measured and marked. When marking, care must be taken to not scratch
the material.
Such scratches can weaken the material or possibly
develop into cracks.
All marks on aluminum except lines to be cut
must be made with an aircraft marking pencil. Exposed metal parts of
a repair and their contact surfaces must be given a coating of epoxy
polymide primer.
Rivet Selection.
In general, the rivet size and alloy in the
repair must be the same as in the original.
After reworking an
enlarged or deformed rivet hole, the next larger size rivet must be
used.
When such rivets are used, proper edge distance must be
maintained.
Where blind rivets must be used in repairs, the
applicable manual must be consulted for the type, size, spacing, and
number of blind rivets needed to replace the original rivets.
Rivet Spacing and Edge Distance.
The instructions in the
applicable manual covering rivet patterns for a repair must be
followed.
As a general rule, existing rivet patterns are used when
possible.
However, rivet spacing is generally between 6 to 8 times
the diameter (6D - 8D) of the rivet shank. The spacing must never be
less than 3 times or more than 10 times the diameter (3D - 10D) of
the rivet shank.
For flush head rivets, edge distance must not be
less than 2 1/2D and for all other rivets at least 2D. Edge distance
greater than 4D must never be used.
SUMMARY
The primary objective in aircraft repair is to restore the damaged
part to its original condition. Each damaged part must be studied to
determine if repair or replacement is required.
All damage is not
obvious or limited to an immediate location.
A thorough assessment
of a damaged area includes inspecting supporting structural members
and adjacent rivets and bolts to see if they have sustained damage
also. The assessment of damage also includes looking for weathering
and corrosion.
A white crystalline deposit at loose rivets and
bolts, scratches, dents, and places where moisture can collect is
corrosion.
A visual inspection does not reveal the existence or extent of
cracks.
Methods used to locate cracks and their extent include
ultrasonic waves, X rays, Zy-glo, and penetrating dye.
Because dye
does not need expensive or complicated equipment, it is used in the
field. The area to be dyed must be thoroughly cleaned. The dye is
applied by brushing or swabbing and allowed to penetrate for 3 to 15
minutes.
An even coat of developer is applied and allowed to dry.
Defects are indicated in red and cracks show up as red lines.
Scattered, unpatterned dots indicate porosity.
All the materials
used in the dye test are flammable, and prolonged breathing of the
fumes is injurious.
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AL0992
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