It is proposed to develop an accurate, in-service transmission life-estimation system for the prediction of remaining component and system life for a helicopter transmission system. Once proven in the helicopter environment, this life-estimation system will be of use to a wide variety of airborne and land-based transmission systems. Its use will improve the safety and reduce the maintenance costs of the monitored systems. The transmission-life estimating system will include three separate algorithms: an in-flight service monitoring algorithm, a pre-flight and post-flight transmission analysis algorithm, and a component-life tallying algorithm. The in-flight service monitor will treat the transmission as a whole in response to sampling data of mast torque and speed. The transmission analysis algorithm will determine the transmission's operating parameters from those of its components. It also will determine the life and reliability of the individual components based on the service monitoring algorithm's output. The component-life algorithm will accumulate life and reliability tables. The Phase I effort will develop the life-monitoring and supporting life-estimation and reliability algorithms. In the Phase II effort, the full life-estimating system will be assembled and tested with a helicopter main-rotor transmission.