, speed, weight) and generates warning or protection outputs. On Boeing conventional-control aircraft, stick shaker activation occurs at approximately 1.05 times the stall speed — leaving margin for recovery before the actual stall. On Airbus FBW aircraft, the alpha floor protection commands full TOGA thrust automatically if the pilot commands an excessive angle of attack, and the control law prevents the aircraft from exceeding alpha max regardless of pilot input.

Key Components

• Angle-of-Attack Vanes: Typically two or three sensors on opposite sides of the fuselage; voting logic uses the most conservative (highest alpha) reading.
• Stall Warning Computer / ADC: Processes alpha, airspeed, flap position, and sometimes bank angle to calculate proximity to stall.
• Stick Shaker: Eccentric mass motor on the control column (Boeing, Bombardier, Embraer designs) that vibrates the stick at stall warning threshold — unmistakable tactile warning.
• Stick Pusher: Pneumatic or electric actuator that applies a forward force to the control column if the aircraft reaches the actual stall angle of attack; fitted on some aircraft types.
• Alpha Limiter / FBW Protection: In FBW aircraft, the flight control law enforces a hard alpha maximum; pilots cannot override it regardless of control input.
• Aural Warning: "STALL STALL" or similar voice warning coordinated with the tactile alerts.

Aircraft Applications

• Boeing 737-800: Dual stick shakers on captain and first officer columns; shaker activates at approximately 1.05 Vs. No stick pusher on production aircraft. AOA vanes feed the stall management yaw damper computer (SMYDC).
• Airbus A320: FBW alpha protection in Normal Law prevents alpha from exceeding alpha prot; alpha floor triggers automatic TOGA thrust. In Alternate Law, conventional stall warning takes over.
• Boeing 777: Stick shakers plus a speed trend indicator showing whether the aircraft is accelerating toward or away from stall speed; crew awareness is emphasised. FBW provides some envelope protection but retains pilot override authority.

Advantages and Limitations

Stall protection has prevented countless accidents, particularly during upset recoveries in turbulence or after automation failures. Airbus alpha limiting has near-zero stall accident rate in Normal Law operation. However, the system can be defeated by sensor failures — if angle-of-attack vanes ice over or fail to the same erroneous value, the stall warning may not activate or may activate spuriously. The Air France 447 accident underscored the importance of crew training in recognising and recovering from stall at high altitude where traditional aerodynamic cues are attenuated. Some operators and regulators have responded with enhanced upset prevention and recovery training (UPRT) requirements.