In the layout of a part, you should plan the
bending and forming operations so that each step is
made in the proper sequence. If the steps are not
made in the proper sequence, the part may become so
bulky that it will be impossible to insert in the brake to
make the final bend.
Since layout of replacement parts involves the
interpretation of blueprints, you should review
Blueprint Reading and Sketching, NAVEDTRA
TYPES OF REPAIRS
The type of repair to be made will depend on the
materials, tools, amount of time available, accessi-
bility to the damaged area, and maintenance level.
The types of repair are permanent, temporary, and
one-time flight (ferry). Repairs are also classified as
either internal or external.
A permanent repair is one that restores the
strength of the repaired structure equal to or greater
than its original strength and satisfies aerodynamic,
thermal, and interchangeability requirements, This
ensures the designed capabilities of the aircraft.
The temporary repair restores the load-carrying
ability of the structure but is not aerodynamically
smooth or able to satisfy interchangeability require-
ments. This repair should be replaced by a permanent
type as soon as possible in order for the aircraft to be
restored to its normal condition.
The one-time flight repair restores a limited
load-carrying ability to the damaged structure in order
to fly the aircraft to a depot maintenance activity for a
permanent repair. When this type of repair is made,
the aircraft cockpit should be placarded to limit the
performance of the aircraft.
After the damage has been inspected and
classified on external surfaces, the structural repair
manual for the specific aircraft should be consulted
for the critical areas where aerodynamic smoothness
must be maintained.
An aerodynamic filler is
available for negligible damage, steps, and gaps. In
many sections the skin is Chem-Milled or machined.
Chem-Mill is a process whereby the proper shape and
size are obtained by a chemical acting on the metal.
The proper shape and thickness of machined skin are
obtained with the use of a shaper or milling machine.
Some skin is manufactured with lands on the metal,
which is a thicker portion of the skin where bulkheads
and frames are attached.
One of the factors that determines the exact
procedure to be used in making skin repairs is the
accessibility of the damaged area. Much of the skin
on an aircraft is inaccessible from the inside. The
skin in such areas is referred to as closed skin. Skin
that is accessible from both sides is called open
Repairs to open skin may usually be made in the
conventional manner using specified types of
standard rivets. To repair closed skin, some types of
special blind fasteners must be used. The exact type
of fastener used will depend upon the type of repair
made and the recommendations of the aircraft
Another of the important factors to be considered
when you are making a skin repair is the stress
intensity of the damaged panel. For example, certain
skin areas are classified as highly critical, other areas
as semicritical, while still other areas may be classi-
fied as noncritical. Repairs to damages in highly
critical areas must provide 100-percent strength
replacement; semicritical areas require 80-percent
strength replacement; and noncritical areas require
60-percent strength replacement. When a repair
specifies it must provide 60-percent strength
replacement, this indicates the amount of repair
strength necessary to maintain a margin of safety on
skin areas. The 60-percent stress intensity repair is
specified when production methods and stiffening
requirements have resulted in an overstrength skin
with a high margin of safety. This repair provides
strength and stiffness equivalent to specific design
requirements rather than the original structure of the
The 100-percent stress intensity repair
makes the strength of the repaired skin equal to or
greater than the original undamaged skin. This type
of skin usually has a low margin of safety.
A lap patch is an external patch that has the edges
of the patch and the skin overlapping each other. The
overlapping portion of the patch is riveted to the skin.
On some aircraft, lap patches are permitted in certain
areas, but only where aerodynamic smoothness is not
important. In areas where it is permitted, the lap
patch may be used in repairing cracks as well as small