Selector valves are used in a hydraulic system to direct the flow of fluid. A selector valve directs fluid under system pressure to the desired working port of an actuating unit (double-acting), and, at the same time, directs return fluid from the opposite working port of the actuating unit to the reservoir.

Some aircraft maintenance instruction manuals (MIMs) refer to selector valves as control valves. It is true that selector valves may be placed in this classification, but you should understand that all control valves are not selector valves. In the strict sense of the term, a selector valve is one that is engaged at the will of the pilot or copilot for the purpose of directing fluid to the desired actuating unit. This is not true of all control valves. 

Selector valves may be located in the pilots compartment and be directly engaged manually through mechanical linkage, or they maybe located in some part of the aircraft and be engaged by remote control. Remote-controlled selector valves are generally solenoid operated.

The typical four-way selector valve has four portsa pressure port, a return port, and two cylinder (or working) ports. The pressure port is connected to the main pressure line from the power pump, the return port is connected to the reservoir return line, and the two cylinder ports are connected to opposite working ports of the actuating unit. 

Three general types of selector valves are discussed in this chapter. They are the poppet, slide, and solenoid-operated valves. Practically all selector valves currently in use come under one of these three general types.