Use recirculation cleaning to remove excessive
only that foreign matter that is retained by the filter
particulate matter that results from normal component
elements normally found in the equipment. A key
wear, limited component failure, or external sources.
factor in recirculation cleaning is the use of
Clean the system by powering it with an external
high-efficiency, 3-micron (absolute) filter elements.
portable hydraulic test stand. Operate the aircraft
Absolute filter elements have no fluid bypass when the
systems so maximum interchange of fluid is produced
filter clogs. The filters have a large dirt-holding
between the aircraft and the test stand. View A of figure
capacity in the portable test stands used for this
8-6 shows a flow diagram for recirculation cleaning.
purpose. In a single fluid pass, these filters remove all
particulate matter larger than 3 microns, and a high
Test stands used for recirculation cleaning must be
percentage of the other particles down to submicron
equipped with 3-micron (absolute) filtration. Before
size. Recirculation cleaning is effective in removing
connecting the test stand to the aircraft, the stand itself
hard particulate matter from hydraulic fluid that is
must be recirculation cleaned and deaerated, and its
otherwise serviceable. It must be recognized that the
filters are not capable of removing water, other foreign
Class 3 cleanliness level. If the system has a makeup
fluids, or dissolved solids. Therefore, recirculation
reservoir, drain and reservice the system reservoir prior
cleaning is limited to decontamination of systems
to recirculation cleaning. Makeup reservoirs have a
found to have a particulate level in excess of Navy
single fluid port similar to an accumulator; therefore,
Standard Class 5, whose fluid is considered otherwise
little or no fluid exchange takes place during
acceptable. For specific procedures on recirculation
cleaning, you should refer to the applicable MIM.
Figure 8-6.--Fluid flow during decontamination.