Flight Envelope Protection - Function

Function

Aircraft have a flight envelope that describes its safe performance limits in regard to such things as minimum and maximum operating speeds, and its operating structural strength. Flight envelope protection calculates that flight envelope (and adds a margin of safety) and uses this information to stop pilots from making control inputs that would put the aircraft outside that flight envelope. For example, if the pilot uses the rearward side-stick to pitch the aircraft nose up, the control computers creating the flight envelope protection will prevent the pilot pitching the aircraft beyond the stalling angle of attack. As a result, even if the pilot tried to apply more and more rearward control, the flight envelope protection would cause the aircraft to ignore this command. Flight envelope protection can in this way increase aircraft safety by allowing the pilot to apply in an emergency maximum control forces while not at the same time inadvertently putting the aircraft outside the margins of its operational safety.

Examples of where this might stop air accidents are when it allows a pilot to make a quick evasive maneuver in response to a ground proximity warning system warning, or in quick response to an approaching aircraft and a potential mid air collision. In this case without a flight envelope protection system, "you would probably hold back from maneuvering as hard as you could for fear of tumbling out of control, or worse. You would have to sneak up on it, and when you got there you wouldn't be able to tell, because very few commercial pilots have ever flown 2.5 G. But in the A320, you wouldn't have to hesitate: you could just slam the controller all the way to the side and instantly get out of there as fast as the plane will take you." Thus the makers of the Airbus argue: "envelope protection doesn't constrain the pilot. It liberates the pilot from uncertainty-and thus enhances safety."