Eine Methode zur optimalen Redundanzallokation im Vorentwurf fehlertoleranter Flugzeugsysteme


Virtuelle Integration und Systembewertung



Art der Publikation:



Christian Raksch

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This thesis investigates within the field of aeroservoelasticity the occurrence of faults in primary flight control systems, the detection of the related failures using methods of analytical redundancy and their impact on the structures of the airframe. Particular emphasis is placed on Oscillatory Failure Cases (OFCs), which occur as control surface oscillations. Such system failures must be accounted for in the development process of a flight control system and its integration in a new aircraft because these failures represent design cases for the structural loads of the airframe. Since the introduction of digital flight control systems and the fly-by-wire (FbW) technology the number of potential failure cases has increased due to a higher overall system complexity. Among such unprecedented failures are OFCs that are for example triggered by the electronic flight control system (EFCS), by oscillating air data sensors, or oscillating sensors of the inertial reference system. Their characteristic oscillatory signal signatures are unpredictable, thus belonging to the class of nondeterministic OFCs, while some other OFC causes, e.g., mechanical defects of the servo valve, lead to deterministic oscillations. OFCs become critical when the oscillation frequency matches a dominant and weakly damped structural mode of the airframe. Then an oscillating actuator control system may induce massive structural loads, the so-called failure case loads, due to the aeroservoelastic coupling of air loads, control surfaces, actuators, and the flexible structures of wings, empennage, and fuselage. Still more serious effects are deteriorated aircraft maneuverability after an OFC incident or even the entire loss of flight control. The latter, however, would only happen in a worst case scenario if the actuator oscillations remained undetected while occurring at considerable amplitudes. This is effectively prevented by providing dedicated OFC-sensitive monitoring systems, which are required by the authorities as a mandatory part of any flight control system. This dissertation presents a comprehensive approach to the investigation of oscillatory failure cases with three main fields of interest. First, the causes for the emergence of control surface oscillations are analyzed. In compliance with the experience of aircraft manufacturers and flight control system suppliers, the great variety of conceivable fault scenarios suggests that the OFC problem cannot be tackled by merely tightening the level of design requirements. Still, the investigations offer valuable clues to an OFC-tolerant actuation system design. Moreover, they allow the determination of the necessary and sufficient number of sensors for an OFC detection device. The second part of this work presents a model-based, minimum sensor-dependent OFC monitoring system (OFC-MS), which copes with the given on-board sensor equipment of an exemplary actuator control system. It is demonstrated through hardware-in-the-loop testing that the proposed method complies with all compulsory system requirements. These include robustness to disturbances, a zero false alarm rate, highest possible detection speeds at minimum achievable detection amplitudes, and an automated system reconfiguration after the failure detection. Compared to the reference monitoring system of an exemplary aircraft, the monitoring performance of the new OFC-MS concept enables a considerable reduction in the OFC detection amplitudes. Transient actuator failures like powered runaways are detected much earlier and at lower limit values, too. Thus, the main benefits of the proposed failure detection method, which is based on analytical redundancy, are improved detection amplitudes. It is finally shown in the last part of this thesis that through the availability of the OFC-MS the level of failure case loads, which are decisive for the dimensioning of the structure, can be reduced. This leads to a weight saving potential for the structural airframe design.