The effects of in-service damage and wear on military turbofan performance are investigated. Relationships between measurable physical changes in engine components and losses in engine performance are developed in a two-step process. Measurable damage and wear are related to efficiency and mass flow losses in turbines or compressors from a collection of empirical and experimental data. A digital computer model of a twin-spool mixed-flow turbofan is employed to determine the resulting changes in thrust, specific fuel consumption, turbine life and compressor surge margin. A method is presented to calculate the degradation in mission capability for two representative military tasks. The identification of a deteriorated component by observing easily monitorable parameters is described. Data and procedures are included to allow further investigations of the consequences of in-service deterioration of a military turbofan.