made in such cases to eliminate any possible transfer of fluid from one pressure source to the other. This is done by
means of shuttle valves. Army aircraft equipped with dual brake systems use shuttle valves to isolate the pilot and
copilot master cylinders from each other so as to allow optional operation of the brakes by either set of master cylinders.
Also, Army aircraft having compressed-air equipment for emergency operation of landing gears and wheel brakes use
shuttle valves to isolate the system hydraulic fluid from the compressed air or other authorized gas used in emergency
operation. Two types of shuttle valves are used in Army aircraft the floating-piston type, and the detent-type.
Floating-piston type. In the floating-piston type shown in figure 4-132, fluid entering the shuttle
valve from pressure source 1 moves the piston to block the flow from pressure source 2 so that fluid flows to the unit
being actuated from pressure source 1 only. Conversely, fluid entering from source 2 would reposition the piston to block
the flow from source 1; fluid would then flow to the unit being actuated from pressure source 2 only. The piston is
restrained against random movement by the slight drag of the O-ring against the cylinder walls.
Figure 4-132. Floating-Piston Shuttle Valve
Detent-type. The detent-type shuttle valve operates on the same general principles as the
floating-stop type, with the exception that a spring-loaded detent ball holds the piston in the position dictated by the
Actuating Cylinders (Actuators). Actuating cylinders transform energy in the form of fluid pressure into
mechanical force so that the energy can perform work. They impart powered linear motion to some movable object or
mechanism. A typical actuating cylinder is made of a cylinder housing, one or more pistons and piston rods, and the
necessary seals to prevent leakage between the piston rods and the end of the cylinder. The cylinder housing contains a
polished bore in which the piston operates and one or more ports through which fluid enters and leaves the bore.
Actuating cylinders come in two mayor types single-action and double-action.
Single-action. The single-action (single port) actuating cylinder causes movement in one
direction only. Operation of this type of cylinder controlled by a three-way control valve. In this single-action cylinder
shown in figure 4-133, fluid under pressure enters the port at the left and pushes against the face of the piston, forcing
the piston to the right As the piston moves, alms forced out of the spring chamber through the vent and the spring is
compressed. When pressure on the fluid is released to a point less than the force in the compressed spring, the spring
pushes the piston toward the left. As the piston moves to the left, fluid is forced out of the fluid port and air enters the
spring chamber through the vent.
Double-action. The double-action (two-port) actuating cylinder causes movement in two
directions Operation of this type of cylinder is usually controlled by a four-way selector valve. Placing the selector valve
in the on position (see figure 4-134), admits fluid pressure to the left-hand chamber of the actuating cylinder, thus forcing
the piston to the right. As the piston moves to the right, it pushes return fluid out of the right-hand chamber and through
the selector valve to the reservoir When the selector valve is placed in its other on position (see figure 4-135), fluid
pressure enters the right-hand chamber and forces the piston to the left As the piston moves to the left, it pushes return
fluid out of the left-hand chamber and through the selector valve to the reservoir. Besides having the ability to move a
load into position, a double-action cylinder can hold a load in position in opposition to any force which may tend to
displace it. This capability exists because, when the selector valve used to control operation