relative to the housing The housing tube and head are stationary.
Operation. In operation, as shown In figure 4-93, fluid pressure admitted to the housing tube at
system operating pressure-of example, at 3,000 psi-enters the space formed between the piston and the housing tube
head This pressure tends to force the piston and housing tube head in opposite directions. However, since the housing
tube head cannot move and the piston can, the piston moves toward the right, pushing against the supply fluid on its right
and thus develop- ing pressure In the fluid The surface of the housing tube is exposed to 3,000 psi pressure within a
space that is one-sixtieth as large in area as the piston surface that contacts the supply fluid This means that a pressure
of 50 psi (3,000 psi - by 60) is built up in the supply fluid.
(b) Air-pressurized reservoir. Pressurizing with air is done by forcing air into the reservoir above the level of
the fluid The amount of pressure kept in an air-pressurized reservoir is usually around 15 psi No attempt is made to keep
the air and fluid separated In most cases, the initial source of air pressure In the compressor section of the aircraft
engine. Since pressure within the engine compressor is normally about 100 psi, it has to be reduced before being
delivered to the reservoir This is accomplished by passing the air through an air pressure regulator.
Baffles and Fins. Baffles and/or fins are used in most reservoirs to prevent the fluid within the reservoir from
swirling and surging These conditions could cause fluid to foam and air to enter the pump along with the fluid (see figure
Finger Strainers. Many reservoirs have strainers in the filler neck to keep foreign matter from entering when the
filler cap is off. These strainers are made of fine gauze and are called finger strainers because of their shape Finger
strainers should never be removed or punctured as a means of speeding up the pouring of fluid into the reservoir (see
Filter Elements. Filter elements are Incorporated In some reservoirs either to filter air before It enters the
reservoir or to filter fluid before It leaves the reservoir Figure 4-94 shows these filters Installed When an air vent filter
element Is used, It Is located In the upper part of the reservoir above the fluid level When a fluid filter element Is used, It
Is located at or near the bottom of the reservoir Fluid returning to the reservoir surrounds the filter element and flows
through the wall of the element This leaves any fluid contaminant on the outside of the element The fluid filter elements
commonly used In aircraft reservoirs are made of treated cellulose formed into accordion-like pleats This construction
exposes the fluid to the maximum amount of filter surface within a given space.
Filter Element Bypass Valve. Reservoirs having filter elements have a bypass valve to ensure that the pump will
not be without fluid even If the filter element becomes clogged This valve is normally held closed by a spring that would
be opened by the stronger partial vacuum that would develop If the element be- came badly clogged.
Standpipes. Some aircraft have emergency hydraulic systems that take over If the main system fails In many
such cases, the pumps of both systems obtain fluid from a single reservoir Under such circumstances, fluid for the
emergency pump is drawn from the bottom of the reservoir and the main system draws Its fluid through a standpipe
located at a higher elevation With this arrangement, operation of the emergency system is ensured should the main
Air Cylinders. Air cylinders are the pneumatic reservoirs In an aircraft pneumatic system, which serves as an
emergency source of pressure for the hydraulic system. These cylinders are made of steel and may have a cylindrical or
a spherical shape (see figure 4-95) Cooling of the high-pressure air In the storage cylinders will cause some
condensation to collect In them. To ensure positive operation of systems, storage cylinders must be purged of moisture
periodically This Is done by slightly cracking the moisture drain fitting located on the cylinder manifold.
Maintenance of Reservoirs. A typical reservoir consists essentially of a reservoir shell assembly which houses a
filter element, relief valve, and required gaskets, seals, and O-ring packings.
Drycleaning solvent is flammable and solvent vapors are toxic. Use P-D-680, Type II Solvent in a well-
ventilated area. Keep away from open flames. Avoid prolonged solvent contact with skin.
(1) Wash all parts and flush Inside of tank with drycleaning solvent, Federal Specification P-D-680. Dry with