Wood Structure Repair Methods

Face grain direction of doublers LONGITUDINAL CRACK Note: 1. Make doublers from plywood for longitudinal crack repairs on spar face 2. Make doublers from solid wood (same species as spar) for insert repair of top or bottom of spar 3A3A ¼ A A B B/10 (max) No fitting within these limits Local damageLocal damage Scarf at ends of insert No less than 12 to 15 to 1 slope (minimum) Insert block—same species as spar grain running in the same direction as the original member. Not more than two splices should be made in any one spar. When a satisfactory repair to a spar cannot be accomplished, the spar should be replaced. New spars may be obtained from the manufacturer or the holder of a PMA for that part. An owner-produced spar may be installed provided it is made from a manufacturer-approved drawing. Care should be taken to ensure that any replacement spars accurately match the manufacturer’s original design.

Bolt & Bushing Holes All bolts and bushings used in aircraft structures must fit snugly into the holes. If the bolt or bushing is loose, movement of the structure allows it to enlarge the hole. In the case of elongated bolt holes in a spar or cracks in close proximity to the bolt holes, the repair may require a new section to be spliced in the spar, or replacement of the entire spar. All holes drilled in a wood structure to receive bolts or bushings should be of such size that inserting the bolt or bushing requires a light tapping with a wood or rawhide mallet. If the hole is so tight that heavy blows are necessary, Direction of grain in plywood reinforcement plates to be same as original web 2A 2A 2A 6A6A 15A ½ B ANo fitting within these limits Plywood B A Plywood Solid wood filler block Solid wood filler block Solid wood filler blockSolid wood filler block 6-18 deformation of the wood may cause splitting or unequal load distribution.

For boring accurate smooth holes, it is recommended that a drill press be utilized where possible. Holes should be drilled with sharp bits using slow steady pressure. Standard twist drills can be used in wood when sharpened to a 60° angle. However, a better designed drill was developed for wood boring called a lip and spur or brad point. The center of the drill has a spur with a sharp point and four sharp corners to center and cut rather than walk as a conventional drill sometimes does. It has the outside corner of the cutting edges leading, so that it cuts the periphery of the hole first and maximizes the chance that the wood fibers cut cleanly, leaving a smooth bore.

Forstner bits bore precise, flat bottomed holes in wood, in any orientation with respect to the wood grain. They must be used in a drill press because more force is needed for their cutting action. Also, they are not designed to clear chips from the hole and must be pulled out periodically to do this. A straight, accurate bore-through hole can be completed by drilling through the work piece and into a piece of wood backing the work piece. All holes bored for bolts that are to hold fittings in place should match the hole diameter in the fitting. Bushings made of steel, aluminum, or plastic are sometimes used to prevent crushing the wood when bolts are tightened. Holes drilled in the wood structure should be sealed after being drilled. This can be accomplished by application of varnish or other acceptable sealer into the open hole. The sealer must be allowed to dry or cure thoroughly prior to the bolts or bushings being installed.

Plywood Skin Repairs Plywood skin can be repaired using a number of different methods depending on the size of the hole and its location on the aircraft. Manufacturer’s instructions, when available, should be the first source of a repair scheme. AC 43.13-1 provides other acceptable methods of repair. Some of those are featured in the following section. Fabric Patch A fabric patch is the simplest method to repair a small hole in plywood. This repair is used on holes not exceeding 1-inch in diameter after being trimmed to a smooth outline. The edges of the trimmed hole should first be sealed, preferably with a two-part epoxy varnish. This varnish requires a long cure time, but it provides the best seal on bare wood.

The fabric used for the patch should be of an approved material using the cement recommended by the manufacturer of the fabric system. The fabric patch should be cut with pinking shears and overlap the plywood skin by at least 1-inch. A fabric patch should not be used to repair holes in the leading edge of a wing, in the frontal area of the fuselage, or nearer than 1-inch to any frame member. Face grain of patch parallel to face grain of skin Pressure plate 1∕8" or ¼" plywood Waxed paper or plastic wrap Weights or clamp Trim to circular shape (15T maximum diameter) Minimum distance to frame = 15T T = 1∕10" or less Plywood skin 5T 5T 6-19 Splayed Patch A splayed patch is a flush patch. The term splayed denotes that the edges of the patch are tapered, with the slope cut at a 5:1 ratio to the thickness of the skin. This may be used for small holes where the largest dimension of the hole to be repaired is not more than 15 times the skin thickness and the skin is not more than 1⁄10-inch thick. This calculates to nothing larger than a 1½-inch trimmed hole in very thin plywood.

