FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 9. Aircraft Systems and Components | 2. Hydraulic Systems | 9-29. Hydraulic System Maintenance Practices
AC 43.13-1B
9/8/98
such that the indicator cannot be reset, unless
the filter bowl is removed and the element re
placed.
c. Flushing a Hydraulic System. When
inspection of hydraulic filters or hydraulic
fluid evaluation indicates that the fluid is con
taminated, flushing the system may be neces
sary. This must be done according to the
manufacturer’s instructions; however, a typical
procedure for flushing is as follows:
(1) Connect a ground hydraulic test
stand to the inlet and outlet test ports of the
system. Verify that the ground unit fluid is
clean and contains the same fluid as the air
craft.
(2) Change the system filters.
of fluid seepage may be permitted on actuator
piston rods and rotating shafts. In a hydraulic
system, a thin film of fluid in these areas indi
cates that the seals are being properly lubri
cated. When a limited amount of leakage is
allowed at any point, it is usually specified in
the appropriate manual.
(2) Tubing should not be nicked, cut,
dented, collapsed, or twisted beyond approved
limits. The identification markings or lines on
a flexible hose will show whether the hose has
been twisted. (See figure 9.9.)
(3) All connections and fittings associ
ated with moving units must be examined for
play evidencing wear. Such units should be in
an unpressurized condition when they are
checked for wear.
(3) Pump clean, filtered fluid through
the system, and operate all subsystems until no
obvious signs of contamination are found
during inspection of the filters. Dispose of
contaminated fluid and filter. (Note: A visual
inspection of hydraulic filters is not always ef
fective.)
(4) Disconnect the test stand and cap
the ports.
(5) Ensure that the reservoir is filled to
the FULL line or proper service level.
d. Inspections. Hydraulic and pneumatic
systems are inspected for leakage, worn or
damaged tubing, worn or damaged hoses, wear
of moving parts, security of mounting for all
units, safetying, and any other condition speci
fied by the maintenance manual. A complete
inspection includes considering the age, cure
date, stiffness of the hose, and an operational
check of all subsystems.
(1) Leakage from any stationary con
nection in a system is not permitted, and if
found, it should be repaired. A small amount
(4) Accumulators must be checked for
leakage, air or gas preload, and position. If the
accumulator is equipped with a pressure
gauge, the preload can be read directly.
(5) An operational check of the system
can be performed using the engine-driven
pump, an electrically-operated auxiliary pump
(if such a pump is included in the system), or a
ground test unit. The entire system and each
subsystem should be checked for smooth op
eration, unusual noises, and speed of operation
for each unit. The pressure section of the sys
tem should be checked with no subsystems to
see that pressure holds for the required time
without the pump supplying the system. Sys
tem pressure should be observed during opera
tion of each subsystem to ensure that the en
gine-driven pump maintains the required pres
sure.
e. Troubleshooting. Hydraulic system
troubleshooting varies according to the com
plexity of the system and the components in
the system. It is, therefore, important that the
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