Method for controlling blade tip clearance in a gas turbine

A method to compensate for blade tip clearance deterioration between rotating blade tips (82) and a surrounding shroud (72) in an aircraft gas turbine engine (10) includes determining one or more variables based on at least one or more moving averages of one or more engine operating parameters respectively averaged over a fixed number of operational engine flight cycles and adjusting a flow rate of thermal control air (36) to counter the blade tip clearance deterioration based on the one or more variables. The engine operating parameters may include running number of engine cycles, takcoff and cruise exhaust gas temperature margins, a cruise turbine efficiency, takeoff and cruise maximum turbine speeds, a cruise fuel flow. Some or all of the variables may be differences between the moving averages and corresponding baselines of the engine operating parameters respectively or the variables may all be moving averages. The flow rate may be adjusted incrementally.