Then, a suitable cleaning agent is introduced into the
directly associated with the pump output. Case drains
hydraulic system and circulated as effectively as
should be drained and flushed separately.
possible to dislodge or dissolve contaminating
If the aircraft filters are suspected of being loaded,
substances. The cleaning operation is followed by
install new or cleaned and tested filter elements in the
complete removal of the cleaning agent, and then
aircraft hydraulic filters before flushing. Test stands
replace it with new hydraulic fluid. After purging the
must be set up and operated in according to the
system, flushing and recirculation cleaning is
requirements of the specific aircraft and the system
performed to ensure adequate decontamination.
being flushed. Adjust the test output pressure and the
Purging aircraft hydraulic systems is performed only
flow volume for normal operation of the aircraft system
upon recommendation from, and under the direct
being flushed. Monitor the reservoir level in the
supervision of, the cognizant engineering activity. The
portable test stand continuously during the flushing
cognizant engineering activity is responsible for
operation. Use approved fluid-dispensing equipment to
selecting the required cleaning agents, providing
replenish the reservoir before the level decreases to the
detailed cleaning procedures, and performing tests
half-full point. Depletion of the SE reservoir fluid may
upon completion of purging to ensure satisfactory
result in cavitation or failure of the test stand pump.
removal of all cleaning agents. Whenever possible,
Operate all the circuits (actuators) on the system
purging operations should be accomplished at a naval
undergoing decontamination until the amount of fluid
aviation depot (NADEP).
collected from the aircraft return line is equivalent to
NOTE: Organizational and intermediate main-
approximately three times the fluid capacity of the
tenance activities are not authorized to perform system
affected system. Give particular emphasis to the
purging.
operation of large displacement actuators, such as those
associated with landing gear and wing fold when
Purifying
powered by the affected system. Continuously monitor
all the filter differential pressure indicators on the
Purification is the process of removing air, water,
aircraft and in the SE. Replace any loaded filter
solid particles, and chlorinated solvents (MIL-C-81302
elements. Sample and analyze the system after cycling
and MIL-T-81533) from hydraulic fluids. Con-
of the components. If contaminant level shows
taminated fluid going to the purifier tower is first
improvement but is still unacceptable, continue the
filtered by a 25-micron (absolute) filter. The vacuum
flushing operation. If no improvement is observed, try
applied to the tower removes air, water, and chlorinated
to find the source of contamination and correct it. If
solvents from the contaminated fluid. As fluid comes
extensive system flushing fails to decontaminate the
out of the tower, it is filtered through a 3-micron
affected system adequately, request assistance from the
(absolute) filter to remove solid particles. See figure
cognizant engineering activity.
8-6. This cycle is repeated until a desired level of
cleanliness is attained. For systems contaminated with
Upon successful completion of system flushing,
air, water, and chlorinated solvents MIL-C-81302 and
recirculation clean it for a minimum period to eliminate
MIL-T-81533, you can use a purifier to clean the
possible residual debris and to ensure that the system is
aircraft and support equipment (SE) to reduce the
in acceptable condition. Sample the system after
consumption of fluid and replace the need for flushing.
recirculation cleaning to verify that contaminant level
is satisfactory. If an unsatisfactory condition is again
SELECTION OF METHOD
indicated, repeat the flushing or recirculation cleaning
operation as required. Upon successful completion of
The type of contamination present in a system
system decontamination, service the system to
determines the method by which a system is
establish proper reservoir fluid level and to bleed
decontaminated. Normally, recirculation cleaning is the
entrapped air.
most effective decontamination method, considering
maintenance man-hours and material requirements.
Purging
This method should be used whenever possible.
However, if a system is contaminated by some
Purging is a decontamination process in which the
substance other than readily filterable particles, it
aircraft hydraulic system is drained to the maximum
may be necessary to flush the system, or in certain very
extent practicable and the removed fluid discarded.
8-17
