• An aircraft’s configuration is the physical appearance and weight disposition of the aircraft: it includes a description of the gross weight at take-off, aerodynamic characteristics during the flight as well as such information as whether the aircraft is loaded with external weapons such as bombs, rockets, missiles for the purpose of air interception/ground attack.

• A clean engine is one that is not suffering any performance deterioration.

• Compressor-washing or cleaning means washing or cleaning the blades of the compressor in order to remove deposits (e.g. dust, dirt, ash, soot and carbon particles) using techniques such as with jets of pressurized water (i.e. compressor washing) or blasting (i.e. compressor cleaning) with sand or walnut seeds.

• The deterioration index (DI) is a hypothetical term defined for the purpose of this investigation. It combines the effects of any reduction in (i) flow capacity and/or (ii) efficiency of any gas-path component into a single parameter. It is presumed, for the purpose of this investigation, that a 1% lowering of efficiency accompanied by a 0.5% reduction in flow capacity results in a 1% deterioration index. Linear relationships are assumed and so a 2% DI represents the combined effect of 2 and 1% falls in efficiency and flow capacity respectively.

• A flight-cycle is the total length of flight path covered by the aircraft starting from rolling out the aircraft for take-off until its landing back, taxiing and finally switching-off the engines. A flight cycle consists of all the flight-segments.

• A flight-phase is the flight path covered by the aircraft for two or more consecutive flight-segments and/or phases partially or in full, besides the flight path covered between first and last two points on the mission profile. The path covered between first two points is called take-off phase, whereas between last two points is called as landing phase. Climbing, to a pre-selected altitude (e.g. 6000 m) while accelerating to a pre-selected mach number (=0.7), followed by cruising at Mach number of 0.7 for 30 s, is a flight-phase. Here both the consecutive flight-segments climbing and cruising are covered in full. Whereas, touch-and-go is another flight-phase, however, in this case aircraft covers the landing and take-off phases partially. For the landing phase, the first two segments (i.e. approach and flare) are completed, whereas only a very small part of the third segment (i.e. taxi back) is covered. For the take-off phase only the latter part of ground-roll segment is covered, whereas remaining two segments (i.e. transition and climb to clear obstacle height are satisfied fully.

• A flight-segment is the flight path between two consecutive points on the mission profile except the first two and last two points on the mission profile. First two points specify the take-off phase which itself consists of three flight segments (i.e. ground-roll, transition and climb to clear obstacle height flight segments). The last two points specify the landing phase, which itself consists of three flight-segments (i.e. approach, flare and taxi-back flight-segments). During a flight-segment, the aircraft’s flying attitude remains same and is governed by the values of the characteristics of the consecutive points. For example climb to a pre-selected altitude (=6000 m) while accelerating to a pre-selected Mach number (=0.7) is the first flight-segment in the assumed mission profile. This flight-segment is defined by the values of the characteristics for respective points of the mission profile (as specified by the user through the input file).

• The gas-path is the track through the engine, whereby air travels from the ambient environment via the engine’s entrance until it is ejected once again to the atmosphere. All the engine’s components, through which the air flows, such as the intake, compressor(s), combustor and turbine(s) are called gas-path components.

• The handle is a set operating parameter, whose value is held constant, relative to which all other parameters are measured. The parameters are normally the measured dependent variables, which influence the engine’s performance. They typically include pressure, temperature, power output and fuel flow.

The contribution coefficient (CC) is unity for PMPs contributing positively, whereas it is minus unity for those which affect the mission’s operational-effectiveness adversely. For example, CC will be unity for any reduction in take-off distance or take-off time, whereas it will be minus unity for any reduction in height that is gained during the take-off phase by the aircraft with engines suffering deterioration. It will also be minus unity for any extra time required to take-off or for an additional runway distance required, whereas it will be taken as unity for any additional height that is gained during the take-off phase. The priority factor (PF) is introduced in order to indicate relative importance among the different PMPs. It ranges from zero to 100, such that sum of all the priority factors is always equal to 100. Changes in the performance-monitoring parameters are indicated as percentages relative to the value for clean engine(s) [or any other specified condition of the engine(s), which may be used as a benchmark].

• On-condition maintenance is a term used to indicate that maintenance is not undertaken at regular time intervals or engine running times, but on the basis of the actual condition of the engine.

• The take-off phase consists of the following three flight segments: (i) The ground-roll flight-segment is the distance travelled by the aircraft before the wheels leave the ground; (ii) the transition flight-segment, during which the aircraft accelerates from take-off speed (1.1 times the stall speed) to climb speed (1.2 times the stall speed); and (iii) the climb to clear surrounding obstacles. The required obstacle clearance is typically 15.24 m for military and 10.67 m for commercial aircraft.

• Touch-and-go practice is the activity whereby the aircraft comes in for landing but, just after touch down, it accelerates and takes off again, instead of slowing down gradually and finally switching off.

High-performance aircraft, as used in modern aviation especially for military purposes, are complex in design and required to operate under severe stresses and temperatures. Thus users of these aircraft continually search for greater reliability and availability, improved performance and safety as well as low LCCs. In-service costs consist mainly of those associated with (i) the fuel consumed during the operation and (ii) the replacement of the system’s components. Therefore, any extensions of life expectations or reductions in fuel-usage of an aircraft’s gas-turbine engines directly lower the LCC and depend upon the types of operation or mission undertaken, operating conditions experienced and rate of in-service engine deterioration. Each type of the latter has an adverse effect on the performance and reliable operational-life of the aircraft and therefore contributes to an increased LCC.

Several publications describe engine-performance deterioration and engine diagnostics using gas-path-analysis techniques: a pertinent generic computer-program, called ‘ ’, has been developed. For a JT9D engine, Sallee and Sallee et al. devised mathematical models to predict the reductions in flow capacity and efficiencies of the LPC, HPC, LPT and HPT engine components, arising due to faults such as increased tip-clearance or airfoil erosion.