FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 1. Wood Structure | 4. Repairs | 1-44. Spar Longitudinal Cracks and Local Damage
9/8/98
AC 43.13-1B
Reinforcement plates to be spruce or plywood and should be bonded only,
with the ends feathered off at 5:1 slope as depicted in Figure 1-2.
Solid spars may be replaced with laminated spars or vice versa,
provided the material is of the same high quality.
FIGURE 1-5. Method of splicing solid or laminated rectangular spars.
shrink and causes the basic spar stock to split
(see paragraph 1-2c). Cracks start under the
plywood plates, usually (but not necessarily) at
a bolt hole or cutout, and usually spread in
each direction until they extend a short dis-
tance beyond the ends of the plates where the
resistance to spar shrinkage disappears.
Cracks have also been found in the butt end of
spars. Other factors, which have been found
conducive to the formation of cracks are poor
protective finishes, large cutouts, and metal
fittings that utilize two lines of large diameter
bolts.
b. Repairing Cracks Versus Installing a
New Spar. The presence of cracks does not
necessarily mean that the spar must be dis-
carded. If the crack is not too long or too close
to either edge and can be reinforced properly,
it will probably be more economical and
satisfactory to perform repair rather than install
a new spar or section. However, a generally
acceptable procedure suitable for all airplane
models is not available. Because of the possi-
bility of strength deficiencies contact the
manufacturer. In absence of the manufacturer,
the FAA should be contacted for approval be-
fore making repairs not in accordance with the
manufacturer-approved instructions or the rec-
ommendations of this advisory circular. Lon-
gitudinal cracking or the recurrence of crack-
ing can be minimized by ensuring that the
moisture content of the solid wood portion is
within the proper range before bonding. In
arid desert areas, during bonding the moisture
content should be in the range of 6-8 percent
before bonding, but in other areas
10-12 percent is satisfactory. If solid or ply-
wood repair stock is procured from another
climatic region, it should be allowed to season,
Par 1-44
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