by the normal system fluid flow. Remove contaminated
If contamination is severe, or if aircraft filters are
fluid in these circuits and associated components by
suspected of being loaded or damaged, or if differential
partially disassembling the unit. Drain and totally flush
pressure indicators have been activated, install new (or
the unit.
cleaned and tested) filter elements in the aircraft before
you begin cleaning. Set up and operate the test stand in
Generally, system flushing continues until analysis
a manner compatible with the requirements of the
of the return line fluid from the system being
specific aircraft and system being powered. Adjust the
decontaminated indicates that the fluid is acceptable. If
test stand output pressure and low volume for normal
there is severe contamination, considerable quantities
operation of the aircraft system being recirculation
of hydraulic fluid may be expended, making it
cleaned.
important to closely monitor the portable hydraulic test
stand reservoir level, and replenish it as required.
Operate all circuits (actuators) on the system
Flushing effectively decontaminates systems con-
undergoing decontamination a minimum of 15
taining water, large amounts of gelatinous-type
complete cycles, or according to procedures in the
materials, or fluid that is chemically unacceptable
specific MIM or MRCs. Give particular emphasis to the
(containing chlorinated or other solvents). This type of
operation of large displacement actuators, such as those
fluid contamination or degradation cannot be remedied
associated with landing gear and wing fold, when
by conventional filtration. In severe cases of particulate
powered by the affected system. Continuously monitor
contamination, such as those that result from major
all filter differential pressure indicators, both on the
component failure, flushing techniques may more
aircraft and on the portable hydraulic test stand, during
easily correct the problem than will recirculation
the cleaning process. Replace any loaded filter
cleaning.
elements.
Detailed procedures for flushing hydraulic systems
Sample and analyze the system after the cycling of
are found in the aircraft MIMs. The basic procedures
components. If the contaminant level shows
are discussed in the following text, and will give you
improvement but is still unacceptable, repeat the
some idea of the procedures used when flushing aircraft
recirculation cleaning process. If no improvement is
hydraulic systems. Remember, use the MIM for the
observed, attempt to determine the source of
specific procedures to use when flushing hydraulic
contamination. System flushing may be required.
systems. Use flushing to decontaminate systems that
When successful recirculation cleaning is com-
cannot be cleaned by recirculation cleaning or
plete, service the system, as required, to establish the
purifying. Normally, flushing requires you to remove
proper reservoir fluid level and to eliminate entrapped
fluids that are found to be chemically or physically
air.
unacceptable, or fluids contaminated with water, other
Flushing
foreign fluids, or particulate matter not readily
filterable because of its nature or the quantity involved.
Flushing is a decontamination method in which
Use an external portable hydraulic test stand to power
contaminated system fluid is removed to the maximum
the contaminate system and accomplish flushing.
extent practicable and then discarded. It is a draining
Allow return fluid from the aircraft to flow overboard
process that is generally accomplished by powering the
into a waste container for disposal. Aircraft subsystems
aircraft system with a portable hydraulic test stand. See
should be operated to produce maximum displacement
figure 8-6. The contaminated return-line fluid from the
of aircraft fluids by cleaned, filtered fluid from the
aircraft is then allowed to flow overboard into a suitable
portable test stand. View B of figure 8-6 shows fluid
receptacle for disposal. In effect, filtered fluid from the
flow during system flushing.
portable hydraulic test stand is used to displace
Test stands used for system flushing must be
contaminated fluid in the system and to replenish it
equipped with 3-micron (absolute) filtration and must
with clean serviceable fluid.
have a minimum internal reservoir of 16 gallons. The
The amount of fluid removed and replaced during
stand itself should be recirculation cleaned and
system flushing varies. It depends upon such factors as
deaerated before it is connected to the aircraft. Drain,
the nature of the contaminant, layout of the system, and
flush, and service the reservoirs or other fluid storage
the ability to produce flow in all affected circuits.
devices in the contaminated system before system
Portions of operating systems are often dead ended.
flushing. If you know that the contamination originated
Fluid found in these portions is static and not affected
at an aircraft pump, drain and flush the hoses and lines
8-16
