FLOW CONTROL UNITEach FCU includes the FCV, 2 solenoids, one torque motor, one positionsensor and 2 pressure sensors.The FCU operates in MAIN or BACK-UP mode, controlled by the ACSCthrough the solenoids.The functions of the components are:- Solenoid 1 controls the ON/OFF (isolation) function. When this solenoidis energized, the FCV is open and regulates when bleed air pressure isavailable.- Solenoid 2 controls the MAIN or BACK-UP operation. When thissolenoid is de-energized, the FCV operates in MAIN mode. The solenoidis energized for BACK-UP operation.