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General Repair Practices During repair of any major structural member, the structure must be firmly supported. The support must enable the work to be completed without any misalignment or distortion. If special fixtures to support the aircraft or any of its components are not available, temporary supports must be made. In addition, repair materials, layout of parts for repair, rivet replacement, and rivet-hole reuse must be given careful selection and assessment. The paragraphs that follow discuss repair material, layout of parts for repair, rivet replacement, and rivet-hole reuse. Repair Material Selection. When repairs must be made, the prime requirement is to duplicate the structure's original strength. To do this, the repair material must be as strong as that used in the original. If it is necessary to substitute a weaker alloy than originally used, a heavier gage metal must be used to secure equivalent cross-sectional strength. A lighter gage material must never be used, even when using a stronger alloy. If substituting a metal becomes necessary, always consult the applicable technical manual. Layout of Parts for Repair. Before fitting them into the aircraft structure, new sections made for repairs must be laid out to the dimensions given in the applicable TM for that aircraft. "Laid out" is used here in the sense of being measured and marked. When marking, care must be taken to not scratch the material. Such scratches can weaken the material or possibly develop into cracks. All marks on aluminum except lines to be cut must be made with an aircraft marking pencil. Exposed metal parts of a repair and their contact surfaces must be given a coating of epoxy polymide primer. Rivet Selection. In general, the rivet size and alloy in the repair must be the same as in the original. After reworking an enlarged or deformed rivet hole, the next larger size rivet must be used. When such rivets are used, proper edge distance must be maintained. Where blind rivets must be used in repairs, the applicable manual must be consulted for the type, size, spacing, and number of blind rivets needed to replace the original rivets. Rivet Spacing and Edge Distance. The instructions in the applicable manual covering rivet patterns for a repair must be followed. As a general rule, existing rivet patterns are used when possible. However, rivet spacing is generally between 6 to 8 times the diameter (6D - 8D) of the rivet shank. The spacing must never be less than 3 times or more than 10 times the diameter (3D - 10D) of the rivet shank. For flush head rivets, edge distance must not be less than 2 1/2D and for all other rivets at least 2D. Edge distance greater than 4D must never be used.
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