Figure 2-9.
Correct and Incorrect Bucking Bars.
SUMMARY
Skin patches are divided into two types. The first, a lap patch, is
applied externally with its edges overlapping the skin.
This patch
is used only where an aerodynamic surface is not needed.
Flush
patches, the second type, are inserted into the damaged area and
riveted to a doubler that is riveted to the skin.
Damaged-area
accessibility and directions in the technical manual applicable to
the aircraft determine the kind of patch to use.
Most of an
aircraft's skin is inaccessible from the inside, and such skin is
designated as closed skin.
That accessible from both sides is open
skin.
An important aspect of making skin repairs is the amount of stress
intensity (strength) to be restored.
Repairs must restore highly
critical areas to 100 percent, semicritical areas to 80 percent, and
noncritical areas to 60 percent of original strength. For stressed-
skin repairs, the applicable technical manual must be consulted and
followed.
Lap patches are used at authorized locations to repair cracks
and small holes. To repair cracks, a small hole is drilled
at both ends of the crack to stop the crack from spreading.
Patch sizes must be large enough to take the number of
rivets prescribed in the applicable manual. Flush plates
are relatively easy to install in locations free of internal
structures. In inaccessible areas, blind rivets are substituted
for standard ones. With these patches the doubler is usually
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