When you are laying out the size of the doubler, the
trim a hole to a rectangular or elliptical shape and round
length should exceed the width. This enables the
the corners to a generous radius. See figure 4-61.
doubler to be slipped in through the skin and positioned
On larger repair areas, it is usually possible to buck
for installation. This eliminates the splitting and
the doubler rivets by inserting and holding the bucking
manipulation of the patch required in installing
bar through the center of the doubler. The filler is then
doublers of square and round flush patch repairs.
riveted in place with blind fasteners. When blind rivets
The filler is fabricated slightly less than the
are used as substitutes for solid rivets, the structural
dimensions of the hole being repaired. Generally, the
repair manual normally specifies the next larger size.
maximum clearance between the skin and the filler is
The proper edge distances for the substitute fasteners
one thirty-second of an inch. This will allow a
must be maintained.
1/64-inch clearance on each end of the filler and
NOTE: Edge distance was discussed earlier. In all
eliminate any possibility of stress developing from
flush patches, the filler should be of the same gauge and
contact between the two parts.
material as the original skin. The doubler, generally,
The doubler is fabricated larger than the hole being
should be of the same material and one gauge heavier
repaired to allow for the specified number of rivets
than the skin. Structural repair manuals will specify the
required to attach the doubler to the skin being repaired.
allowable substitution of materials. This can be in the
The doubler, filler, and attaching skin rivet pattern may
form of a Note on the repair diagram.
Figure 4-61.--Flush rectangular patch.
Figure 4-62.--Flush patch over internal structure.