Textron Lycoming implemented an expert system that supports acceptance testing of gas turbine engines in four independent test cells. The LEADER system, Lycoming Expert Advisor for Diagnosis and Engine Repair, is an expert system that expedites problem diagnosis by automatically analyzing engine parameters to identify faults. LEADER models the reasoning of an experienced engineer for a specific steady-state testing procedure with several hundred rules controlled by a diagnostic procedure. The system evolved from initial prototype to fielded system through an incremental expert system development methodology. Developers used diverse tools through the development lifecycle to effectively perform knowledge acquisition, knowledge base development and verification, and system implementation. This project success is a major milestone for expert system applications at Textron Lycoming.