Aero engine Original Equipment Manufacturers (OEMs) generate more than half of their revenue from aftermarket services. This research investigates the effect of two main categories of sustainment contracts in this industry, namely Time & Material Contracts (T&MCs) and Outcome-Based Contracts (OBCs), on defense aero engine reliability. A Rolls-Royce dataset with military aero engine repairs is analyzed by using Ordinary Least Squares (OLS) regression and survival analysis with the semi-parametric Cox model and the parametric Weibull model. Results from an existing study on the civil sector are used to compare the defense with the civil aero engine aftermarket.