the hydraulic liquid, the hydraulic system must be
NOTE: The presence of foreign particles in the
hydraulic system indicates a possible component
malfunction that you should correct before flushing the
A hydraulic system in which the liquid is con-
taminated should be flushed according to current
applicable technical instructions. Flushing procedures
are normally recommended by the manufacturer. The
procedure varies with different hydraulic systems. One
method is as follows:
Drain out as much of the contaminated liquid as
possible. Drain valves are provided in some systems for
this purpose; while on other systems, lines and fittings
must be disconnected at the low points of the system to
remove any trapped fluid in the lines and components.
Close all the connections and till the system with the
applicable flushing medium. Any of the hydraulic
liquids approved for use in power-transmission systems
may be used for flushing purposes.
Diesel fuel oil must not be used for flushing
hydraulic systems in active service, because of
its poor lubricating qualities and its
contaminating effect on the subsequent till of
While being flushed with an approved hydraulic
liquid, power-transmission systems can be operated at
full load to raise the temperature of the liquid.
Immediately following the warming operation, the
system should be drained by opening all drain outlets
and disconnecting the hydraulic lines to remove as much
of the flushing medium as possible. All filter elements,
screens, and chambers should be cleaned with new fluid
before filling the system with the required service liquid.
The system should not be operated while or
after draining the liquid.
Power-transmission systems and their intercon-
nected hydraulic controls whose inner surfaces have
been inactivated and treated with a corrosion prevention
or preservation compound must be flushed to remove
the compound. The latest current instructions for
flushing and other operations required to reactivate a
particular system must be strictly followed to prevent
Some hydraulic systems are flushed by forcing new
liquid into the system under pressure, forcing out the
contaminated or decomposed liquid.
Hydraulic liquid which has been contaminated by
continuous use in hydraulic equipment or has been
expanded as a flushing medium must not be used again
but should be discarded according to the prevailing
Never permit high-pressure air to be in
direct contact with petroleum base liquids in a
closed system, because of the danger of
ignition. If gas pressure is needed in a closed
system, nitrogen or some other inert gas should
Aviation Hydraulics Manual, NAVAIR 01-1A-17,
Commander, Naval Air Systems Command,
Washington, D.C., 1989.
Aviation Support Equipment Technician, Naval
Education and Training Program Management
Support Activity, Pensacola, Fla., 1990.
Base Vehicle Equipment Mechanic, Extension Course
Institute, Air University, Gunter Air Force Station,
Montgomery, Ala., 1986.
Fluid Power, Naval Education and Training Program
Management Support Activity, Pensacola, Fla.,