Engine performance diagnostics play a major role in keeping track of gas turbine condition and performance and identification of possible errors or faults. One of the key performance indicators used in engine performance diagnostics is the Hot Day Exhaust Gas Temperature Margin (EGTM). Ambient humidity affects the EGTM, however humidity is not measured on current civil aircraft and consequently introduces inaccuracy in the reported EGTM. Research has been performed investigating the effect of humidity on turbofan performance, to provide corrections for the reported EGTM for ambient humidity. Test-cell corrections, simulations and historical data have been reflected and were found to be very comparable. Corrections have been developed increasing reported EGTM accuracy. However not negligible, the overall effect of humidity on the inaccuracy of the EGTM is found to be small. It is therefore concluded that correcting the EGTM for humidity is possible but will increase EGTM accuracy only slightly.

After maintenance, turbofan engines are subjected to a performance acceptance test in an indoor test-cell to demonstrate that corrected performance thresholds are met. The same indicators are also monitored after on-wing installation. Despite corrections for operation conditions, differences are observed between test-cell and subsequent on-wing performance. A comprehensive list of potential root causes for those differences was investigated using data-driven analyzes, theory and simulations. The main root causes are thermal effects, resulting from the lack of thermal stabilization during on-wing operation, and seal run-in, resulting from the initial decrease of effectiveness of replaced seals. Aircraft sensor bias and test-cell correction factors are expected to also contribute considerably. Engine bleed air and power extraction effects are negligible. The impact of inaccurate or missing throttle, temperature and humidity corrections was eliminated by application of proposed engine-specific customized corrections, which served as a successful proof of concept for improved on-wing monitoring accuracy.