Dual pump system with isolating valve.
Valves in the fuel or hydraulic systems to separate and isolate subsystems in case of any failure. For example, a fuel-isolation valve isolates the pump from its servocontrol system, except the overspeed governor. The valve permits the pump to operate at a full stroke in the event of a sudden drop in the engine RPM caused by a failure of the servosystem, which leads to the piston moving to the minimum stroke. It also is used as a safeguard against the total failure of the system during takeoff and is switched off once a safe height and speed have been achieved. If the isolation valve remains on in flight, it will result in significantly higher fuel consumption. Similarly, in some aircraft, the hydraulic-isolation valve isolates the main systems, like those lowering the undercarriage and the flaps, from the operation of other hydraulically operated systems in case of any leaks.