While the hot‐end of a gas turbine, with its high thermal stresses and vulnerability to excessive exhaust gas temperatures, tends to attract most attention from the aircraft engineer, it is the compressor which is the dominant factor in aircraft gas turbine performance. This is particularly true of smaller types of engine and older designs. In these gas turbines any reduction in compressor efficiency can have a substantial effect on the thermal efficiency and service life of an engine. Between overhauls the turbine will be running in relatively clean combustion gases, but the compressor, from the moment it enters service, will be subject to continuous fouling.