Active clearance control (ACC) is a method used in gas turbines to improve fuel efficiency. This is achieved by controlling the tip clearance. It is particularly effective on turbofans since they are required to respond quickly to changes of the thrust setting.
The active clearance control in incorporated with ACC valve which receives the air from the by pass airflow and routes the air to flow over the pipes surrounding the turbine casing which in turn reduces the thermal expansion of the turbine case and maintaining the accurate clearance between the turbine case and the blade tip. This clearance should be maintained accurately which is essential for the engine efficiency and its performance. this ACC valve opening is adjusted automatically by the FADEC system in order with the throttle position.