marked with "forced removal" dates in accordance with
CONTAMINATION CONTROL
NAVAIR 01-17-1A. These particles can vary from large
Filters provide adequate control of the
chunks of packing and hose material down to steel
contamination problem during all normal hydraulic
shavings of microscopic dimensions that are beyond the
system operations. Control of the size and amount of
retention potential of system filters.
contamination entering the system from any other
source must be the responsibility of the personnel who
The chief source of chemical contaminants in
service and maintain the equipment. Therefore,
hydraulic fluid is oxidation. These contaminants are
precautions must be taken to ensure that contamination
formed under high pressure and temperatures and are
is held to an absolute minimum during servicing.
promoted by the catalytic action of water and air and of
Remove and clean or replace the filter element before a
metals like copper or iron oxides. Oil-oxidation
system becomes excessively contaminated.
products appear initially as organic acids, gums, and
varnishes--sometimes combined with dust particles as
As an aid to exercising contamination control,
sludge. Fluid soluble oxidation products tend to
adhere at all times to the following maintenance and
increase fluid viscosity, while insoluble types form
servicing procedures:
sediments and precipitates, especially on colder
Maintain all tools and work areas (work benches
elements such as heat exchanger coils.
and test equipment) in a clean, dirt-free
Fluids containing antioxidants have little tendency
condition.
to form gums under normal operating conditions.
Open hydraulic systems only when necessary.
However, as the temperature increases, resistance to
Before removing or installing a component,
oxidation diminishes. Hydraulic fluids that have been
subjected to excessively high temperatures (above
make sure the unit is clean and the fittings have
250F) break down in substance, leaving minute
been wiped down.
particles suspended in the fluids. The fluid changes to
Do not drain and reuse hydraulic fluid.
brown in color and is called decomposed fluid. This
explains the importance of keeping the hydraulic fluid
Before removing a defective component, have a
temperature below specified levels.
replacement available (to minimize the amount
of time you must leave the system open).
The second contaminant producing chemical
Recap tubing, hoses, fittings, and components
action in hydraulic fluids is one that permits these
fluids to establish a tendency to react with certain types
immediately after opening.
of rubber. This causes structural changes in the rubber,
Ensure all internal and external hose assemblies
turning it brittle, and finally causing its complete
are properly annotated with applicable data as
disintegration. For this reason, the compatibility of
required by the Aviation Hose and Tube Manual.
system fluid with seals and hose material is a very
important factor.
Ensure all internal and external hose assemblies
are monitored to comply with forced removal
PARTICLES INTRODUCED BY FOREIGN
date (FRD) requirements.
FLUIDS. One of the most common foreign-fluid
Use metal caps on male fittings; metal plugs in
contaminants is water, especially in hydraulic systems
female fittings.
that require petroleum-based fluids. Water, which
enters even the most carefully designed systems by
Replace filter elements immediately after
condensation of atmospheric moisture, normally settles
removing; if this is not possible, place a
to the reservoir bottom. Fluid movement in the reservoir
protective covering over the opening.
disperses the water into fine droplets; agitation of the
Before installing filters, fill filter bowl with the
fluid in the pump and in high-speed passages forms an
correct hydraulic fluid (this reduces air in the
oil-water-air emulsion. Such an emulsion normally
system).
separates out during the rest period in the system
If a hydraulic filter warning light is on, do not
reservoir; but when fine dust and corrosion particles are
operate the equipment.
present, the emulsion is catalyzed by high pressures into
Service hydraulic reservoirs with an approved
sludge. The damaging action of sludge explains the
need for water separating qualities in hydraulic fluids.
dispenser, providing the proper micron filtering.
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