Flight Envelope Protection (FEP): Stall, Overspeed, and G-Load Limits

022-08-03Pablo Asensio Martínez2026-03-272 min

Concept and Purpose

Flight Envelope Protection (FEP) systems monitor and restrict aircraft flight parameters to ensure they remain within safe structural and aerodynamic limits.

  • Objective: To allow the pilot to use the full capabilities of the aircraft without fear of entering an unsafe state (stall, overstress, overspeed).
  • Implementation:
    • Conventional Aircraft: Often limited to warnings (Stick Shaker) or simple mechanical interventions (Stick Pusher). Corrective action relies heavily on the pilot.
    • Fly-By-Wire (FBW) Aircraft: The Flight Control Computers (FCC) process pilot inputs and actively prevent the control surfaces from moving in a way that would exceed limits. This is known as "Hard Protection".

Key Protections

  1. Stall Protection (High Angle of Attack)

    • Warning: Stick Shaker (vibrates column to simulate pre-stall buffet).
    • Prevention (Conventional): Stick Pusher (mechanically pushes nose down).
    • Prevention (FBW):
      • Alpha Prot: System actively commands nose down to prevent AoA increasing.
      • Alpha Floor: Autothrust automatically commands TO/GA (Take-off/Go-around) power if AoA is critical.
      • Alpha Max: The aircraft cannot exceed maximum AoA even with full back stick.

  2. Overspeed Protection (High Speed)

    • Prevents exceedance of $V_{MO}$ (Maximum Operating Velocity) or $M_{MO}$ (Maximum Operating Mach).
    • Action:
      • Inhibits nose-down trim.
      • Commands a pitch-up maneuver to trade speed for altitude.
      • Reduces thrust (if A/THR engaged).
    • Override: In emergency descent scenarios, pilots may need to override "soft" protections, but "hard" protections typically remain to prevent structural damage.

  3. Attitude Protection (Pitch and Bank)

    • Pitch Limits: Prevents excessive nose-up (stall risk) or nose-down (dive risk) attitudes (e.g., +30° / -15°).
    • Bank Limits: Prevents excessive banking (spiral dive risk), typically limited to 67° in FBW normal law. If the stick is released, the aircraft returns to a standard bank angle (e.g., 33°).

  4. Load Factor Protection (G-Load)

    • Prevents overstressing the airframe.
    • Limits commanded G-force (e.g., Clean configuration: +2.5g to -1.0g).

Soft vs. Hard Protection

  • Soft Protection: Returns the aircraft to the safe envelope or warns the pilot, but can be overridden by applying more force to the controls.
  • Hard Protection: Cannot be overridden by the pilot in Normal Law. The computer strictly limits control surface deflection to stay within the envelope.

Note: In the event of system failures (sensor loss, computer failure), FBW systems may revert to Alternate Law or Direct Law, where some or all protections are lost, leaving only basic stability or direct control.