Wood Wing and Spar Repairs

16A Plywood face plates Top view A 3A 10A 3A Side view Face grain of plywood side platesA Spruce block The scarf length bevel is 10 times dimension A (thickness of the rib cap strip) with the spruce reinforcement block being 16 times dimension A (the scarf length plus extension on either end of the scarf). The plywood splice plates should be of the same material and thickness as the original plates used to fabricate the rib. The spruce block should have a 5:1 bevel on each end. [Figure 6-15] These specific rib repairs describing the use of one scarf splice implies that either the entire forward or aft portion of the cap strip beyond the damage can be replaced to complete the repair and replace the damaged section. Otherwise, replacement of the damaged section may require a splice repair at both ends of the replaced section of the cap strip using the indicated dimensions for cutting and reinforcing of each splice.

When a cap strip is to be repaired at a point where there is a joint between it and cross members of the rib, make the 3A 5A C 3A A E B D Direction of faceA B C D and areE grain of plywoodoriginal dimensions. Reinforcement plates shall be plywood glued and nailed. Splice plate repair by reinforcing the scarf joint with plywood gussets, as shown in Figure 6-16. If a cap strip must be repaired where it crosses a spar, reinforce the joint with a continuous gusset extending over the spar, as shown in Figure 6-17. The scarf joints referred to in the rib repairs are the most satisfactory method of fabricating an end joint between two solid wood members. When the scarf splice is used to repair a solid wood component, the mechanic must be aware of the direction and slope of the grain. To ensure the full strength of the joint, the scarf cut is made in the general direction of the grain on both connecting ends of the wood and then correctly oriented to each other when glued. [Figure 6-18] The trailing edge of a rib can be replaced and repaired by removing the damaged portion of the cap strip and inserting a softwood block of white pine, spruce, or basswood. The entire repair is then reinforced with plywood gussets and nailed and glued, as shown in Figure 6-19.

Compression ribs are of many different designs, and the proper method of repairing any part of this type of rib is specified by the manufacturer. All repairs should be performed using recommended or approved practices, materials, and adhesives. the I section type (i.e., wide, shallow cap strips, and a center plywood web with a rectangular compression member on each side of the web). The rib damage suggests that the upper 6-13 A Incorrect B Incorrect C Correct Damaged area Top Damaged areaSpruce block Plywood, nail, and glue A 10A A B C D and E are original dimensions. B C Face grain of plywood D E and lower cap strips, the web member, and the compression members are cracked completely through. To facilitate this repair, cut the compression members as shown in sections to the rear spar. Cut the damaged cap strips and repair as shown in Figure 6-20, replacing the aft section of the cap strips. Plywood side plates are then bonded on each side diagonally to reinforce the damaged web as shown in a standard rib with rectangle compression members added to one side and a plywood web to the other side. The method used in this repair is essentially the same as in Figure 6-20A, except that the plywood reinforcement plate, shown in spars.

a rectangular vertical member on each side of the web. The method of repair is essentially the same as in Figure 6-20A, except the plywood reinforcement plates on each side, shown in Figure 6-20C-C, are continued the full distance between the spars. Wing Spar Repairs Wood wing spars are fabricated in various designs using solid wood, plywood, or a combination of the two. [Figure 6-21] When a spar is damaged, the method of repair must conform to the manufacturer’s instructions and recommendations. In the absence of manufacturer’s instructions, contact the FAA for advice and approval before making repairs to the spar and following recommendations in AC 43.13-1. If instructions are not available for a specific type of repair, it is highly recommended that you request appropriate engineering assistance to evaluate and provide guidance for the intended repair.

Shown in Figure 6-22 is a recommended method to repair either a solid or laminated rectangle spar. The slope of the scarf in any stressed part, such as a spar, should not be steeper than 15 to 1. 6-14 6A 12A recommended 10A minimum 6A 2A ¼ A A Plywood reinforcement same thickness and face grain direction as original Repair See A B C D 3A 3A A B C DAMAGE A A- B B- C C- Unless otherwise specified by the aircraft manufacturer, a damaged spar may be spliced at almost any point except at wing attachment fittings, landing gear fittings, engine mount fittings, or lift-and-interplane strut fittings. These fittings may not overlap any part of the splice. The reinforcement plates of the splice should not interfere with the proper attachment or alignment of the fittings. Taper reinforcement plates on the ends at a 5:1 slope [Figure 6-23].

The use of a scarf joint to repair a spar or any other component of an aircraft is dependent on the accessibility to the damaged section. It may not be possible to utilize a scarf repair where recommended, so the component may have to be replaced. A scarf must be precisely cut on both adjoining pieces to ensure an even thin glue line; otherwise, the joint may not achieve full strength. The primary difficulty encountered in making this type of joint is obtaining the same bevel on each piece. [Figure 6-24] The mating surfaces of the scarf must be smooth. You can 6-15 Box I Double I C Plain rectangular Routed 6A recommended 5A minimum 2A A 6A recommended 15A minimum 5A minimum A No fittings within these limits 1/4 A Direction of grain if spruce or outer face grain if plywood machine smooth a saw cut using any of a variety of tools, Once the two bevels are cut for the intended splice, clamp the such as a plane, a joiner, or a router. For most joints, you pieces to a flat guide board of similar material. Then, work a need a beveled fixture set at the correct slope to complete sharp, fine-tooth saw all the way through the joint. Remove the cut. Figure 6-25 illustrates one method of producing an the saw, decrease pressure, and tap one of the pieces on the accurate scarf joint. end to close the gap. Work the saw again through the joint.

Continue this procedure until the joint is perfectly parallel 6-16 Feathered end Undamaged section 5:1 slope New section to be spliced in Slope 10 to 1 in solid wood A Correctly beveled pieces Gap B Incorrect beveled pieces with matching surfaces. Then, make a light cut with the grain, using a sharp plane, to smooth both mating surfaces. Another method of cutting a scarf uses a simple scarf-cutting fixture that you can also fabricate for use with a router. Extend the work piece beyond the edge so the finished cut results in a feathered edge across the end of the scarf. [Figure 6-26] There are numerous tools made by individuals, and there are commercial plans for sale with instructions for building scarf- cutting tools. Most of them work, but some are better than others. The most important requirement for the tool is that it produces a smooth, repeatable cut at the appropriate angle.

Local damage to the top or bottom edge of a solid spar may be repaired by removing the damaged portion and fabricating a replacement filler block of the same material as the spar. Full width doublers are fabricated as shown and then all three pieces are glued and clamped to the spar. Nails or screws should not be used in spar repairs. A longitudinal crack in a solid spar may be repaired using doublers made from the proper thickness plywood. Care must be taken to ensure the doublers extend the minimum distance beyond the crack. Guide board Routed scaft Edges are guide for router base Slope fixed as appropriate 10:1 to 12:1, etc.

Clamp work piece to fixture [Figure 6-27] A typical repair to a built-up I spar is illustrated using plywood reinforcement plates with solid wood filler blocks. As with all repairs, the reinforcement plate ends should be feathered out to a 5:1 slope. [Figure 6-28] Repair methods for the other types of spar illustrated at the start of this section all follow the basic steps of repair. The wood used should be of the same type and size as the original spar. Always splice and reinforce plywood webs with the same type of plywood as the original. Do not use solid wood to replace plywood webs because plywood is stronger in shear than solid wood of the same thickness. The splices and scarf cuts must be of the correct slope for the repair with the face 6-17