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 | 9-28. Handling Hydraulic Fluid
AC 43.13-1B CHG 1
9/27/01
airframe manufacturers dubbed these new gen
erations of hydraulic fluid as “types” based on
their performance. Today, types IV and V
fluids are used. Two distinct classes of type
IV fluids exist based on their density: class I
fluids are low density and class II are standard
density. The class I fluids provide weight
savings advantages versus class II. Monsanto
and Exxon are the suppliers of the type IV
phosphate ester-based aviation hydraulic flu
ids.
(2) In addition to the type IV fluids that
are currently in use, type V fluids are being
developed in response to industry demands for
a more thermally stable fluid at higher operat
ing temperatures. Type V fluids will be more
resistant to hydrolytic and oxidative degrada
tion at high temperature than the type IV flu
ids.
d. Materials of Construction. Hydraulic
systems require the use of special accessories
that are compatible with the hydraulic fluid.
Appropriate seals, gaskets, and hoses must be
specifically designated for the type of fluid in
use. Care must be taken to ensure that the
components installed in the system are com
patible with the fluid. When gaskets, seals,
and hoses are replaced, positive identification
should be made to ensure that they are made of
the appropriate material.
9-28. HANDLING HYDRAULIC FLUID.
In addition to any other instructions provided
in the aircraft maintenance manual or by the
fluid supplier, the following general precau
tions must be observed in the handling of hy
draulic fluids:
a. Ensure that each aircraft hydraulic
system is properly identified to show the kind
of fluid to be used in the system. Identification
at the filler cap or valve must clearly show the
type of fluid to be used or added.
b. Never allow different categories of hy
draulic fluids to become mixed. Chemical re
actions may occur, fire resistant fluids may
lose their fire resistance, seals may be dam
aged, etc.
c. Never, under any circumstances, serv
ice an aircraft system with a fluid different
from that shown on the instruction plate.
d. Make certain that hydraulic fluids and
fluid containers are protected from contamina
tion of any kind. Dirt particles may cause hy
draulic units to become inoperative, cause seal
damage, etc. If there is any question regarding
the cleanliness of the fluid, do not use it.
Containers for hydraulic fluid must never be
left open to air longer than necessary.
(1) Phosphate ester-based hydraulic
fluids have good solvency properties and may
act as plasticizer for certain polymers. Care
should be taken in handling to keep the fluid
from spilling on plastic materials and paint
finishes.
(2) If a small amount of the fluid is
spilled during handling, it must be cleaned up
immediately with a dry cloth. When larger
quantities are spilled, an absorbent sweeping
compound is recommended. A final cleaning
with an approved solvent or detergent should
remove any traces of fluid.
e. Do not expose fluids to high tempera
ture or open flames. Mineral-based fluids are
highly flammable.
f. The hydrocarbon-based hydraulic
fluids are, in general, safe to handle. To work
with these fluids, reasonable handling proce
dures must always be followed. Take precau
tion to avoid fluid getting in the eyes. If fluid
contacts the eye, wash immediately with water.
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