be hastened by the use of infrared lamps or hot
The repair of any damaged component made of
sandbags. After the resin has been cured, remove the
reinforced plastic requires the use of identical
cellophane and sand off the excess resin; then, lightly
materials, whenever they are available, or of approved
sand the entire repaired area to prepare it for
substitutes for rebuilding the damaged portion. Abrupt
refinishing.
changes in cross-sectional areas must be avoided by
tapering joints, by making small patches round or oval
P LY D A M A G E ( S A N D W I C H L A M I -
instead of rectangular, and by rounding the corners of
NATES).--When the damage has penetrated more
all large repairs. Uniformity of thickness of core and
than one ply of the cloth in sandwich-type laminates,
facings is exceedingly important in the repair of
the repair may be made by using the scarfed method,
radomes. Repairs of punctured facings and fractured
shown in figure 5-8. This repair is made in the
cores necessitate removal of all the damaged material,
following manner: Clean the area thoroughly, and then
followed by replacement with the same type of material
sand out the damaged laminate plies, as shown in view
and in the same thickness as the original. All repairs to
B. The area should be sanded to a circular or oval shape,
components housing radar or radio gear must be made
and then the area should be tapered uniformly down to
in accordance with the manufacturer's recommenda-
the deepest penetration of the damage.
tions. This information may be found in the aircraft
The diameter of the scarfed (tapered) area should
structural repair manual or in drawings and specifica-
be at least 100 times the depth of the penetration. You
tions.
should exercise care when using a mechanical sander.
Before a thorough inspection of the damage can be
Excess pressure on the sander can cause the sandpaper
made, the area should be cleaned with a cloth saturated
to grab, resulting in the delamination of undamaged
with methyl ethyl ketone (MEK). After drying, the
plies.
paint should be removed by sanding lightly with No.
280 grit sandpaper, and then clean the sanded area with
MEK. The extent of damage can then be determined by
CAUTION
tapping the suspected areas with a blunt instrument.
You could use a coin as a blunt instrument, such as a
The sanding of glass cloth reinforced laminates
quarter, to perform the tap test. This is referred to as the
produces a fine dust that may cause skin irritation. In
"coin tap" method. You should never use a hammer as a
addition, if you breathe an excessive amount of this
blunt instrument. The damaged areas will have a dull or
dust, it may be injurious; precautions as to skin, eyes,
dead sound, while the undamaged areas will have a
and respiration protection must be observed.
clear metallic sound.
Damages are divided into four general classes:
surface damage, facing and core damage, puncture
damage (both facings and core), and damage requiring
replacement.
Repairing Surface Damage
The most common types of damage to the surface
are abrasions, scratches, scars, dents, cuts, and pits.
Minor surface damages may be repaired by applying
one or more coats of room-temperature catalyzed resin
to the damaged area. More severe damages may be
repaired by filling with a paste made from
room-temperature resin and short glass fibers. Over this
coated surface, apply a sheet of cellophane, extending 2
or 3 inches beyond the repaired area. After the
cellophane is taped in place, start in the center of the
repair and lightly brush out all the air bubbles and
excessive resin with your hand or a rubber squeegee
towards the outer ridge of the repair. Allow the resin to
cure at room temperature, or if necessary, the cure can
Figure 5-8.--Ply repair (scrafed method).
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