Abstract: A method of controlling tip clearance of rotor blades from a surrounding casing in a gas turbine engine. The method comprising the steps of performing a first run of the gas turbine engine, shutting down the gas turbine engine and starting the gas turbine engine for a second run. The method further comprises measuring at least one performance parameter during or after the first engine run but before a time selected at which air within the gas turbine engine, soaked to the approximate temperature of one or more surrounding engine components, is substantially displaced as a consequence of the second run. The thermal condition of at least part of the engine in accordance with the measured performance parameters is then recorded. Finally the recorded thermal condition is retrieved and the temperature of the casing is controlled during at least part of the second run in a manner dependent on it.