Method of monitoring a gas turbine engine

A method of monitoring a gas turbine engine performance parameter at a predetermined power setting for the engine as a determinant of engine efficiency, the method characterized in that a value for the engine performance parameter is adjusted for operational deterioration in the performance parameter, the value for the change in the parameter is given by a relationship:

• S=(a×change in the parameter at core constant pressure ratio)+(b×change in the parameter at by-pass pressure ratio);
• whereby a and b are constants determined for a specific engine type by analysis through a computer model of the specific engine with the steps of:
• a) Determination of engine performance parameter values using a computer model with deterioration assumed only in the low pressure system for the engine to provide a low data set;
• b) Determination of engine performance parameter values using a computer model with deterioration assumed only in the core pressure system for the engine to provide a core data set;
• c) Determination of engine performance parameter values using a computer model with deterioration assumed in all pressure systems for the engine to provide a combined data set;
• d) Scaling the low data set, the core data set and the combined data set to an equivalent of a predetermined value of deterioration in the value of S and plotting a relational variation for the relative changes in parameters due to the low pressure system and the core pressure system;
• and
• e) Comparing the relative changes in the parameters due to the low pressure system and the core pressure system as solutions for the constants a, b in the relationship above through use of simultaneous equation analysis and solution of the respective relational variations;
• thereby determining engine deterioration and subsequently improving the engine's operational efficiency.