Using the sample 1⁄10-inch thick plywood and a maximum trimmed hole size of 1½-inches, and cutting a 5:1 scarf, results in a 2½-inches round section to be patched. The patch should be fabricated with a 5:1 scarf, from the same type and thickness plywood as the surface being repaired. Glue is applied to the beveled edges and the patch is set with the grain parallel to the surface being repaired. A pressure plate of thicker plywood cut to the exact size of the patch is centered over the patch covered with waxed paper. A suitable weight is used for pressure until the glue has set. The repair is then sanded and finished to match the original surface.

[Figure 6-29] Surface Patch Plywood skins not over 1⁄8-inch thick that are damaged between or along framing members may be repaired with a surface or overlay patch. Surface patches located aft of the 10 percent chord line, or which wrap around the leading edge and terminate aft of the 10 percent chord line, are permissible. You can use surface patches to patch trimmed holes up to a 50-inch perimeter, and may cover an area as large as one frame or rib space. Trim the damaged area to a rectangle or triangular shape with rounded corners. The radius of the corners must be at least 5 times the skin thickness. Doublers made of plywood at least ¼-inch thick are reinforcements placed under the edge of the hole inside the skin. Nail and glue the doublers in place.

Extend the doublers from one framing member to another and strengthen at the ends by saddle gussets attached to the framing members. [Figure 6-30] The surface patch is sized to extend beyond the cutout as indicated. All edges of the patch are beveled, but the leading edge of the patch should be beveled at an angle at least 4:1 of the skin thickness. The face-grain direction of the patch must be in the same direction of the original skin. Where possible, weights are used to apply pressure to a surface patch until the glue has dried. If the location of the patch precludes the use of weight, small round head wood screws can be used to apply glue pressure to secure the patch. After a surface patch has dried, the screws can be removed and the holes filled. The patch should be covered with fabric that overlaps the original surface by at least 2-inches. The fabric should be from one of the approved fabric covering systems using the procedures recommended by the manufacturer to cement and finish the fabric.

Plug Patch Two types of plug patch, oval and round, may be used on plywood skins. Because the plug patch is only a skin repair, use it only for damage that does not involve the supporting structure under the skin. Cut the edges of a plug patch at right angles to the surface of the skin. Cut the skin also to a clean round or oval hole with edges at right angles to the surface. Cut the patch to the exact size of the hole; when installed, the edge of the patch forms a butt joint with the edge of the hole. You can use a round plug patch where the cutout repair is no larger than 6-inches in diameter. Sample dimensions for holes of 4-inches and 6-inches in diameter appear in Figure 6-31.

The following steps provide a method for making a round plug patch: 1. Cut a round patch large enough to cover the intended repair. If applicable for size, use the sample dimensions in Figure 6-31. The patch must be of the same material and thickness as the original skin. 2. Place the patch over the damaged spot and mark a circle of the same size as the patch. 3. Cut the skin inside the marked circle so that the plug patch fits snugly into the hole around the entire perimeter. 4. Cut a doubler of soft quarter-inch plywood, such as poplar. A small patch is cut so that its outside radius is 5⁄8-inch greater than the hole to be patched and the inside radius is 5⁄8-inch less. For a large patch the dimensions would be increased to 7⁄8-inch each.

If the curvature of the skin surface is greater than a rise of 1⁄8-inch in 6-inches, the doubler should be preformed to the curvature using hot water or steam. As an alternative, the doubler may be laminated from two pieces of 1⁄8-inch plywood. 5. Cut the doubler through one side so that it can be inserted through the hole to the back of the skin. Place the patch plug centered on the doubler and mark around its perimeter. Apply a coat of glue outside the line to the outer half of the doubler surface that will bear against the inner surface of the skin. 6. Install the doubler by slipping it through the cutout 6-20 Spar 30T 12T 8T (1" minimum) Rib cap Plywood saddle gusset Minimum thickness = T Nailed and glued in place Patch Plywood skin Rib cap 4T 12T 12TPatch Patch Section A-A Section B-B Section C-C Unsupported lap A C CB B B A C T T T T B A Damage B C A B Front Spar Rear Spar Ribs Trimmed Opening Minimum Radius 5T Saddle Gusset 3T (¼" Minimum) 6-21 Outer edge of doubler Nail holes Screw holes—to be filled before finishing Butt joint of patch to skin Plywood skin Saw cut in doubler Plywood doubler Plug patch Inner edge of doubler Saw cut in doubler Butt joint of patch to skin Grain direction of skin, patch, and doubler A B C (Laminate doubler from two pieces of 1∕8" ply in areas of skin curvature.) ¼" 2 5˜8" 3" 1 ³˜8" Small circular plug patch (Two rows of screws and nails are required for a large patch.) A B C DIMENSIONS 6-22 Large circular plug patch 3 7⁄8" 2" 2 1⁄8"