The direct connection between hydraulic SE and
hydraulic system is opened for repairs, the hydraulic
the systems or components being checked or serviced
system must be bled of air to the maximum extent
is necessary to minimize the introduction of external
possible upon repair completion.
contaminants. Test units that are not properly
Hydraulic fluid can hold large amounts of air in
c o n fi g u r e d , m a i n t a i n e d , o r u s e d m a y s eve r e l y
solution. Fluid, as received, may contain dissolved air
contaminate hydraulic systems in operational aircraft.
or gasses equivalent to 6.5 percent by volume, which
It is your responsibility to make sure that hydraulic SE
may rise to as high as 10 percent after pumping.
i s m a i n t a i n e d a n d u s e d a c c o r d i n g t o ex i s t i n g
Dissolved air generates no problem in hydraulic
contamination control requirements.
systems so long as it stays dissolved, but when it comes
out of solution (as extremely minute bubbles), it
CONFIGURATION
becomes entrained or free air. Free air could enter a
system during component installation, filter element
All support equipment used to service or test
installation, or opening the system during repairs.
aircraft hydraulic systems or components is equipped
with adequate output filtration that has a rating of 3
Free air is harmful to hydraulic system
microns (absolute). The 3-micron filter assembly is a
performance. The compressibility of air acts as a soft
nonbypass variety, preferably equipped with a
spring in series with the stiff spring of the oil column in
d i ff e r e n t i a l p r e s s u r e i n d i c a t o r. I t i s i n s t a l l e d
actuators or tubing, resulting in degraded response.
immediately upstream of the major fluid discharge
Also, because free air can enter fluid at a very high rate,
ports.
the rapid collapse of bubbles may generate extremely
high local fluid velocities that can be converted into
Portable hydraulic test stands are equipped with
impact pressures. This is the phenomenon known as
recirculation cleaning manifolds and fluid sample
cavitation. Cavitation causes pump pistons and slide
valves for self-cleaning and fluid analysis before they
valve metering lands to wear rapidly, commonly
are connected to equipment under test.
causing component failure.
CLEANLINESS
Any maintenance operation that involves breaking
into the hydraulic system introduces air into the
Hydraulic SE is maintained in a clean state. All
system. The amount of such air can be minimized by
hydraulic SE is maintained as clean as practicable,
prefilling replacement components with new, filtered
consistent with its construction and use. Always keep
hydraulic fluid. Because some residual air may still be
external fluid connections, fittings, and openings clean
introduced, all maintenance of this type is followed by
and free of contamination. When not in use, protect
a thorough air bleed of the system. Most hydraulic
fittings or hose ends using metal dust caps or other
systems in high-performance aircraft are of the closed,
approved closures. You can use clean, polyethylene
airless type; they are designed to self-scavenge free air
bags if you do not have the approved metal closures,
back to the system reservoir. Air bleed valves are
providing the bags are adequately secured and are
provided at the reservoir to remove this air. Because
protected from physical damage and the entrance of
free air resulting from maintenance actions or other
water. When equipment is not being used, store it in
causes may enter the system at a point remote from the
clean, dry areas. Minimize exposure of in-service
system reservoir, the system should be extensively
equipment to precipitation, wind-driven sand, or other
cycled with full power to transfer air to the reservoir,
environmental contaminants.
where it can be bled off.
Air bleed valves are sometimes found at high
AIR BLEEDING
points in the aircraft circulatory system, filter
assemblies, and remote system components such as
Air bleeding is a service operation. In this
actuators. These valves make the removal of free air
operation, entrapped air is allowed to escape from a
easier. Refer to the applicable MIMs for the location
closed hydraulic system. For specific air bleed
and use of additional bleed points. In systems not
procedures for each model aircraft, you should refer to
equipped with additional bleed points, you may have to
the applicable MIM. Excessive amounts of free or
loosen line connections temporarily at strategic points
entrained air in an operating hydraulic system results
in the system, which permits removal of entrapped air
in degraded performance, chemical deterioration of
from remote or dead-end points. When you bleed a
fluid, and premature failure of components.
Therefore, when a component is replaced or a
system in this manner, be careful to avoid excessive
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