The difference is caused by the particle counter not
ment method used at all levels of mainte-
counting those particles smaller than 5 microns, while
many of them are retained by the patch-test filter
membrane, causing it to discolor proportionately.
When is the best time to take a hydraulic fluid
When test results conflict, the equipment tested is
considered unacceptable if it fails either test method.
Before taking a hydraulic fluid sample from a
The equipment should then be subjected to
hydraulic test stand, you must recirculate the
fluid for how long?
You need to recognize that the differing test results
How many samples should you take from each
may indicate system deficiencies and justify a request
for an engineering investigation of the equipment. Poor
correlation between particle counts and patch tests can
When sampling hydraulic fluid, what size test
result from improper sample-taking procedures,
filter should you use?
procedures. These possibilities must be carefully
investigated if a correlation problem is encountered.
LEARNING OBJECTIVE: Identify de-
contamination methods used on naval aircraft
and their purpose.
The halogen leak detector (fig. 8-5) is used to test
System decontamination is a maintenance opera-
hydraulic fluid samples for MIL-C-81302A (Freon) or
tion performed when a system contains fluid that is
other chlorinated solvents. The detector is a
battery-powered, self-contained instrument. The in-
be contaminated with foreign matter or it is not
strument provides an audible indication, varying from a
considered acceptable for service for some other
slow ticking sound to a loud squeal, to indicate the level
reason. The purpose of decontamination is to remove
of the vapor concentration.
foreign matter from the operating fluid or to remove the
You can determine the acceptability of unknown
contaminated fluid itself. Before you can de-
hydraulic fluid samples by using the HDL-440 leak
contaminate an affected system, replace any failed or
detector. To do this, you compare the vapor level of a
known contamination-generating components. Other
known hydraulic fluid to that of the unknown hydraulic
components of the system are not to be disturbed,
fluid and determine whether the unknown sample
contains more or less than 200 ppm (parts per million)
in the HDL-440 is hydraulic fluid MIL-H-5606 or
MIL-H-83282, which contains a known amount
There are four basic methods used to de-
(200 ppm) of MIL-C-81302.
contaminate aircraft hydraulic systems. The methods
are recirculation cleaning, flushing, purging, and
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
Figure 8-5.--HDL-440 halogen leak detector in operation.