FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 1. Wood Structure | 4. Repairs | 1-51. Types of Patches
AC 43.13-1B
9/8/98
(5) As there is no reinforcement behind
this patch, care must be used so that pressure is
not great enough to crack the skin. On hori-
zontal surfaces, weights or sandbags will be
sufficient. On patches too far from any edge
for the use of standard hand clamps, jaws of
greater length may be improvised. Table 1-2,
columns (1) and (3), may also be used for de-
termining the maximum thickness of single
laminations for curved members.
on the trimmed opening is greater than
100 times the skin thickness, a scarf patch may
be installed.
(1) Scarf cuts in plywood may be made
by hand plane, spoke shave, scraper, or accu-
rate sandpaper block. Rasped surfaces, except
at the corners of scarf patches and sawn sur-
faces, are not recommended as they are likely
to be rough or inaccurate.
(6) Fill, sand, and refinish the patch.
b. Surface Patch. Plywood skins that are
damaged between or along framing members
may be repaired by surface or overlay patches
as shown in figure 1-14. Surface patches lo-
cated entirely aft of the 10 percent chord line,
or which wrap around the leading edge and
terminate aft of the 10 percent chord line, are
permissible. Surface patches may have as
much as a 50 inch perimeter and may cover as
much as 1 frame (or rib) space. Trim the dam-
aged skin to a rectangular or triangular shape
and round the corners. The radius of rounded
corners must be at least 5 times the skin thick-
ness. Bevel the forward edges of patches lo-
cated entirely aft of the 10 percent chord line
to 4 times the skin thickness. The face-grain
direction must be the same as the original skin.
Cover completed surface patches with fabric to
match surrounding area. The fabric must
overlap the original fabric at least 2 inches.
c. Scarf Patch. A properly prepared and
inserted scarf patch is the best repair for dam-
aged plywood skins and is preferred for most
skin repairs. Figure 1-15 shows the details and
dimensions to be used when installing typical
scarf skin patches, when the back of the skin is
accessible. Follow figure 1-16 when the back
of the skin is not accessible. The scarf slope
of 1 in 12, shown in both figures, is the steep-
est slope permitted for all kinds of plywood. If
the radius of curvature of the skin at all points
(2) Nail strip or small screw clamping
is often the only method available for bonding
scarf joints in plywood skin repairs. It is es-
sential that all scarf joints in plywood be
backed with plywood or solid wood to provide
adequate nail holding capacity. The face-grain
direction of the plywood patch must be the
same as that of the original skin.
(3) If the back of a damaged plywood
skin is accessible (such as a fuselage skin), it
should be repaired with a scarf patch, follow-
ing the details shown in figure 1-15. When-
ever possible, the edges of the patch should be
supported as shown in section C-C of fig-
ure 1-15. When the damage follows or ex-
tends to a framing member, the scarf may be
supported as shown in section B-B of fig-
ure 1-15. Damages that do not exceed
25 times the skin thickness in diameter after
being trimmed to a circular shape and are not
less than 15 times the skin thickness to a
framing member, may be repaired as shown in
figure 1-15, section D-D.
(a) The backing block is carefully
shaped from solid wood and fitted to the inside
surface of the skin, and is temporarily held in
place with nails.
(b) Use waxed paper or plastic wrap
to prevent bonding of the backing block to the
skin.
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