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 | 9-25. General
9/8/98
SECTION 2. HYDRAULIC SYSTEMS
AC 43.13-1B
9-25. GENERAL. Hydraulic systems in
aircraft provide a means for the operation of
aircraft components. The operation of landing
gear, flaps, flight control surfaces and brakes is
largely accomplished with hydraulic power
systems. Hydraulic system complexity varies
from small aircraft that require fluid only for
manual operation of the wheel brakes to large
transport aircraft where the systems are large
and complex. To achieve the necessary redun
dancy and reliability, the system may consist
of several subsystems. Each subsystem has a
power generating device (pump) reservoir, ac
cumulator, heat exchanger, filtering system,
etc. System operating pressure may vary from
a couple hundred psi in small aircraft and ro
torcraft to several thousand psi in large trans
ports. Generally, the larger the aircraft, the
more mechanical work is required to control
the aircraft’s various functions. Consequently,
the system operating pressure increases ac
cordingly. Primarily, hydraulic power is gen
erated by either engine driven or electric motor
driven pumps. The majority of hydraulic
pumps are pressure compensated to provide a
constant output pressure at a flow-rate de
manded by the system. Some constant dis
placement pumps with a relief valve are used
on the smaller aircraft.
MIL-H-83282 with MIL-H-5606 may render
the MIL-H-83282 non fire-resistant.
a. Mineral-Base Fluids. MIL-H-5606,
mineral oil-based hydraulic fluid is the oldest,
dating back to the 1940’s. It is used in many
systems, especially where the fire hazard is
comparatively low. MIL-H-6083 is simply a
rust-inhibited version of MIL-H-5606. They
are completely interchangeable. Suppliers
generally ship hydraulic components with
MIL-H-6083.
b. Polyalphaolefin-Based Fluids.
MIL-H-83282, is a fire-resistant hydrogenated
polyalphaolefin-based fluid developed in the
1960’s to overcome the flammability charac
teristics of MIL-H-5606. MIL-H-83282 is sig
nificantly more flame resistant than
MIL-H-5606, but a disadvantage is the high
viscosity at low temperature. It is generally
limited to -40 �F. However, it can be used in
the same system and with the same seals, gas
kets, and hoses as MIL-H-5606.
MIL-H-46170 is the rust-inhibited version of
MIL-H-83282. Small aircraft predominantly
use MIL-H-5606 but some have switched to
MIL-H-83282, if they can accommodate the
high viscosity at low temperature.
9-26. PURPOSES OF HYDRAULIC
SYSTEMS. Hydraulic systems make possible
the transmission of pressure and energy at the
best weight per horsepower ratio.
9-27. TYPES OF HYDRAULIC FLUID.
There are three principal categories of hydrau
lic fluids; mineral base fluids, polyalphaolefin
base, and phosphate ester base fluids. When
servicing a hydraulic system, the technician
must be certain to use the correct category of
replacement fluid. Hydraulic fluids are not
necessarily compatible. For example, con
tamination of the fire-resistant fluid
c. Phosphate Ester-Based Fluid (Sky
drol/Hyjet). These fluids are used in most
commercial transport category aircraft, and are
extremely fire-resistant. However, they are not
fireproof and under certain conditions, they
will burn. The earliest generation of these
fluids was developed after World War II as a
result of the growing number of aircraft hy
draulic brake fires which drew the collective
concern of the commercial aviation industry.
(1) Progressive development of these
fluids occurred as a result of performance re
quirements of newer aircraft designs. The
Par 9-25
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