FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 3. Fiberglass and Plastics | 2. Metallic Sandwich Secondary Structure Repairs | 3-10. Repairs to Metallic Sandwich Secondary Structure
9/8/98
AC 43.13-1B
SECTION 2. METALLIC SANDWICH SECONDARY STRUCTURE REPAIRS
3-10. REPAIRS TO METALLIC SAND-
WICH SECONDARY STRUCTURE. Mag
nesium, titanium, or stainless steel facings re
quire special procedures that are not included
in the following methods of repair. Aluminum
alloys such as 7075-T6, 2024-T3, and 2014-T6
are commonly used for the repair of facings for
sandwich structural parts having aluminum
facings. For maximum corrosion resistance,
use only clad aluminum for repairs to clad
aluminum alloy facings.
a. Dents, scratches, or fractures, not ex
ceeding 1/4 inch in largest dimension in alu
minum facings, may be repaired with a suit
able filler such as viscous epoxy resin. Dents
that are delaminated shall not be filled but re
paired. Thoroughly clean the repair area with
fine sandpaper and acetone before applying the
filler. After the resin has partially cured, re
move any excess resin with a sharp plastic
scraper. When the resin has completely cured,
sand to the original contour. If the damage in
cluded a fracture, reclean the area around the
filled hole and apply a surface patch.
b. Fractures or punctures in one facing
and partial damage to the core of an alumi
num-faced laminate may be repaired by several
different methods. The technique used will
depend upon the size of the damage, the
strength required, and the aerodynamic loads
of the area involved. If the repair requires
aerodynamic smoothness, the facing sur
rounding the repair core cavity may have to be
step cut to one-half its thickness. This can be
done by using a router with an end mill bit and
a template.
d. After locating the extent of the total
damaged area by tapping or other nondestruc
tive test methods, remove the damaged facing
and that portion of the core material that is also
affected. The depth to which the core must be
removed will depend upon the type of core
material and the method of repair. The re
placement core material must be the same
material and core cell size as the original. Fab
ricate core material to shape, keeping the same
core ribbon or grain direction. When a sub
stitution is permissible, wood or glass-fabric
honey-comb cores are sometimes used in the
repair of aluminum honeycomb cores, as they
are generally easier to shape. Typical types of
core replacements are shown in figure 3-10.
Resin fills can be used to replace the core and
facing where smaller core damage exists.
Phenolic microballoons, low-density insulating
materials, and/or other ingredients are added to
lower the density and give greater flexibility.
e. For the repair of larger holes in which
it is inconvenient to use a face patch because
of aerodynamic smoothness requirements in
that area, both the core and facing are some
times replaced with glass-fiber fabric discs and
resin. Undercut the core, as shown in fig
ure 3-11, in order to obtain a better bonding of
the fill with the facing. Fill the core cavity
with accurately shaped resin-saturated glass
cloth discs, and press each ply down to remove
any air bubbles. Special care should be taken
that the final plies fit well against the under
side of the top facing. When the core cavity is
filled, close the cutout in the facing with resin-
impregnated glass fiber fabric discs that have
been precut to size.
c. Damage that extends completely
through the core and both facings may be re
paired using the same general techniques as
those used for repairing fiberglass laminates
when both facings are accessible.
f. Overlap repairs, typically called scab
patches, have a long history of use in repairing
aircraft structures. These repairs simply cover
the damaged area with patch material. Overlap
repairs can be bonded and/or mechanically
Par 3-10
Page 3-9