| Skin Replacement Damage to an aircraft's skin is sometimes so extensive that an entire panel must be replaced. Also, a section or area of skin may have such a number of patches or minor repairs that a panel may have to be replaced. In either case, the first step, as in all other repairs, is to thoroughly inspect the damaged area to determine the full extent of the damage. This includes inspecting internal structures for damage or signs of strain. These members must be repaired or replaced if they are bent, fractured, or wrinkled. All rivets in the vicinity of such members must be inspected for signs of failure. They can be sheared without such a condition being externally evident. Under such circumstances, rivets at various points in the damaged area must be drilled out and examined. For this procedure the applicable TM for the particular aircraft must be consulted. The paragraphs that follow discuss these aspects of skin replacement and the mechanics and techniques in measuring, cutting, and aligning new metal sheets, drilling new rivet holes, and bucking rivets. Measuring, Cutting, and Aligning. In removing a panel, care must be taken to avoid further skin damage because frequently the old skin can be used as a template for the new skin. The gage and alloy of material to replace the panel is shown in the applicable manual for the specific aircraft. The size of the panel can be determined in either of two ways. The dimensions can be measured during the inspection, or the old skin can be used as a template for the layout of the new sheet of skin and the location of the holes. Because the latter method is more accurate, it is the preferred one. Whichever method is used, the new sheet must be large enough to replace the damaged area; and it can be cut with an allowance of 1 to 2 inches of material outside the rivet holes. If the old sheet is not too badly damaged, it must be flattened out and can be used as a template. The new sheet, cut 1 to 2 inches larger than the old, is drilled near the center using the holes in the old sheet as a guide. The two sheets are then fastened together with sheet metal fasteners. Sheet metal screws are not recommended because they damage the edges of the rivet holes. Drilling must always start at the center and work to the outside of the sheet with sheeting metal fasteners inserted at frequent intervals. If the old sheet cannot be used as a template, the holes in the new sheet must be drilled from the inside of the structure. The holes in the reinforcing members are used as guides for the drilling. Fasteners are inserted in the same manner as described before; this technique is called back drilling. Before the new sheet is placed on the framework to drill the holes, the reinforcing members must be in proper alignment and flush at the points where they intersect. Therefore, the new sheet must have the same contour as the old sheet. Drilling New Rivet Holes. Much care must be taken in duplicating holes from reinforcing members, or the frame and skin can be ruined. Such care is necessary because most ribs, stringers, and bulkheads depend on the skin for some of their rigidity and they can be forced out of alignment during drilling. The pressure from drilling forces the skin away from the frame and causes the holes to be out of alignment. This can be prevented by holding a block of wood firmly against the skin while drilling. Also the drill must be held at a 90° angle to the skin during drilling or the holes will be out-of-round. A smaller pilot drill must be used first when drilling through anchor nuts. Care must be taken to avoid damaging the anchor nut threads. The pilot holes are then enlarged to the proper size. Marking New Holes. An angle attachment or flexible-shaft drill may be necessary where a straight drill cannot be inserted. If neither of these drills can be used, mark the new section with a soft pencil through the holes in the old section. Another method for marking new hole locations is with a transfer or prick punch, shown in Figure 2-7. Center the punch in the old hole and then hammer lightly on the outside of the sheet with a mallet. The resulting mark can be used to locate the hole in the new sheet. Another way to locate the rivet holes without a template is to use a hole finder similar to the one shown in Figure 2-8. This device makes a perfect alignment with the holes in the old section possible while drilling holes in the new section of skin. The hole finder illustrated is made in two sections, an upper and a lower part, bolted together at one end. A guide rivet, at the free end of the bottom section of the hole finder, drops into the old holes in the sheet still in place. The free end of the hole finder's top section has a hole that exactly matches the position of the guide rivet. The new hole is drilled through this opening. As the hole finder is moved along, the guide rivet drops into an old hole and automatically determines the new hole's location. 
Figure 2-7. Transfer Punch.
Figure 2-8. Hole Finder.
Removing Burrs. The temporary fasteners are taken out and the sheet is removed from the framework after all the holes are drilled. Burrs left by drilling must be removed from both sides of all holes in the skin, stringers, and rib flanges. Deburring can be done with a few light turns of a countersink drill. If the burrs are not removed, the joint may be tight; and the rivets can expand, or flash, between the parts being riveted. Bucking Bar. Selecting and using a bucking bar of the correct type and weight is important to a successful riveting job. For example, a bucking bar for 1/8-inch rivets must weigh at least 2 pounds. Larger rivets require proportionately heavier bucking bars. A light bar requires too many blows to complete the riveting, and the blows tend to deform the rivet head. A straight bar is preferred because its weight can be applied directly in line with the rivet's shank. If flanges on ribs or stringers do not permit using a straight bar, one that allows straight-line pressure must be used. Figure 2-9 shows two such bars, (A and B). The bar at (C) or a similar bar can, at best, only create more damage. 
Figure 2-9. Correct and Incorrect Bucking Bars.
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