On the effects of monitoring system on redundant hydraulic flight controls in failure conditions

An active monitoring system is generally required to detect failures of the on-board hydraulic system and overcome its incorrect behaviour. Indeed, several kinds of failures can affect the flight control systems, and sometimes they can be safety-critical. This work is focused on electrohydraulic actuators equipped with proper equalization devices acting when failure modes are present. It studies the most commonly used architecture of redundancy based on the torque-sum arrangement (hydraulic motors generate torques that are summed within the gear reducer connecting the power drive unit with the motion transmission). To simulate and evaluate different behaviours of the system affected by various failure mode conditions, the author developed a dedicated computer program based on a physical-mathematical model of the whole actuation system. It is equipped with a hydraulic motor, electro-hydraulic servovalve, position feedback, and equalization control law. The author pointed out the limits of each kind of failure linked to the related equalization device and concluded that, in case of failure, the disengagement of the equalization device allows a correct actuation operation, thus preventing all critical situations. Equalization devices represent an obstacle because of the reduced contrasting action performed by the operational valve against the failed one.