and temperatures acting with the catalytic action
open containers. Hydraulic fluid absorbs dust and
of water, air, and copper or iron oxides. Oil-
oxidation products appear first as organic acids,
problems. Keep hydraulic parts and servicing
asphaltenes, gums, and varnishes. These products
equipment clean. Should the system become con-
combine with dust particles and appear as sludge.
taminated, minimize damage by taking prompt
Oxidation products that dissolve in liquid increase
a liquid's resistance to flow. Products that do not
dissolve in liquid form sediments and precipitates,
HYDRAULIC SYSTEM MAINTENANCE
especially on colder elements such as heat
exchanger coils. A precipitate is a solid substance
Hydraulic system maintenance consists of
that was chemically separated from a solution.
Liquids containing antioxidants have little
parts. External leaks, where fluid is escaping from
tendency to form gums under normal operating
a cylinder, valve, or fitting, are usually easy to
conditions. However, as the temperature in-
Hydraulic fluids that are subjected to high
temperatures (above 250°F) will break down,
leaving particles of asphaltene suspended in the
liquid. The red fluid changes to brown, and is
A pinhole leak in a 3,000-psi hydraulic
referred to as decomposed liquid. This explains
system can force fluid through your skin.
the importance of keeping the hydraulic fluid
Do not use your hand to feel for a leak.
temperature below specified levels.
The second chemical reaction that can produce
Inspect the area around the leak, A leak may
not be directly above the accumulation of fluid.
react with certain types of rubber. This reaction
Fluids often follow the structure or tubing to a
causes the structure of the rubber to change,
lower point before dropping off. When you notice
turning it brittle, and causing the rubber to fall
leaks, trace them to the source, and then repair
apart. Make sure that system fluids are compatible
or replace the bad unit or part.
with the seals and hoses, and that those parts are
Internal leaks are caused by fluid under
appropriate for the system.
pressure slipping past an unseated valve or worn
packing ring. Normally, fluids flow into the return
4. Particles introduced by foreign liquids.
line back to the reservoir. The signs of internal
Water is the most common foreign fluid contami-
leakage are sluggish operation of an actuating
nant, especially in petroleum-based hydraulic
system or a drop-off in system pressure. A drop
in gauge pressure or an indication of insufficient
atmospheric moisture and normally settles at the
pressure on the gauge may be caused by an
bottom of the reservoir. Fluid movement in the
internal leak. When internal leakage is suspected
reservoir disperses the water into fine droplets.
or known to be in the hydraulic system, the
These water droplets form an oil-water-air
symptoms should be noted to aid in locating the
emulsion because of the mixing action created in
leak. Follow the aircraft technical instruction
the pumps and passages. This emulsion normally
for specific troubleshooting and maintenance
separates during the rest period in the system
procedures. For more information on hydraulic
maintenance procedures, consult the Aviation
Filters provide adequate control of the con-
FUEL AS A HYDRAULIC FLUID
tamination problem during all normal hydraulic
Fuel, and the means to regulate fuel pressure,
system operations. Contamination control from
is readily available within the aircraft. It is an ideal
o u t s i d e sources are the responsibility of
fluid to use for hydraulic control of systems.
maintenance personnel. Therefore, take all
Especially those systems controlling certain engine
precautions to be sure contamination is held to
a minimum during service and maintenance. Do
not reuse fluid drained from hydraulic systems or
geometry actuation, and operation of the after-
burner variable nozzles.
equipment. Do not use hydraulic fluid stored in