Spline couplings utilizing the one-piece, nonmetallic bushing concept (MS14169) have completed over a decade of laboratory and flight certification on military aircraft. Developed specifically to improve drive spline reliability on engine-mounted accessories, these couplings have proven effective in reducing aircraft operating costs and downtime.Faced with rising operating costs and competitive pressures, the airline industry must continually investigate alternative design modifications to equipments and systems. Airline engineers often employ payback period analyses to gauge the cost-effectiveness and financial impact of a proposed modification. This paper reviews nonmetallic spline technology and examines a typical hydraulic pump-drive spline-wear problem on JT3D and JT8D commercial aircraft engines. The analysis is based on a Net Present Value (NPV) approach, which considers both inflated and discounted cash flow. Maintenance cost analyses are presented and payback values are calculated by comparing unmodified and modified spline couplings. Key parameters affecting payback period, such as nonmetallic spline cost and reliability, are also described.