For an aircraft flying similar routes each day the engine baseline characteristic can be predicted and compared to the actual characteristic. The engine and aircraft parameters are monitored and entered into a programmable pocket calculator which normalises the gas generator speed (Ng), inter turbine temperature (ITT) and fuel flow (Wf) and corrects for the baseline altitude and speed. The delta Ng, ITT, Wf between the actual and baseline at the normalised output power are plotted against engine running time to give a trend plot. Deviations can be interpreted to determine when engine maintenance, including hot section inspections (HSI's), is required. HSI's scheduling based on trend plots is providing a maintenance cost saving.