Some in-service deterioration in any mechanical device, such as an aircraft's gas-turbine engine, is inevitable. However, its extent and rate depend upon the qualities of design and manufacturing, as well as on the maintenance/repair practices followed by the users. Each deterioration has an adverse effect on the performance and shortens the reliable operational life of the engine. The adverse effect on the life-cycle cost can be reduced by determining the realistic fuel and life-usage and by having a better knowledge of the effects of each such deterioration on operational-performance. Subsequently improvements can be made in the design and manufacturing of adversely-affected components as well as maintenance and repair practices. For a military aircraft's mission-profile (consisting of several flight-segments), using a bespoke computer simulation, the consequences of engine deterioration upon the operational effectiveness and fuel consumption are predicted. This will help in making wiser management decisions (such as whether to remove the aero-engines from the aircraft for maintenance or to continue using them with some changes in the aircraft's mission profile), as well as planning different take-off times for aircraft with the various types and extents of engine deterioration. Hence improved engine utilization, lower total fuel consumption and the optimal mission operational effectiveness for the squadron of aircraft can be achieved.