Analysis and Design of Control Systems for Impaired Aircraft

Sevket Fatih Catpinar

doi:10.17918/00001082

Designing an automatic control system for an aircraft with a jammed elevator during flight is a complex challenge since it is impossible to predict the exact jam position before it occurs. Once a jam failure has occurred, there is often a short time frame to respond, and the situation may deteriorate rapidly as time goes by. An aircraft with an elevator jam will lose more than 50% of its ability to maneuver its flight in pitch direction. Furthermore, the aerodynamic force exerted upon the jammed elevator surface will cause persistent disturbance against the flight. These two factors will make the control of the impaired aircraft very difficult or even impossible. The nominal flight equilibrium, which is associated with the nominal flight condition, will disappear if the elevator moves away its equilibrium and jam elsewhere. The system may become unstable and uncontrollable quickly if no appropriate action is taken in time. The flight of the impaired aircraft is unpredictable, which can be a nose down, tail spin, heading to a crash. The most challenging issues are that the location of the elevator jam is uncertain and the mitigation time window is limited. Without knowing the elevator jam position, it seems impossible to know what flight equilibrium the aircraft would flight and thus there is no aircraft flight dynamics model available for the control system designer before the event occurs. The control system design and implementation may be carried out on board right after the jam occurs; however, it will waste much of the rescuable time window in accurately estimating the jam position, finding a safe flight equilibrium, assembling the flight dynamics model at the equilibrium, and designing/implementing a failure mitigation controller. The solution we propose is a single predesigned fixed controller for the equilibrium of a safe flight after an elevator jam occurs. The design does not require the exact knowledge of the jam position. It only requires the time of jam to switch to the new predesigned mitigation controller. It seems to good to be true, but it becomes possible due to the following three reasons: (1) The regulator control theory can be employed to address the uncertain persistent disturbance on the jammed elevator control surface, (2) The diversified redundancy provided by the engine thrust control can be employed to compensate the loss control authority of the elevator; and most importantly, (3) The geometry of the equilibrium manifold can be employed to determine a feasible safe flight equilibrium without knowing the elevator jam position.

Analysis and Design of Control Systems for Impaired Aircraft

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Analysis and Design of Control Systems for Impaired Aircraft

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