The aim of this investigation is to describe a way of setting up a fuzzy-logic process to achieve diagnoses, based on gas-path measurements, for modern military turbofan engines. Each engine's gas-path components' faults are quantified, taking into account measurement noise, through a non-linear analysis. A simplified version of this theoretical tool is explained by focusing on one engine-component and assuming that only a single fault occurs at a given time. Eventually the concept is generalised to cater for multiple-component fault diagnoses. (c) 2004 Elsevier Ltd. All rights reserved.