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 10077-F1. 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. External 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 skin.” 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 manufacture. 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 material. 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. Lap Patches 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 holes. 13-42