What is "flight envelope protection," which the A320 pioneered?

An aircraft's flight envelope corresponds to the various critical operating limits beyond which its controllability and structural integrity may be at risk. Flight envelope protection calculates that envelope, adds a safety margin to it, and uses that information to prevent pilots from making control inputs that would place the aircraft outside it. The protections are embedded in the flight control laws generated by the onboard flight control computers, and are intended to keep the aircraft within preset parameter limits. Their purpose is to allow the best possible performance and control authority while minimizing the risk of overcontrolling, losing control, overstressing, or damaging the aircraft. One of them is high angle-of-attack protection, against the risk of an aerodynamic stall, even in wind shear situations. Low energy and alpha floor protections ensure that an adequate level of in-flight energy is maintained, with corrections via manual or automatic thrust increases.

The A320 was the first commercial aircraft to incorporate full flight envelope protection in its flight control software. The initiative came from Bernard Ziegler, Airbus's former senior vice president of engineering. On the Airbus, the protection cannot be fully overridden, although the crew can fly beyond the envelope limits by selecting an alternative "control law." Boeing took a different approach on the 777, allowing the crew to override the envelope limits by applying excessive force to the controls.

Loss-of-control-in-flight (LOC-I) accidents have fallen by 89% in the latest generations of commercial aircraft equipped with this envelope protection.