Some aircraft hydraulic systems use fluid
pressure for pressurizing the reservoir. The reservoir
shown in figure 7-12 is a fluid-pressurized reservoir.
This reservoir is divided into two chambers by a
floating piston. The floating piston is forced down-
ward in the reservoir by a compression spring within
the pressurizing cylinder and by system pressure
entering the pressurizing port of the cylinder.
The pressurizing port is connected directly to the
pressure line. When the system is pressurized,
pressure enters the pressure port, thus pressurizing the
reservoir. This pressurizes the pump suction line and
the reservoir return line to the same pressure.
Positive pressure prevents pump starvation.
The reservoir shown in figure 7-12 has five
ports-pump suction, return, pressurizing, overboard
drain, and bleed port. Fluid is supplied to the pump
through the pump suction port. Fluid returns to the
reservoir from the system through the return port.
Pressure from the pump enters the pressurizing
cylinder in the top of the reservoir through the
pressurizing port. The overboard drain port is for the
purpose of draining the reservoir, when necessary,
while performing maintenance. The bleed port is
used as an aid in servicing the reservoir.
When you service a system equipped with this
type of reservoir, place a container under the bleed
drain port. The fluid should then be pumped into the
reservoir until air-free fluid flows through the bleed
The reservoir fluid level is indicated by the
markings on the part of the pressurizing cylinder that
moves through the reservoir dust cover assembly. See
figure 7-12. There are three fluid level markings
indicated on the cover: full at zero system pressure
(FULL ZERO PRESS), full when system is
pressurized (FULL SYS PRESS), and REFILL.
When the system is unpressurized and the pointer on
the reservoir lies between the two FULL marks, a
marginal reservoir fluid level is indicated. When the
system is pressurized and the pointer lies between
REFILL and FULL SYS PRESS, a marginal reservoir
fluid level is also indicated.
Figure 7-12.Typical fluid-pressurized reservoir.