positions to extend and retract the cylinder and one OFF position.
Hydraulic systems are classified as either open-center or closed-center. Open-center systems have only
open-center selector valves and closed-center systems only closed-center valves.
Hydraulic servos are physical combinations of actuators and selector valves. They are used when
precise control of movement is required and normally found in the flight control system of an aircraft.
Irreversible valves are used in line with servos to prevent feedback to the flight controls.
Ratchet valves are locking devices for actuating cylinders; they
hold the cylinders in any desired position.
If full fluid flow in one direction only is required, a simple in-line check valve is used. When full flow
in one direction and restricted flow in the opposite direction is desired, an orifice check valve is used.
When more than one function must be performed in a hydraulic system and a definite order must be
followed, sequence valves are used. Sequence valves ensure that the proper order of operations is
maintained. In a reduction of pressure or fluid flow, certain components can be cut out of the hydraulic
system to ensure an adequate supply of fluid for the essential components, such as flight controls.
Priority valves are used to automatically shut off the supply of fluid to nonessential components.