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Summary

All repairs in aviation maintenance must maintain the original strength of the repaired part or member. For example, if the fuselage skin is subject to compression, put the patch on the outside; this ensures greater resistance to such loads. The patch cross-sectional area must be equal to, or greater than, the original damaged area or section. To reduce the danger of cracks starting at the corners, use circular or oval patches. If a rectangular patch must be used, you should ensure that the curvature radius at each corner is no smaller than 1/2 inch. Buckled or bent members are replaced or reinforced by putting splices over the affected areas. Replacement or reinforcement material must be similar to the original material. If substitutions must be made, be sure that the material's gage is heavy enough to give equivalent cross-sectional strength. Using a lighter gage, stronger material is dangerous. One material may have more tensile strength but less compressive strength than another. Buckling and torsional strengths of some sheet metal and tubular parts depend primarily upon thickness rather than allowable compressive and shear strengths. So, even though a thinner material has higher compressive and shear strength, it can have less buckling and torsional strength. Care must be used when forming heat-treated and cold-worked alloys. When subjected to much bending, such alloys will crack. Although soft alloys are easily formed without danger of cracking, they lack strength for use in primary structures. Strong alloys can be formed in their annealed state first and then heat treated, in the desired shape, to develop their strength. When using a brake to form metal, be sure that a thin piece of soft metal is placed on the brake jaws to prevent marring the metal's surface.

The best method for determining the size and number of rivets to use on a repair is to refer to the direct and general support manual applicable to the aircraft being repaired. This manual gives the essential information needed to perform direct and general support and depot maintenance on the complete airframe.

All repairs must be made so that the aircraft's original contour is maintained. In addition, all repairs must be kept to a minimum weight. This is accomplished by using patches as small as practicable and limiting the number of rivets to what is necessary. When weight is added in each repair, the aircraft's balance may be so disturbed that trim and balance tabs may have to be adjusted. A patch on the propeller spinner requires a balancing patch to keep the propeller assembly in balance.

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