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The Aircraft Energy Efficiency Program within NASA is developing technology required to improve the fuel efficiency of commercial subsonic transport aircraft. One segment of this program, the Engine Component Improvement (ECI) Project, includes Engine Diagnostics which is directed toward determining the sources and causes of performance deterioration in the Pratt & Whitney Aircraft JT9D and General Electric CF6 high-bypass ratio turbofan engines and developing technology for minimizing the performance losses. This paper presents the results of engine performance deterioration investigations based on historical data, special engine tests, and specific tests to define the influence of flight loads and component clearances on performance. The results of analyses of several damage mechanisms that contribute to performance deterioration such as blade tip rubs, airfoil surface roughness and erosion, and thermal distortion are also included.