There are four basic methods used to decontaminate
aircraft hydraulic systems. The methods are recircu-
lation cleaning, flushing, purging, and purifying.
Recirculation cleaning is a decontamination
process in which the system to be cleaned is powered
from a clean external power source. The system is
cycled so it produces a maximum interchange of fluid
between the powered system and the SE used to
power it. When decontaminating a system, the
contaminated fluid is circulated through the hydraulic
filters in the aircraft system and in the portable
hydraulic test stands.
Decontamination that uses the recirculation
cleaning method is a filtration process. It can remove
only that foreign matter that is retained by the filter
elements normally found in the equipment. A key
factor in recirculation cleaning is the use of
high-efficiency, 3-micron (absolute) filter elements.
Absolute filter elements have no fluid bypass when
the filter clogs. The filters have a large dirt-holding
capacity in the portable test stands used for this
purpose. In a single fluid pass, these filters remove
all particulate matter larger than 3 microns, and a high
percentage of the other particles down to submicron
size. Recirculation cleaning is effective in removing
hard particulate matter from hydraulic fluid that is
otherwise serviceable. It must be recognized that the
filters are not capable of removing water, other
foreign fluids, or dissolved solids. Therefore,
recirculation cleaning is limited to decontamination
of systems found to have a particulate level in excess
of Navy Standard Class 5, whose fluid is considered
For specific procedures on
recirculation cleaning, you should refer to the
Use recirculation cleaning to remove excessive
particulate matter that results from normal component
wear, limited component failure, or external sources.
Clean the system by powering it with an external
portable hydraulic test stand. Operate the aircraft
systems so maximum interchange of fluid is produced
between the aircraft and the test stand. View A of figure
4-6 shows a flow diagram for recirculation cleaning.
Figure 4-6.-Fluid flow during decontamination.