Intermittent fault detection on an experimental aircraft fuel rig: Reduce the No Fault Found rate

In the context of aircraft engineering and maintenance, No Fault Found (NFF) is a chain of events that develops from a pilot experiencing a system malfunction with post-flight maintenance failing to reproduce the reported symptoms. Without any repair being undertaken, the malfunction may be experienced again on subsequent flights. This present significant cost impacts to the industry that includes financial, reduced operational achievement, airworthiness challenges and potential flight safety issues. One of the major causes identified for NFF occurrence within electronic, mechanical and hydraulic products are faults that are intermittent in nature. This makes it difficult to use systematic fault detection techniques effectively, as system are subject to unknown disturbances and model uncertainties. The philosophy behind this criterion is that the designed model-based Fault Detection (FD) observer should be robust to disturbances but sensitive to intermittent faults where the occupance of intermittent faults can be alarmed by the use of an adaptive threshold. The aim of this paper is to demonstrate the development of such methodologies and to examine its performance in a real-world test bed. The test bed consists of an aircraft fuel system simulation rig which simulates by hardware the components of an aircraft fuel system.