In the context of aircraft engineering and maintenance,No Fault Found (NFF) is a chain of events that developsfrom a pilot experiencing a system malfunction with postflightmaintenance failing to reproduce the reported symptoms.Without any repair being undertaken, the malfunction may beexperienced again on subsequent flights. This present significantcost impacts to the industry that includes financial, reducedoperational achievement, airworthiness challenges and potentialflight safety issues. One of the major causes identified forNFF occurrence within electronic, mechanical and hydraulicproducts are faults that are intermittent in nature. This makes itdifficult to use systematic fault detection techniques effectively,as system are subject to unknown disturbances and modeluncertainties. The philosophy behind this criterion is that thedesigned model-based Fault Detection (FD) observer shouldbe robust to disturbances but sensitive to intermittent faultswhere the occupance of intermittent faults can be alarmed bythe use of an adaptive threshold. The aim of this paper isto demonstrate the development of such methodologies and toexamine its performance in a real-world test bed. The test bedconsists of an aircraft fuel system simulation rig which simulatesby hardware the components of an aircraft fuel system.
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