Since oil monitoring is an effective diagnostic method in aeroengine troubleshooting,an expert system OSTM is developed,which means"Oil System Troubleshooting Master".Its goal is to help maintenance personnel to analyze the complex information acquired from the oil monitoring.This software is implemented in Turbo Prolog language and customized for use in portable personal computers.An exhaustive back-chain inference algorithm is adopted.The rule ignition superiority is determined by rule sequence in the knowledge base.Hence,common faults can be picked out at first.An explanation mechanism is included as well.Further,a menu query interface is programmed for quick manipulation of particular faults.For this purpose,the fault symptoms are classified according to their respective diagnostic domains.Moreover,knowledge acquisition and knowledge base management modules have been created.Until now,the diagnostic knowledge is mainly adopted from aircraft maintenance manual.