Figure 12-12.--Typical fluid-pressurized reservoir.
emergencies; that is, for operation of actuating units
When you service a system equipped with this type
of reservoir, place a container under the bleed drain
when the engine-driven pump is inoperative. Hand
port. The fluid should then be pumped into the
pumps are generally installed for testing purposes as
reservoir until air-free fluid flows through the bleed
well as for use in emergencies.
drain port.
Hand Pumps
The reservoir fluid level is indicated by the
markings on the part of the pressurizing cylinder that
Hand pumps are used in hydraulic systems to
moves through the reservoir dust cover assembly. See
supply fluid under pressure to subsystems, such as the
figure 12-12. There are three fluid level markings
landing gear, flaps, canopy, and bomb-bay doors, and
indicated on the cover: full at zero system pressure
to charge brake accumulators. Systems using hand
( F U L L Z E RO P R E S S ) , f u l l w h e n s y s t e m i s
pumps are classified as emergency systems. Most of
pressurized (FULL SYS PRESS), and REFILL. When
t h e s e s y s t e m s m a y b e u s e d e ff e c t ive l y d u r i n g
the system is unpressurized and the pointer on the
preventive maintenance.
reservoir lies between the two FULL marks, a marginal
Double-action type of hand pumps are used in
reservoir fluid level is indicated. When the system is
pressurized and the pointer lies between REFILL and
hydraulic systems. Double action means that a flow of
FULL SYS PRESS, a marginal reservoir fluid level is
fluid is created on each stroke of the pump handle
also indicated.
instead of every other stroke, as in the single-action
type. There are several versions of the double-action
PUMPS
hand pump, but all use the reciprocating piston
principle, and operation is similar to the one shown in
All aircraft hydraulic systems have one or more
figure 12-13.
power-driven pumps and may have a hand pump as an
This pump consists of a cylinder, a piston
additional source of power. Power-driven pumps are
containing a built-in check valve (A), a piston rod, an
the primary source of energy, and may be either
operating handle, and a check valve (B) at the inlet
engine-driven or electric-motor driven. As a general
port. When the piston is moved to the left in the
rule, motor-driven pumps are installed for use in
12-12
