A gas turbine engine comprising an actuation system, said actuation system comprising: a fluid supply conduit; a piezoelectric actuator; and a position control actuator, wherein: the fluid supply conduit is in fluid communication with the position control actuator, such that the position control actuator can be controlled by the supply of fluid thereto; and the piezoelectric actuator is positioned in the fluid supply conduit, such that the flow in the fluid supply conduit can be regulated by the piezoelectric actuator; the gas turbine engine further comprising a turbine case cooling system, wherein the position control actuator is connected to a valve for regulating the flow of cooling fluid to a turbine case, and/or a variable vane control system, wherein the position control actuator is used to control the position of the variable vanes; and wherein: the piezoelectric actuator is transformable between a first configuration and second configuration; and when the piezoelectric actuator is in the first configuration, fluid in the fluid supply conduit can pass from an upstream side of the piezoelectric actuator to a downstream side of the piezoelectric actuator, whereas in the second configuration fluid is blocked from passing from the upstream to the downstream side of the piezoelectric actuator. a fluid supply conduit; a piezoelectric actuator; and a position control actuator, wherein: the fluid supply conduit is in fluid communication with the position control actuator, such that the position control actuator can be controlled by the supply of fluid thereto; and the piezoelectric actuator is positioned in the fluid supply conduit, such that the flow in the fluid supply conduit can be regulated by the piezoelectric actuator; the fluid supply conduit is in fluid communication with the position control actuator, such that the position control actuator can be controlled by the supply of fluid thereto; and the piezoelectric actuator is positioned in the fluid supply conduit, such that the flow in the fluid supply conduit can be regulated by the piezoelectric actuator; the gas turbine engine further comprising a turbine case cooling system, wherein the position control actuator is connected to a valve for regulating the flow of cooling fluid to a turbine case, and/or a variable vane control system, wherein the position control actuator is used to control the position of the variable vanes; and wherein: the piezoelectric actuator is transformable between a first configuration and second configuration; and when the piezoelectric actuator is in the first configuration, fluid in the fluid supply conduit can pass from an upstream side of the piezoelectric actuator to a downstream side of the piezoelectric actuator, whereas in the second configuration fluid is blocked from passing from the upstream to the downstream side of the piezoelectric actuator. the piezoelectric actuator is transformable between a first configuration and second configuration; and when the piezoelectric actuator is in the first configuration, fluid in the fluid supply conduit can pass from an upstream side of the piezoelectric actuator to a downstream side of the piezoelectric actuator, whereas in the second configuration fluid is blocked from passing from the upstream to the downstream side of the piezoelectric actuator.