In general, however, closed-skin repairs require some kind of special
fastener.
The exact fastener depends on the kind of repair and the
manufacturer's
recommendation.
This
section
discusses
stress
intensity, patching procedures, repairs on watertight and pressurized
areas, flush access doors, and skin replacement.
STRESS INTENSITY
An important part of making a skin repair is the amount of stress
intensity (strength) to be restored to the damaged panel.
For
example, some skin areas are designated as highly critical, other
areas as semicritical, and still others as noncritical.
Damage
repairs to highly critical areas must restore 100 percent of original
strength; semicritical areas must have 80 percent restoration of
original strength; and noncritical areas need 60 percent restoration
of original strength.
To determine the requirements for making
stressed-skin repairs, the applicable technical manual for the
particular aircraft must be consulted and its instructions followed.
PATCHING PROCEDURES
By comparison, lap patching appears to be less complex than flush
patching.
However, all patch repairs must be made with careful
attention to detail, and the technical manual appropriate to the
aircraft must be consulted. The paragraphs that follow describe some
details in both patching procedures.
Lap Patches.
Lap patches can be used at authorized locations to
repair cracks and small holes (Figure 2-3).
When repairing cracks,
always drill a small hole at each end of the crack with two stop
holes, one at each end.
To drill the holes use a Number 40 drill.
These holes are important because they stop the crack from spreading.
The patch size must be large enough to take the required number of
rivets.
The number is determined from the rivet schedule indicated
for the gage of the material in the damaged area.
The recommended
patch can be cut in on a circular, square, rectangular or diamond
shape. Patch edges must be chamfered, beveled, to an angle of 45.
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