Research On Safety Analysis And Assessment Techniques For Digital Control System Of Civil Turbofan Engine

The safety is very important requirement for digital control system of civil turbofan engine in thesystem lifecycle. With the development of digital control system, safety analysis and assessment couldimprove the safety of the system. The safety assessment process of civil turbofan engine controlsystem is studied and the technical methods such as Functional Hazard Assessment (FHA), FailureMode and Effect Analysis (FMEA), Fault Tree Analysis (FTA) and Markov Analysis (MA) areanalyzed in this thesis.Based on analysis of related standards and references of aerospace safety, the relationshipbetween safety and reliability is analyzed. The appropriate measurement index of digital controlsystem of turbofan engine is obtained. The FHA method of digital control system is studied. Thedefinition of the system functions and failure conditions are declared and the failure effect on theengine is analyzed. The classification of failure effect and the primary safety requirements are given.The FMEA method of digital control system is researched. The digital control system of engineis divided into four subsystems: fuel distribution unit, actuators and sensors, electronic control unitand electrical system. For each component of the subsystems, the main failure modes, failure causes,fault modes effects and risk level are given.The FTA method of digital control system is studied. According to the safety requirement fromFHA, three top events “excessive fuel supply”,“excessive VSV opening” and “error to open the9thstage air valve” are analyzed, and the fault trees according to the engine control schedule and theFMEA of digital control system is built. The failure probability of the top event “error to open the9thstage air valve” is calculated.The MA method of digital control system is studied. And a reduced reliability model isestablished. A Markov model is built for the double channels and sensors of the digital control systemand the safety index is calculated.The method of assuring safety based on system theory is studied. According to the study ofhistory accidents and analysis of typical faults of digital control system, STAMP model is built andthe common failure is analyzed. Software requirement specifications and completeness is discussed.