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 consists of the ACC valve which mixes hot and cold air from the compressor exit and the bypass duct, respectively, to a desired temperature. The air is routed to flow through tubes surrounding the casing at each turbine stage. This air expands or contracts the turbine case and in doing so, it maintains 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. The ACC valve opening is adjusted automatically by the FADEC system depending on the throttle position.