FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 1. Wood Structure | 3. Inspection | 1-29. Inspection Methods
9/8/98
AC 43.13-1B
(d) Check ribs on either side of strut
attach points for missing or loose rib-to-spar
attach nails.
(e) Check ribs on either side of strut
attach points for cracks where the cap strips
pass over and under the spars.
(f) Check for cracked leading edge
skin and/or failed nose ribs in the area directly
in front of the jury strut.
(g) Check the brackets which attach
the struts to the spars for cracks.
(h) Check the aileron, flap hinge, and
hinge brackets for cracks and loose or missing
rivets.
(i) Check all exposed end grain
wood, particularly the spar butts, for cracking
or checking. Checking, or splitting, of wood
spar butts is common on aircraft based in arid
areas.
physical damage or decay. When removing
metal fasteners from wood, check for evidence
of corrosion. Any corrosion present indicates
the presence of moisture and the strong prob-
ability of decay in the adjoining wood
structure.
(k) Any wooden member that has
been overstressed is subject to compression
failure (e.g., ground loop). Compression
cracking and failure of the wood spars in cer-
tain aircraft are a continuing problem. Com-
pression failures are defined as failure of wood
fibers on a plane perpendicular to the wood fi-
ber’s longitudinal axis. If undetected, com-
pression failures may result in structural failure
of the wing during flight. Compression cracks
have been found emanating from the upper
surfaces of the wing spars and progressing
downward.
(l) The usual locations for cracks
have been the front spar at both ends of the
reinforcement plate for the lift strut and the
front spar rib attach points, both inboard and
outboard of the spar reinforcement plate; and
the rear spar lift strut and rib attach points. An
inspection of both the front and rear spars for
compression cracks is recommended.
FIGURE 1-3. Likely areas to incur structural damage.
(j) Also check for any cracks that in-
dicate a bond line failure or structural failure
of the wood member. Any evidence of move-
ment of fittings, bushings, or fasteners should
be cause for concern, and further inspection is
warranted. Splits in fabric covering the ply-
wood, especially on upper surfaces exposed to
ultraviolet light and water, dictate that the me-
chanic remove the fabric around the split so
the underlying plywood may be inspected for
(m) The two areas where it is possi-
ble to identify a compression crack are on the
face and top surface of the spar. Using a
borescope through existing inspection holes is
one method of inspection. An alternate
method is to cut inspection holes in the skin.
If inspection holes are cut, they should be
made on the aft side of the front spar and the
forward side of the rear spar. This will allow
the fabric to be peeled away from the spar.
Longitudinal cracks may also be detected dur-
ing this inspection. Loose or missing rib nails
may indicate further damage and should be
thoroughly investigated. The mechanic may
shine a light, at a low angle and parallel with
the grain, in the area of the member
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