A multi-mode upset recovery flight control system for large transport aircraft is presented. The flight control system uses a state machine to reduce the angular rates and recover the aerodynamic envelope, then recover the attitude envelope, and finally recover from overspeed. Angular rate recovery and aerodynamic envelope recovery is performed by using the natural rate damping and aerodynamic stability of the aircraft. Once the aerodynamic envelope is recovered, the control surfaces produce predictable forces and moments again, and the normal flight control laws and protection laws are available to perform the attitude envelope recovery and the overspeed recovery. Attitude envelope recovery is performed by using roll rate control and bank angle control to return the aircraft to wings level, and by using normal load factor control and flight path angle control to return the aircraft to level flight. An angle of attack protection controller is used to prevent the aircraft from exiting the aerodynamic envelope again, and the normal load factor control is used to constrain the load factors and prevent structural failure and injury to the pilot and passengers. Once the bank angle and flight path angle have been returned to the attitude envelope, an ascending flight path angle is commanded and gravity is used to rapidly recover from overspeed. Once overspeed is recovered, a level flight path angle is commanded and the airspeed controller automatically increase the engine throttle to maintain the airspeed. The multi-mode upset recovery system has been implemented and verified on the NASA GTM simulation model. The aircraft is placed in a severe stall and steep spin upset condition, and then the upset recovery flight control system is activated. The results show that the upset recovery state machine is able to recover the aircraft to normal flight, given sufficient altitude, and that the normal load factor remains within the structural integrity envelope during the entire upset recovery process.
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