Abstract: The main control servo valve responsive to electrical input signals modulates a power piston to position a throttle valve that varies the flow of fluid during normal operation. A transfer servo valve responsive to the pressure in the servo system moves hard over upon sensing a low pressure signal that is normally lower than the system's pressure to cause the power piston to position the throttle valve closed. In the preferred embodiment the system throttles compressor air of a gas turbine engine to provide an anti-icing function and activates the transfer servo valve upon a hard over malfunction.