Abstract: An artificial neural network based expert system has been developed and is applied to aircraft turbofan engine fault diagnosis.The case study of a high bypass ratio turbofan engine has verified the feasibility of a three-layer back propagation network.The very encouraging diagnostic results prove that the fuzzy association feature, the distributed memorizing and parallel processing ability,and the random error depressing capability make it an effective and quick method which has a very bright perspective and can be used extensively for aircraft engine diagnosis.