especially true where there is an access door or plate
minimum number of rivet holes in the patch; normally
through which the rivets can be bucked. In inaccessible
four will suffice at an angle of 90 degrees to each other.
areas, the flush patch may be made by substituting
Position the patch over the surface being repaired, and
blind rivets for standard rivets, where permissible, and
ensure that the correct edge distances are being
devising a means of inserting the doubler through the
maintained. Drill four holes in the surface being
opening.
repaired, using the predrilled holes in the patch as a
pattern for alignment. As each hole is drilled, using the
One method, shown in figure 4-60, has a doubler
proper temporary fasteners, secure the patch in place.
that has been split. To insert the doubler, slip the split
When the patch is temporarily secured, drill the
edge under the skin and twist the doubler until it slides
remaining rivet holes through the patch and the surface
in place under the skin. The screw in the center of the
being repaired. Remove the patch and deburr all rivets
doubler is temporarily installed to serve as a handle for
holes with a deburring tool or a large drill bit. Prime the
inserting the doubler through the hole. This type of
repair materials with the proper corrosion-preventive
patch is normally recommended for holes up to 1 1/2
material before the riveting operation. Secure the patch
inches in diameter. In holes larger than 1 1/2 inches,
in position with temporary fasteners to maintain
alignment during riveting.
Holes may be repaired in either stressed or
nonstressed skin that is less than three-sixteenths of an
inch in diameter by filling with a rivet. Drill the hole
and install the proper size rivet to fill the hole. For holes
three-sixteenths of an inch and larger, you should
consult the applicable structural repair manual for the
necessary repair information. The damaged area is
removed by cutting and trimming the hole to a circular,
square, rectangular, or diamond shape. The corners of
the hole should be rounded to a minimum of one-fourth
of an inch in radius. The lap patch is fabricated and
installed in the same manner as previously explained
for repairing cracks.
Flush Patches
A flush patch consists of a filler patch that is flush
with the skin after it is inserted. It is backed up and
riveted to a reinforcement plate that, in turn, is riveted
to the inside of the skin. This reinforcement plate is
usually referred to on some repair diagrams as
the doubler or the backup plate. On some
high-performance aircraft, only the flush patch is
permitted in making skin repairs.
Flush patches should be used where aerodynamic
smoothness is required. The type of flush patch used
depends on the location of the damaged area. One type
is clear of internal structures, and the other is not. Like
all types of repairs, you must consult the applicable
structural repair manual for the necessary repair
information. The repairs discussed next are typical of
most repairs.
FLUSH PATCH CLEAR OF INTERNAL
STRUCTURES.--In areas that are clear of internal
Figure 4-60.--Repair of small holes in skin with flush patch.
structure, the repair is relatively simple to make. This is
4-37