Abstract: The present invention provides a method of optimising engine wash event scheduling. An underlying deterioration gradient and a total deterioration gradient are determined. A number of engine wash events, x, are set. Engine deterioration for x engine wash events is calculated. The washed engine cost of engine deterioration for x engine wash events is calculated. The calculation of engine deterioration is iterated for different x to optimise the number of engine wash events with respect to the cost of engine deterioration. Advantageously the method systematically optimises scheduling of wash events taking into account the underlying deterioration of the engine which cannot be recovered by engine washing and the cost of the engine wash events.