| Maintain Original Strength In so far as aviation maintenance is concerned, any repair must maintain the repaired member's original strength. Failure to maintain this strength can put a weakening strain on another member or members. The paragraphs that follow discuss five basic considerations in maintaining original strength. Compression or Bending. If a member or fuselage skin is subject to compression and needs patching, put the patch on the outside to ensure higher resistance to compression or bending loads. If the patch cannot be placed on the outside, use material one gage thicker than the original material, and put the patch on the inside. The patch must have a cross-sectional area equal to, or greater than, the original damaged section. The general rule here is to regain, as near as possible, the original strength. Cracking. Circular or oval patches must be used to reduce the danger of cracks starting at the corners. If a rectangular patch must be used, make the curvature radius at each corner no smaller than 1/2 inch. Buckled or bent members must be replaced or reinforced by putting splices over the affected areas. Similarity of Material. Be certain that all replacement or reinforcement material used is similar to the original material used. If substitutions are necessary, use material of a gage heavy enough to give an equivalent cross-sectional strength. Substituting a lighter gage but stronger material for the original is dangerous. The danger is that one material can have greater tensile strength than another but less compressive strength. For example, the buckling and torsional strengths of some sheet metal and tubular parts are dependent primarily upon the thickness rather than the allowable compressive and shear strengths of the material. Therefore, a substitute thinner than the original will reduce the buckling and torsional strength of a part, even though the thinner substitute material has higher compressive and shear strengths. NOTE: Never substitute a material thinner than the original material or with cross-sectional area less than the original. The buckling and torsional strengths of many sheet metal and tubular parts are dependent primarily upon the thickness, rather than the allowable compressive and shear strength of the material. Therefore, a substitute thinner than the original will reduce the bucking and torsional strengths of a part considerably, even though the thinner substitute material has higher allowable compressive and shear strengths. Forming. Be particularly careful when forming (shaping) alloys. Heat-treated and cold-worked alloys cannot take much bending without cracking. On the other hand, soft alloys can be easily formed without cracking; however, they are not strong enough for primary structures. Strong alloys can be formed in their annealed state first and then heat treated, in the desired shape, to develop their strength. In some cases, if annealed metal is not available, the metal can be heated and quenched according to regular heat-treating practices and formed before hardening sets in. However, forming must be completed in approximately half an hour after quenching because the material may become too hard to work. When a brake is used to form the metal, be sure to use a thin piece of soft metal on the brake jaws to prevent the metal's surface from being marred. Rivet Size and Number. The rivets in the next parallel row inboard on the wing or forward on the fuselage can be used to determine the size of rivets needed for any repair. Another method is to multiply skin thickness by three and use the nearest larger size rivet corresponding to that result. For example, if skin thickness is 0.040 inch, multiply that by three. The result is 0.120; use the next larger size rivet, 4/32 inch (0.125 inch). Each repair takes a specific number of rivets to restore the original strength. This number will vary with the thickness of the material being repaired and the size of the damage. However, the direct and general support (DS and GS) maintenance manual applicable to the particular aircraft gives essential information to accomplish direct or general support and depot maintenance on the complete airframe, including the kind, size, and number of rivets.
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