In this work a robust design method for observer-supported fault detection filters by the example of a modern aviation actuator and regarding an Inflight application is examined. The robustness against external influences and the stability of the filter with failing suggestion represent core characteristics for the employment in flight. To take account of that, a filter structure directed toward additives faults was considered. Therefore, apart from the question of the representable robustness, concepts are to be sketched, in order to be able to treat multiplicative, typically internal errors.