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For actuation of high lift surfaces in modern airplanes, complex mechanical shaft transmission systems powered by central drive units are deployed. The design of mechanical actuation systems, which have a major share in the weight of secondary flight controls, is a complex and challenging engineering task. Especially for specification of essential component and system design parameters within the preliminary design phase, engineering skill and experience are of significant importance owing to many uncertainties in component data and boundary conditions. Extensive trade-offs, as well as an evaluation of the system requirements and constraints lead to an iterative and time-consuming design process. Utilizing an integrated design assistance tool, mathematical functions and constraints can be modeled on system and component level and formalized as a constraint satisfaction problem (CSP). Thus, automated consistency checking and pruning of the solution space can be achieved.

System failures represent critical design cases for the development of an actuation control system and its integration in the surrounding structures of any new aircraft. With the focus on oscillatory and transient actuation system failures, this paper presents a method that uses analytical redundancy for the robust detection of such failure cases with minimum sensor equipment. Additional background information is given about the causes for the occurrence of such flight control system failures. Particular focus is here on failures leading to control surface oscillations, so-called Oscillatory Failure Cases (OFCs). Compared to classical signal-based concepts, the benefits of the proposed model-based detection approach are lowered detection amplitudes and reduced detection times. This allows for a considerable reduction in the corresponding failure case load levels that can determine the dimensioning of the airframe structure.

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.