of pressures between ambient (outside) air and
reservoir air pressures.
AIR RELIEF VALVE.An air relief valve is
normally incorporated in the air portion of the
hydraulic power system to relieve excessive air
pressure entering the reservoir due to a mal-
functioning air pressure regulator. The relief valve
shown in figure 7-10 is cylindrical in shape and
consists of a housing, poppet, spring, and adjusting
screw. This valve may be mounted directly to the
reservoir or in a line leading from the reservoir,
depending on the aircraft system design.
During operation, air pressure enters the inlet port
and contacts the poppet surface. When system air
pressure increases to 50 psi, the poppet is forced off
its seat, which allows excessive air pressure to be
exhausted to the atmosphere. When system pressure
is lowered to 49 psi, the poppet spring tension
overcomes system pressure and reseats the poppet,
thus closing the valve.
Maintenance of the valve usually includes the
replacement of all seals and the adjustment of its
This valve is designed to
relieve at a cracking (just open) pressure of 50 psi; the
reseating pressure is 49 psi. The valve will operate at
full flow when the pressure reaches 60 psi.
All pressure adjustments of relief valves must be
performed on a test bench. You can control valve
pressures by adjusting the adjusting screw on the
valve until the proper settings are obtained.
AIR BLEEDER VALVE.During hydraulic
system maintenance, it is necessary to relieve
reservoir air pressure to assist in the installation and
removal of components, lines, etc. An air bleeder
valve is incorporated within the reservoir air system
to avoid disassembly of lines or units. A similar valve
may be incorporated in reservoir return lines to
provide a means for bleeding air from returning fluid.
This type of valve is small in size and has a push
button installed in the outer case. Figure 7-11 shows
a full view schematic drawing of a bleeder valve. The
valve is made up of a body, spring, poppet, and push
When the bleeder valve push button is
depressed, pressurized air from the reservoir flows
through the valve to an overboard vent, until the air
pressure is depleted or the button is released. When
the button is released, the internal spring causes the
poppet to return to its seat. In case of malfunction,
this type of valve is replaced with a new valve.
SYSTEM OPERATION.During normal
operation, the pressurizing air source comes from
engine bleed air. See figure 7-7. This bleed air is
routed through a poppet-type, one-way check valve to
the chemical drier. The chemical drier conditions the
air by absorbing its moisture. Conditioned air is then
routed through a poppet check valve to the system air
The regulator decreases engine
bleed air pressure to a desired working pressure.
As air pressure leaves the regulator, it enters the
reservoir and acts on its piston, which, in turn,
transmits force to the fluid. If malfunction of the
regulator causes excessive reservoir air pressure, an
air relief valve will open at a preset pressure and
exhaust excessive air overboard. Fluid under
pressure in the reservoir provides a positive flow of
fluid through a one-way check valve to the suction
port of the hydraulic pump, thus preventing pump
cavitation or starvation.
Figure 7-10.Air relief valve.
Figure 7-11.Air bleeder valve.