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
AC 43.13-1B
9/8/98
Splicing under minor fittings such as drag
wire, antidrag wire, or compression strut fit-
tings is acceptable under the following condi-
tions:
a. The reinforcement plates of the splice
should not interfere with the proper attachment
or alignment of the fittings. Do not alter the
locations of pulley support brackets, bellcrank
support brackets, or control surface support
brackets. Plates are to be tapered off, as de-
picted in figure 1-2.
b. The reinforcement plate may overlap
drag wire, antidrag wire, or compression strut
fittings, if the reinforcement plates are on the
rear face of the rear spar or the front face of the
front spar. In such cases, it will be necessary
to install slightly longer bolts. The front face
reinforcement plate should not overlap drag
strut fittings, except when it does not require
sufficient shortening of compression struts or
changes in drag-truss geometry, to prevent
adjustment for proper rigging. Even though
take up is sufficient, it may be necessary to
change the angles on the fittings. (Acceptable
methods for splicing the various types of spars
are shown in figure 1-4 through figure 1-9.)
Reinforcement plates must be used as indi-
cated on all scarf repairs to spars and the
slopes of scarves shown are minimum slopes.
1-41. SPAR REPLACEMENT. Replace-
ment of spars is a major repair. Spars may be
replaced by new parts made by the manufac-
turer or the holder of a Parts Manufacturer Ap-
proval (PMA) for that part. Owner-produced
spars may be installed providing they are made
from a manufacturer-approved drawing. Also,
a spar may be made by reference to an existing
spar providing sufficient evidence is presented
to verify that the existing spar is an original
part, and that all materials and dimensions can
be determined. The dimensions and type of
wood used are critical to the structural strength
of the aircraft. Care should be taken that any
replacement spars accurately match the manu-
facturer’s original design.
1-42. SPLICING OF BOX SPAR WEBS.
Always splice and reinforce plywood webs
with the same type of plywood as found on the
original part. Do not use solid wood to replace
plywood webs. Plywood is stronger in shear
than solid wood of the same thickness due to
the grain direction of the individual plies. The
face-grain of plywood replacement webs and
reinforcement plates must be in the same di-
rection as the original member to ensure that
the new web will have the required strength.
(The method of splicing plywood webs is
shown in figure 1-9.)
1-43. REPLACING SOLID-TYPE SPARS
WITH LAMINATED-TYPE SPARS. Solid
spars may be replaced with laminated spars or
vice versa, provided the material is of the same
high quality. External reinforcements (ply-
wood or solid) must always be replaced as on
the original member.
1-44. SPAR LONGITUDINAL CRACKS
AND LOCAL DAMAGE. Cracked spars
(except box spars) may be repaired by bonding
plates of spruce or plywood of sufficient
thickness to develop the longitudinal shear on
both sides of the spar. Extend the plates well
beyond the termination of the cracks, as shown
in figure 1-10. A method of repairing small
local damage to either the top or bottom side
of a spar is also shown in figure 1-10.
a. Longitudinal Cracking of Wood
Wing Spars of Aircraft Operating in Arid
Regions. Aircraft having wood spars and op-
erating in arid regions may develop longitudi-
nal spar cracks in the vicinity of the plywood
reinforcement plates. These cracks result from
the tendency of the spar to shrink when drying
takes place. Plywood resists this tendency to
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