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 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.  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.  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  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. 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] 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.  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] 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. skin is not more than  1 ⁄ 10 -inch thick. This calculates to nothing  larger than a 1½-inch trimmed hole in very thin plywood. Plywood Skin Repairs Fabric Patch Splayed Patch Surface Patch 6-20