World developments have led the armed forces of many countries to become more aware of how their increasingly-stringent financial budgets are spent. One major expenditure for military authorities is upon aero-engines. Deterioration of an engine generally results in a lower specific thrust and higher specific fuel-consumption for the same spool speeds and turbine-entry temperatures. In order to meet the thrust requirement for the same aircraft＇s performance, the engines are ran at higher spool speeds and/or turbine-entry temperatures. This results in greater rates of fuel consumption, and a shorter life (due to the greater creep damage and low-cycle fatigue incurred). Possessing a better knowledge, of the impacts of engine deterioration on an aircraft＇s operational performance as well as its fuel and residual life-expectancy, helps the users to improve engine utilization.