Impact of Oscillatory Failure Cases in Electro-Hydraulic Actuation Systems on an Aeroelastic Aircraft

Malfunctioning of primary flight control (PFC) systems, as classified in JAR/FAR 25.671c [8, 3], represents critical design cases in the development of fault tolerant actuation systems. Besides a potential loss of control, oscillations of the control surfaces due to Oscillatory Failure Cases (OFC) may induce massive structural loads - the failure case loads - in the flexible structures of an aeroelastic aircraft (AC), thus deteriorating the fatigue life of e.g. wing, fuselage, and empennage. The approach to this problem, as outlined in [14], comprises both an analysis of the causes that may trigger such oscillations and suitable means for their reliable and fast detection. The results presented hereafter illustrate the impact of OFC on a flexible AC and to what extent the availability of an OFC sensitive monitoring system (MS) allows to alleviate these adverse effects by reducing the failure case loads level. Experimental verification is accomplished by means of a test rig representing a real PFC system.