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A system, method and apparatus for monitoring the performance of a gas turbine engine. A counter value Indicative of the comparison between the engine condition and the threshold condition is adjusted. The aircraft operator is warned of an impending maintenance condition based on the counter value a

A system, method and apparatus for monitoring the performance of a gas turbine engine. A counter value Indicative of the comparison between the engine condition and the threshold condition is adjusted. The aircraft operator is warned of an impending maintenance condition based on the counter value and determines an appropriate course of action.

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What is claimed is: 1. A method of monitoring the performance of an aircraft-mounted gas turbine engine, the method comprising the steps of: selecting at least one engine parameter to monitor, wherein the engine parameter is indicative of a deterioration condition of the engine as the engine is ope

What is claimed is: 1. A method of monitoring the performance of an aircraft-mounted gas turbine engine, the method comprising the steps of: selecting at least one engine parameter to monitor, wherein the engine parameter is indicative of a deterioration condition of the engine as the engine is operated; selecting an engine at-limit point corresponding to the parameter; selecting an engine warn point corresponding to the at-limit point, where the warn point is different than the at-limit point and provides an operating margin for the parameter between the warn point and the at-limit point; monitoring the engine parameter; determining a difference in the engine parameter between an actual value and an expected value; adjusting a counter value based on the engine parameter actual-expected difference; comparing the counter value to the warn point; setting a warning flag indicative of an impending maintenance condition when the counter value meets at least a first criterion based on said comparison; and indicating to an operator of the aircraft that the warning flag has been set. 2. The method of claim 1, wherein the step of adjasting the counter includes adjusting the counter value based on the magnitude of the difference. 3. The method of claim 2, wherein the counter is adjusted in a first direction if a non-zero difference is determined, and adjusted in a second direction opposite to the first direction if zero difference is determined. 4. The method of claim further wherein the steps of sensing and adjusting are iterated until the step of setting a warning flag is achieved. 5. The method of claim 1, wherein the deterioration condition of the engine is affected by an ambient operating environment of the engine, and the method further comprises the step of altering a flight pat of the aircraft from a first path to a second path based on the warning flag indication to thereby select a flight path permitting continued operation of the engine within at least one maintenance limit. 6. The method of claim 5, wherein the second path includes a destination having lower ambient temperature conditions than ambient temperature conditions at a destination of the first path. 7. The method of claim 1, further comprising the step of providing information to the operator on a remaining operating margin within which the engine may be operated prior to performance of a maintenance operation. 8. The method of claim 7, wherein the deterioration condition of the engine is affected by an ambient operating environment of the engine, and the method further comprises the step of altering a flight path of the aircraft from a first path to a second path to thereby operate the engine within the remaining operating margin. 9. The method of claim 8, wherein the second path includes a destination having lower ambient temperature conditions than ambient temperature conditions at a destination of the first path. 10. The method of claim 1 wherein the at one engine parameter includes a parameter of a line replaceable unit of the engine. 11. The method of claim 1, wherein the engine condition is selected from a set of engine conditions susceptible to control by reason of a selection of ambient operating environments available to the operator. 12. The method of claim 11, wherein the set of engine conditions includes at least one of an engine temperature, an engine turbine shaft speed, engine oil pressure and an engine inlet guide vane angle. 13. The method of claim 1, further comprising the step of altering the schedule of a maintenance task for said engine. 14. The method of claim 13, further comprising the step of performing said re-scheduled maintenance task. 15. The method of claim 11 where in the engine is an auxiliary power unit (APU) and wherein operating parameter is the variable inlet guide vane (IGV) angle on a APU load compressor. 16. The method of claim 15 wherein said difference in IGV angle is present when engine exhaust gas temperature (EGT) exceeds a selected reference value. 17. The method of claim 16 wherein the step of determining occurs during a on-ground main engine start driven by the APU. 18. The method of claim 11 where in the engine is a prime mover of the aircraft and wherein the operating parameter is one of a turbine shaft torque and a turbine shaft speed. 19. The method of claim 18 wherein said difference in said one of turbine torque and turbine speed is present when an engine gas path temperature downstream of a turbine exceeds a selected reference value. 