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 | 9-30. Hydraulic Lines and Fittings
AC 43.13-1B CHG 1
9/27/01
technician should be guided by local, state, and
federal regulations, with regard to means of
disposal of used hydraulic fluid. Presently, the
most universally accepted procedure for dis
posal of phosphate ester-based fluid is incin
eration.
9-30. HYDRAULIC LINES AND FIT-
TINGS. Carefully inspect all lines and fittings
at regular intervals to ensure airworthiness.
Investigate any evidence of fluid loss or leaks.
Check metal lines for leaks, loose anchorage,
scratches, kinks, or other damage. Inspect fit
tings and connections for leakage, looseness,
cracks, burrs, or other damage. Replace or re
pair defective elements. Make sure the lines
and hoses do not chafe against one another and
are correctly secured and clamped.
a. Replacement of Metal Lines. When
inspection shows a line to be damaged or de
fective, replace the entire line or, if the dam
aged section is localized, a repair section may
be inserted. In replacing lines, always use
tubing of the same size and material as the
original line. Use the old tubing as a template
in bending the new line, unless it is too greatly
damaged, in which case a template can be
made from soft iron wire. Soft aluminum
tubing (1100, 3003, or 5052) under ¼-inch
outside diameter may be bent by hand.. For all
other tubing use an acceptable hand or power
tube-bending tool. Bend tubing carefully to
avoid excessive flattening, kinking, or wrin
kling. Minimum bend radii values are shown
in table 9-2. A small amount of flattening in
bends is acceptable, but do not exceed
75 percent of the original outside diameter.
Excessive flattening will cause fatigue failure
of the tube. When installing the replacement
tubing, line it up correctly with the mating part
so that it is not forced into alignment by tight
ening of the coupling nuts.
b. Tube Connections. Many tube con
nections are made using flared tube ends with
standard connection fittings: AN-818
(MS 20818) nut and AN-819 (MS 20819)
sleeve. In forming flares, cut the tube ends
square, file smooth, remove all burrs and sharp
edges, and thoroughly clean. The tubing is
then flared using the correct 37-degree aviation
flare forming tool for the size of tubing and
type of fitting. A double flare is used on soft
aluminum tubing 3/8-inch outside diameter
and under, and a single flare on all other tub
ing. In making the connections, use hydraulic
fluid as a lubricant and then tighten. Over-
tightening will damage the tube or fitting,
which may cause a failure. Under-tightening
may cause leakage which could result in a
system failure.
CAUTION:
Mistaken use of
45-degree automotive flare forming
tools may result in improper tubing
flare shape and angle; causing misfit,
stress and strain, and probable system
failure.
c. Repair of Metal Tube Lines. Minor
dents and scratches in tubing may be repaired.
Scratches or nicks not deeper than 10 percent
of the wall thickness in aluminum alloy tubing,
that are not in the heel of a bend, may be re
paired by burnishing with hand tools. Replace
lines with severe die marks, seams, or splits in
the tube. Any crack or deformity in a flare is
unacceptable and cause for rejection. A dent
less than 20 percent of the tube diameter is not
objectionable unless it is in the heel of a bend.
A severely-damaged line should be replaced;
however, it may be repaired by cutting out the
damaged section and inserting a tube section
of the same size and material. Flare both ends
of the undamaged and replacement tube sec
tions and make the connection by using stan
dard unions, sleeves, and tube nuts. If the
damaged portion is short enough, omit the in
sert tube and repair by using one union and
two sets of connection fittings.
Page 9-18
Par 9-29