| Patching Procedures By comparison, lap patching appears to be less complex than flush patching. However, all patch repairs must be made with careful attention to detail, and the technical manual appropriate to the aircraft must be consulted. The paragraphs that follow describe some details in both patching procedures. Lap Patches. Lap patches can be used at authorized locations to repair cracks and small holes (Figure 2-3). When repairing cracks, always drill a small hole at each end of the crack with two stop holes, one at each end. To drill the holes use a Number 40 drill. These holes are important because they stop the crack from spreading. The patch size must be large enough to take the required number of rivets. The number is determined from the rivet schedule indicated for the gage of the material in the damaged area. The recommended patch can be cut in on a circular, square, rectangular or diamond shape. Patch edges must be chamfered, beveled, to an angle of 45°. 
Figure 2-3. Lap Patch for a Crack.
Flush Patches. Flush patches in locations free of internal structures are relatively easy to install. This is especially true where there is an access door or plate through which rivets can be bucked. In inaccessible areas, the flush patch can be used by substituting blind rivets for standard rivets, where permitted. However, ways of inserting the doubler through the opening must be devised. A doubler is the piece of material used to back up, double, or strengthen the filler patch. One such method is illustrated in Figure 2-2. The doubler has been split. To insert the doubler, slip one edge under the skin and turn the doubler until it slides into place under the skin. Notice in the figure that a screw has been installed temporarily as a handle to help insert the doubler. This type of patch is generally recommended for holes up to 1 1/2 inches in diameter. Usually holes larger than 1 1/2 inches are trimmed to a rectangular or square shape. All corners must be rounded to at least a 1/2-inch radius. Figure 2-4 shows such rounded corners. In all flush patches the filler must be of the same gage and material as the original skin. Generally, the doubler must be of material one gage heavier than the skin. Figure 2-5 shows a flush patch over an internal structure. 
Figure 2-4. Rectangular Flush Patch.
Figure 2-5. Internal Structure Flush Patch
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