to prevent corrosion if no breaks, holes, or cracks
exist. The procedures and materials used in replacing
the original protective coating are outlined later in this
Facing-to-core voids of less than 2.5 inches in
diameter can usually be repaired by drilling a series of
holes 0.06 to 0.10 inch in diameter in the upper facing
over the void area. An expandable forming resin,
such as Thermofoam 607 or equivalent, is then
injected through the holes with a pressure-type
caulking gun. When the void is on the lower surface
of the panel, only sufficient resin must be injected so
as to completely fill the void. With voids on the upper
surface, the core area should be filled until the resin
comes out of the injection holes. These holes should
be sealed with a thermosetting epoxy resin adhesive,
and the entire assembly cured with lamps, as required
for the adhesive system.
When the void areas are large, it is necessary to
remove the facing over the damaged area and
follow the repair procedures for a puncture. See
A puncture is defined as a crack, break, or hole
through one or both skin facings with resulting
damage to the honeycomb and/or balsa wood core.
The size of the puncture, amount of damage to the
core, assembly to be repaired (rudder, elevator, etc.),
and previous repairs to the damaged assembly are
factors to be considered in determining the type of
repair to be made. Damage to a honeycomb and/or
balsa wood core assembly that exceeds a specified
length or diameter in inches or the total number of
repairs exceeds a specified percentage of the total
bonded area necessitates replacement of the assembly.
NOTE: These figures are found in the
applicable structural repair manual.
HONEYCOMB CORE.The repair shown in
figure 14-16, view A, is used when a puncture through
one skin facing has caused only minor damage to the
core material. To repair this type of damage, proceed
Cover the component with a suitable protective
covering (polyvinyl sheet or kraft paper). Cut out a
section of the protective covering that will extend
approximately 2 inches beyond the damaged area.
Use masking tape to hold the cutout in place.
Stop-drill as necessary through the skin facing
Strip the paint and protective coating 1 1/2 inches
beyond the stop drilled holes. Then, clean the
stripped area with a special cleaning paste. Fill the
void with the specified filler material to within
approximately 0.063 inch of the skin facing, and cure
Prepare a round or oval patch large enough to
overlap the damaged area at least 1 inch. Apply
sealant to the undersurface of the patch and to the
filler and skin surface.
Install the repair patch,
maintaining correct overlap, and clamp to the
assembly to assure contact with the skin facing. Cure
as directed. Remove the excess adhesive, and refinish
The repair shown in figure 14-16, view B, is used
when a puncture through one skin facing has caused
extensive damage to the honeycomb core. When the
core has been damaged extensively, the damaged
material must be replaced.
Prepare the assembly as previous] y described.
Cut out the damaged skin facing with a hole saw or
aviation snips. File the edges of the hole smoothly.
Using a pocket knife, carefully cut out the damaged
Do not damage the opposite facing. Install
a new core filler and complete the repair as
previously described for view A of figure
The repair shown in figure 14-16, view C, is used
when both skin facings and the core have been
damaged. Use the same procedures as described
above for views A and B to make this repair.