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
SECTION 3. INSPECTION
AC 43.13-1B
1-27. GENERAL. Inspection of wooden
structure includes some methods, equipment,
and awareness of failure modes which are
unique to wooden aircraft.
service. Although none of the older adhesives
have been specifically found to fail by simple
aging, the mechanic is advised to inspect all
accessible joints carefully.
1-28. TYPES OF DETERIORATION
AND DAMAGE.
a. Wood Decay. Wood is an organic
product which is subject to attack by fungi.
Fungi are plants that grow on and in wood.
The moisture content of the wood nominally
will have to be 20 percent or greater to sustain
fungus growth. The result of this growth is
called decay. Decayed wood exhibits softness,
swelling if still wet, excessive shrinkage when
dry, cracking, and discoloration. Repair or re-
place wood if any amount or form of decay is
found.
b. Splitting. Splits or cracks in wooden
members occur along grain lines. When the
moisture content of wood is lowered, its di-
mensions decrease. The dimensional change is
greatest in a tangential direction (across the fi-
bers and parallel to the growth rings), some-
what less in a radial direction (across the fibers
and perpendicular to the growth rings), and is
negligible in a longitudinal direction (parallel
to the fibers). These dimensional changes can
have detrimental effects upon a wood struc-
ture, particularly when two parts are bonded
together with grains in different directions.
This effect can often be seen where a plywood
doubler is bonded to a spruce member. As the
spruce member dries, it attempts to shrink, but
is restrained by the plywood, which shrinks
less. The resulting stress in the spruce member
exceeds its cross-grain strength, and a split oc-
curs.
c. Bond Failure. Bond joint failure is
generally due to improper fabrication tech-
nique or prolonged exposure to moisture in
d. Finish Failure. The finish coat on
wood structure (usually varnish) is the last line
of defense to prevent water entry into wood
and the resulting decay. Finish failure can be
the result of prolonged water exposure, wood
splitting, ultraviolet light exposure, or surface
abrasion.
e. Damage. Stress, impact, or mechanical
damage to a wood structure is caused by ex-
cessive aerodynamic loads or impact loads oc-
curring while the aircraft is on the ground.
Overtightening of fittings can also cause
crushing of the underlying wood member and
possible bending of the metal fitting.
1-29. INSPECTION METHODS. When-
ever possible, the aircraft should be kept in a
dry, well-ventilated hangar, with all inspection
covers, access panels, etc., removed for as long
as possible before final inspection. The air-
craft should be given a preliminary inspection
when first removing the inspection covers and
access panels and inspected with a moisture
meter at this time. If the moisture content is
high, the aircraft should be thoroughly dried.
If the aircraft is dry, this will facilitate later in-
spection, especially when determining the
condition of bonded joints.
a. Likely locations for wood structure
deterioration should be given special attention.
Most damage is caused by external influence
such as moisture, temperature extremes, or
sunlight. Care should be taken to note all pos-
sible entry points for moisture, (i.e., cracks or
breaks in the finish, fastener holes, inspec-
tion/access openings, control system openings,
drain holes, and the interfaces of metal fittings
Par 1-27
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