20. The method of claim 19 wherein the step of determining occurs during take-off during engine full power operation. 21. A method of tracking the cumulative effect of ambient operating temperature on an aircraft-mounted gas turbine engine, the method comprising the steps of: selecting at least one parameter to be trended, said parameter having a relationship to said effect of ambient operating temperature on a gas turbine engine; selectively comparing a measured value of the parameter with an expected value of the parameter to determine a difference therebetween and adjusting a counter based on the difference determined; and warning an operator when the counter reaches a margin limit, the margin limit selected to provide a remaining operating margin for the counter between the margin limit and a final limit the final limit associated with a maximum amount of engine deterioration permissible before a maintenance operation is required. 22. The method of claim 21 wherein operation of the step of selectively adjusting the counter includes the step of only adjusting the counter if a threshold criterion for counter adjustment has been met. 23. The method of claim 22 wherein said at least one threshold criterion includes the criteria of (a) ambient temperature outside the engine is within engine operating envelope and (b) the aircraft is on the ground. 24. The method of claim 21 wherein the engine is a prime mover of the aircraft and wherein the at least one parameter to be trended is selected from the group consisting of a shaft speed of the engine and a temperature downstream of at least one turbine of the engine. 25. The method of claim 24 wherein the step of comparing occurs during aircraft take-off while the engine is at frill power. 26. The method of claim 21 wherein the at least one parameter to be trended is used as a proxy to trend another parameter, the at least one parameter to be wended having a known relationship to said another parameter. 27. The method of claim 26 wherein a reference point for said another parameter is selected, and wherein when said another parameter exceeds said reference point the at least one parameter to be trended changes from said expected value to said measured value, thereby resulting in said difference between measured and expected values. 28. The method of claim 27 wherein reference point is substantially fixed. 29. The method of claim 27 wherein reference point is calculated based on ambient environmental operating conditions of the engine. 30. The method of claim 29 where the reference point is calculated using at least one of ambient temperature and ambient pressure outside the aircraft. 31. The method of claim 26 wherein said another parameter is an exhaust gas temperature of the engine. 32. The method of claim 31 wherein the at least one parameter to be trended is an angle of variable inlet guide vane (IGV) of the engine. 33. The method of claim 32 wherein a reference temperature for exhaust gas temperature is selected, and wherein when the exhaust gas temperature exceeds said reference temperature the IGV angle changes from said expected value to said measured value, thereby resulting in said difference between measured and expected values. 34. The method of claim 21 wherein a positive difference is indicative of engine operating in a hot environment causing accelerated engine deterioration. 35. The method of claim 21 wherein the counter is incremented if a positive difference is determined. 36. The method of claim 35 wherein the counter is decremented if a non-positive difference is determined. 37. The method of claim 21 where in the counter is adjusted an amount based on a magnitude of the difference. 38. The method of claim 37 wherein the counter is adjusted upwardly a greater amount for a second difference positive magnitude than for a first difference positive magnitude, the second difference positive magnitude being greater than the first difference positive magnitude. 39. The method of claim 37 wherein an amount of counter adjustment is selected from a pre-determined schedule of difference magnitude versus counter adjustment values. 40. The method of claim 21 wherein the counter is adjusted by an amount reflective of rate of actual engine deterioration represented by said difference. 41. The method of claim 21 wherein the at least one parameter to be trended is selected from the group consisting of an operating parameter of the engine parameter and an operating parameter of line-replaceable unit (LRU) of the engine. 42. The method of claim 21 wherein the at least one parameter to be tended is an angle of a variable inlet guide vane (IGV). 43. The method of claim 42 wherein the IGV is on a load compressor and wherein the engine is an auxiliary power unit (APU) including said load compressor. 44. The method of claim 43 wherein the step of comparing occurs during an on-ground main engine start (MES) powered by the APU. 45. The method of claim 27 further comprising the step of adjusting the reference point by a selected amount after a period of engine operation to thereby provide a new reference point against which engine deterioration is measured. 46. The method of claim 33 further comprising the step of adjusting the exhaust gas temperature reference temperature by a selected amount after a period of engine operation thereby providing a new reference temperature, said new reference temperature thereby affecting IGV angle and thus affecting said difference determined. 47. The method of claim 21 wherein the at least one parameter to be trended is an oil pressure of the engine.