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 manufacture, as well as on the maintenance/repair practices followed by the users. As a result of experiencing deterioration (of the engine as a whole or any of its components individually), an engine will seek a different steady operating point relative to that for an engine with no deterioration. This results in changes in the available thrust from the engine(s) at the same TETs and/or rotational-speeds. Any reduction in the available thrust at specified engine settings, especially maximum available thrust from the engines, will have a significant adverse effect upon the aircraft's operational-performance. These adverse effects can be reduced by having a better knowledge of the effects of each such deterioration on the aircraft's operational-performance. Subsequently improvements can be made in the design and manufacture of adversely-affected components as well as in maintenance and repair practices. For a military aircraft's mission-profile (consisting of several flight-segments), using a bespoke computer simulation, the consequences of low-pressure compressors' fouling of a turbofan upon the aircraft's mission operational-effectiveness have been 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, or planning different take-off times and/or carrying different fuel/weapon loads for aircraft with the various extents of low-pressure compressors' fouling. Hence improved engine utilization and the optimal mission operational-effectiveness for a squadron of aircraft can be achieved. (C) 2007 Elsevier Ltd. All rights reserved.