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9/8/98 AC 43.13-1B Reinforcement plates to be spruce or plywood and should be bonded only, with the ends feathered off at 5:1 slope as depicted in Figure 1-2. Solid spars may be replaced with laminated spars or vice versa, provided the material is of the same high quality. FIGURE 1-5. Method of splicing solid or laminated rectangular spars. shrink and causes the basic spar stock to split (see paragraph 1-2c). Cracks start under the plywood plates, usually (but not necessarily) at a bolt hole or cutout, and usually spread in each direction until they extend a short dis- tance beyond the ends of the plates where the resistance to spar shrinkage disappears. Cracks have also been found in the butt end of spars. Other factors, which have been found conducive to the formation of cracks are poor protective finishes, large cutouts, and metal fittings that utilize two lines of large diameter bolts. b. Repairing Cracks Versus Installing a New Spar. The presence of cracks does not necessarily mean that the spar must be dis- carded. If the crack is not too long or too close to either edge and can be reinforced properly, it will probably be more economical and satisfactory to perform repair rather than install a new spar or section. However, a generally acceptable procedure suitable for all airplane models is not available. Because of the possi- bility of strength deficiencies contact the manufacturer. In absence of the manufacturer, the FAA should be contacted for approval be- fore making repairs not in accordance with the manufacturer-approved instructions or the rec- ommendations of this advisory circular. Lon- gitudinal cracking or the recurrence of crack- ing can be minimized by ensuring that the moisture content of the solid wood portion is within the proper range before bonding. In arid desert areas, during bonding the moisture content should be in the range of 6-8 percent before bonding, but in other areas 10-12 percent is satisfactory. If solid or ply- wood repair stock is procured from another climatic region, it should be allowed to season, Par 1-44 Page 1-17PDF Image | AFS-640
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