FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 11. Aircraft Electrical Systems | 18. Conduits | 11-251. Conduit Installation
9/8/98
SECTION 18. CONDUITS
AC 43.13-1B
11-248. GENERAL. Conduit is manufac-
tured in metallic and nonmetallic materials
and in both rigid and flexible forms. Primar-
ily, its purpose is for mechanical protection of
cables or wires. Conduit should be inspected
for: proper end fittings; absence of abrasion at
the end fittings; proper clamping; distortion;
adequate drain points which are free of dirt,
grease, or other obstructions; and freedom
from abrasion or damage due to moving ob-
jects, such as aircraft control cables or shifting
cargo.
11-249. SIZE OF CONDUIT. Conduit
size should be selected for a specific wire
bundle application to allow for ease in main-
tenance, and possible future circuit expansion,
by specifying the conduit inner diameter (I.D.)
about 25 percent larger than the maximum di-
ameter of the wire bundle.
11-250. CONDUIT FITTINGS. Wire is
vulnerable to abrasion at conduit ends. Suit-
able fittings should be affixed to conduit ends
in such a manner that a smooth surface comes
in contact with the wire. When fittings are not
used, the end of the conduit should be flared
to prevent wire insulation damage. Conduit
should be supported by use of clamps along
the conduit run.
11-251. CONDUIT INSTALLATION.
Conduit problems can be avoided by follow-
ing these guidelines:
a. Do not locate conduit where passen-
gers or maintenance personnel might use it as
a handhold or footstep.
b. Provide drainholes at the lowest point
in a conduit run. Drilling burrs should be care-
fully removed.
c. Support conduit to prevent chafing
against structure and to avoid stressing its end
fittings.
11-252. RIGID CONDUIT. Conduit sec-
tions that have been damaged should be re-
paired to preclude injury to the wires or wire
bundle which may consume as much as
80 percent of the tube area. Minimum accept-
able tube bend radii for rigid conduit are
shown in table 11-23. Kinked or wrinkled
bends in rigid conduits are not recommended
and should be replaced. Tubing bends that
have been flattened into an ellipse and the mi-
nor diameter is less than 75 percent of the
nominal tubing diameter should be replaced
because the tube area will have been reduced
by at least 10 percent. Tubing that has been
formed and cut to final length should be de-
burred to prevent wire insulation damage.
When installing replacement tube sections
with fittings at both ends, care should be
taken to eliminate mechanical strain.
TABLE 11-23. Bend radii for rigid conduit.
Nominal Tube O.D.
(inches)
1/8
3/16
1/4
3/8
1/2
5/8
3/4
1
1 1/4
1 1/2
1 3/4
2
Minimum Bend Radii
(inches)
3/8
7/16
9/16
15/16
1 1/4
1 1/2
1 3/4
3
3 3/4
5
7
8
Par 11-248
Page 11-101