5. Specification number and color of fluid
up of the various hydraulic and pneumatic components
will vary with the design of the hydraulic system. You
6. Position of operating cylinders during filling
should consult the current applicable technical
7. System pressure (accumulator charged or
publication for the latest information on items such as
location, pressure, temperature, and tolerances.
discharged)
8. Instructions regarding air bleeding
RESERVOIRS
Additional information may be added, when
The reservoir is a tank in which an adequate supply
required, such as the following:
of fluid for the system is stored. Fluid flows from the
1. Additional full and refill levels under various
reservoir to the pump, where it is forced through the
conditions of system pressure
system and eventually returned to the reservoir.
2. Safety precautions
The reservoir not only supplies the operating needs
of the system, but it also replenishes fluid lost through
3. Filter element servicing information
leakage. Furthermore, the reservoir serves as an
4. Total fluid capacity of the system
overflow basin for excess fluid forced out of the system
by thermal expansion (the increase of fluid volume
There are two classes of hydraulic reservoirs--
class I and class II. Class I reservoirs are constructed in
caused by temperature changes), the accumulators,
such a manner that the air and hydraulic fluid are not
and by piston and rod displacement. The reservoir also
separated. Class II reservoirs are constructed in such a
furnishes a place for the fluid to purge itself of air
manner that the pressurizing agent and fluid chambers
bubbles that may enter the system. Foreign matter
are separated. This is accomplished by installing a
picked up in the system may also be separated from the
piston between the chambers.
fluid in the reservoir, or as it flows through line filters.
Nonpressurized reservoirs are vented to the
Most nonpressurized reservoirs contain filters to
atmosphere so the reservoir can "breathe." This is done
maintain the hydraulic fluid in a clean state, free from
to prevent a vacuum from being formed as the fluid
foreign matter. They are usually located in filler necks
level in the reservoir is lowered. The vent also makes it
and internally within the reservoir. The mesh-type
possible for air that has entered the system to find a
filter (finger strainer), usually installed in the filler
means of escape.
neck, removes foreign particles from fluid that is added
to the reservoir. Internally installed filters clean the
The reservoir on aircraft designed for high-altitude
fluid as it returns to the reservoir from the system. This
flying is usually pressurized. Pressurizing assures a
t y p e o f i n s t a l l a t i o n m a y h ave a b y p a s s va l ve
positive flow of fluid to the pump at high altitudes
incorporated to allow fluid to bypass the filter if it
when low atmospheric pressures are encountered.
becomes clogged. Some modern aircraft hydraulic
On some aircraft, the reservoir is pressurized by
reservoirs do not incorporate this feature. All
bleed air taken from the compressor section of the
reservoirs containing filters are designed to permit
engine. On others, the reservoir may be pressurized by
easy removal of the filter element for cleaning or
hydraulic system pressure.
replacement.
A reservoir instruction plate is usually attached to
Nonpressurized Reservoirs
the reservoir, or it may be attached to the aircraft
Nonpressurized reservoirs are used in several
structure adjacent to the filler opening. Navy
transport, patrol, and utility aircraft. These aircraft are
specifications designate the minimum information that
not designed for violent maneuvers; in some cases,
must be contained on this plate. Figure 12-3 shows the
they do not fly at high altitudes. Those aircraft that
reservoir instruction plate. Information on an
incorporate nonpressurized reservoirs and fly at high
instruction plate must include the following:
altitudes have the reservoirs installed within a
1. Simple and complete instructions for filling
pressurized area. High altitude in this situation means
an altitude where atmospheric pressure is inadequate
2. Reservoir fluid capacity at full level
to maintain sufficient flow of fluid to the hydraulic
3. Full level indication
p u m p s . M o s t n o n p r e s s u r i z e d r e s e r vo i r s a r e
constructed in a cylindrical shape. The outer housing
4. Refill level indication
12-6
