It is proposed to develop an accurate, in-service transmission life-use estimation system for the prediction of remaining component and system life for a helicopter transmission system. Once proven in the helicopter environment, this life- use estimation system will be of use to a wide variety of transmission 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. 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. The component-life algorithm will accumulate life and reliability tables. The Phase I effort developed the algorithms. In the Phase II effort, the life-use monitor algorithm will be placed in its appropriate hardware and flown in a commercial helicopter to provide an improvement in that aircraft's safety. Maintenance record comparisons will be made between the predicted maintenance intervals and the present maintenance pattern for the aircraft. Any increase in flight safety will be documented. The deliverable product of the Phase II effort will be the on-board life-use monitor.