A means of storing hydraulic fluid and minimizing contamination is necessary to any aircraft hydraulic
system. These functions are performed by reservoirs and filters. The component which causes fluid
flow in a hydraulic system--the heart of any hydraulic system--can be a hand pump, power-driven pump,
accumulator, or any combination of the three. Finally, a means of converting hydraulic pressure to
mechanical rotation is sometimes necessary, and this is accomplished by a hydraulic motor.
The hydraulic reservoir is a container for holding the fluid required to supply the system, including a
reserve to cover any losses from minor leakage and evaporation. The reservoir can be designed to
provide space for fluid expansion, permit air entrained in the fluid to escape, and to help cool the fluid.
Figure 1-1 shows two typical reservoirs. Compare the two reservoirs item by item and, except for the
filters and bypass valve, notice the similarities.
Filling reservoirs to the top during servicing leaves no space for expansion. Most reservoirs are
designed with the rim at the filler neck below the top of the reservoir to prevent overfilling. Some
means of checking the fluid level is usually provided on a reservoir. This may be a glass or plastic sight
gage, a tube, or a dipstick. Hydraulic reservoirs are either vented to the atmosphere or closed to the
atmosphere and pressurized. A description of each type follows.
Vented Reservoir. A vented reservoir is one that is open to atmospheric pressure through a vent line.
Because atmospheric pressure and gravity are the forces which cause the fluid to flow to the pump, a
vented reservoir is mounted at the highest point in the hydraulic system. Air is drawn into and
exhausted from the reservoir through a vent line. A filter is usually installed in the vent line to prevent
foreign material from being taken into the system.
Pressurized Reservoir. A pressurized reservoir is sealed from the atmosphere. This reservoir is
pressurized either by engine bleed air or by hydraulic pressure produced within the hydraulic system
itself. Pressurized reservoirs are used on aircraft intended for high altitude flight, where atmospheric
pressure is not enough to cause fluid flow to the pump.
In reservoirs pressurized by engine bleed air, the amount of air pressure is determined by an air
pressure regulator--usually 10 to 15 pounds per square inch (psi) gage. An example of a