Par 1-49 Page 1-30 should be allowed to dry completely on the form. When bending plywood over a leading edge, drying may be hastened by laying a piece of coarse burlap over the leading edge before using it as a bending form. To speed drying, a fan may be used to circulate air around the repair part.
b. In bending pieces of small radii or to speed up the bending of a large number of parts of the same curvature, it may be necessary to use a heated bending form. The surface temperature of this form may be as high as 149° C (300° F), if necessary, without danger of damage to the plywood. The plywood should be left on the heated form only long enough to dry to room conditions.
1-50. REPAIRS TO DOUBLE CURVATURE PLYWOOD SKIN. The molded plywood necessary for a repair to a damaged plywood skin of double curvature cannot be made from flat plywood unless the area to be repaired is very small or is of exceedingly slight double curvature;
therefore, molded plywood of the proper curvature must be on hand before the repair can be made. If molded plywood of the proper curvature is available, the repair may be made using the same procedure as on single curvature skins.
1-51. TYPES OF PATCHES. There are four types of patches: splayed patch, surface (or overlay) patch, scarf patch, and plug patch. They are all acceptable for repairing plywood skins.
a. Splayed Patch. Small holes with their largest dimensions not over 15 times the skin thickness, in skins not more than 1/10 inch in thickness, may be repaired by using a circular splayed patch as illustrated in Figure 1-13. The term “splayed” is used to denote that the edges of the patch are tapered, but the slope is steeper than is allowed in scarfing operations.
FIGURE 1-13. SPLAYED PATCH
Par 1-51 Page 1-31
(1) Lay out the patch according to Figure 1-13. Tack a small piece of plywood over the hole for a center point and draw two circles with a divider, the inner circle to be the size of the hole and the outer circle marking the limits of the taper. The difference between the radii is 5T (5 times the thickness of the skin). If one leg of the dividers has been sharpened to a chisel edge, the dividers may be used to cut the inner circle.
(2) Taper the hole evenly to the outer mark with a chisel, knife, or rasp.
(3) Prepare a circular tapered patch to fit the prepared hole, and bond the patch into place with face-grain direction matching that of the original surface.
(4) Use waxed paper or plastic wrap (cut larger than the size of the patch) between the patch and the plywood pressure plate. This prevents excess adhesive from bonding the pressure plate to the skin. Center the pressure plate carefully over the patch.
(5) As there is no reinforcement behind this patch, care must be used so that pressure is not great enough to crack the skin. On horizontal surfaces, weights or sandbags will be sufficient.
On patches too far from any edge for the use of standard hand clamps, jaws of greater length may be improvised. Table 1-2, columns (1) and (3), may also be used for determining the maximum thickness of single laminations for curved members.
(6) Fill, sand, and refinish the patch.
b. Surface Patch. Plywood skins that are damaged between or along framing members may be repaired by surface or overlay patches as shown in Figure 1-14. Surface patches located entirely 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. Surface patches may have as much as a 50 inch perimeter and may cover as much as 1 frame (or rib) space. Trim the damaged skin to a rectangular or triangular shape and round the corners. The radius of rounded corners must be at least 5 times the skin thickness. Bevel the forward edges of patches located entirely aft of the 10 percent chord line to 4 times the skin thickness. The face-grain direction must be the same as the original skin. Cover completed surface patches with fabric to match surrounding area. The fabric must overlap the original fabric at least 2 inches.
FIGURE 1-14. SURFACE PATCHES
FRONT SPAR
• TRIMMED OPENING
- MINIMUM RADIUS 51
- RIBS DAMAGE
- SADDLE GUSSET
REAR SPAR 30T
- 12T → - PLYWOOD SKIN PATCHI 8T 3T (¼" MINIMUM) RIB CAP (1" MINIMUM)
- PLYWOOD SADDLE GUSSET MINIMUM THICKNESS = T.
SECTION A-A NAILED AND GLUED IN PLACE.
12 T - PATCH - 12 T- SPAR
UNSUPPORTED RIB CAP LAP
SECTION B-B SECTION C-C
c. Scarf Patch. A properly prepared and inserted scarf patch is the best repair for damaged plywood skins and is preferred for most skin repairs. Figure 1-15 shows the details and dimensions to be used when installing typical scarf skin patches, when the back of the skin is accessible. Follow Figure 1-16 when the back of the skin is not accessible. The scarf slope of 1 in 12, shown in both figures, is the steepest slope permitted for all kinds of plywood. If the radius of curvature of the skin at all points on the trimmed opening is greater than 100 times the skin thickness, a scarf patch may be installed.
Par 1-51 Page 1-32
FIGURE 1-15. SCARF PATCHES (BACK OF SKIN ACCESSIBLE)
SADDLE GUSSET STRIPS
MAXIMUM DIAMETER 25T
MINIMUM DISTANCE 15T
TEMPORARY
BACKING 30T
PATCH - 12T
BACKING
PLYWOOD SADDLE GUSSET MINIMUM THICKNESS • T
BONDED IN PLACE Section A - A (1/4" min.)
PATCH PATCH 12T *
FRAMING FRAMING
MEMBEF MEMBER
BACKING BACKING
CLAMP AND BOND BACKING CLAMP AND BOND BACKING
TO FRAME AND SKIN TO FRAME AND SKIN Section B - B NAILING STRIPS Section C - C 12T →
WAXED PAPER OR PLASTIC WRAP / LOCK SHARE TO KIT SKIN Section D - D Par 1-51 Page 1-33
Par 1-51 Page 1-34 FIGURE 1-16. SCARF PATCHES (BACK OF SKIN NOT ACCESSIBLE) (
1) Scarf cuts in plywood may be made by hand plane, spoke shave, scraper, or accurate sandpaper block. Rasped surfaces, except at the corners of scarf patches and sawn surfaces, are not recommended as they are likely to be rough or inaccurate.
(2) Nail strip or small screw clamping is often the only method available for bonding scarf joints in plywood skin repairs. It is essential that all scarf joints in plywood be backed with plywood or solid wood to provide adequate nail holding capacity. The face-grain direction of the plywood patch must be the same as that of the original skin.
(3) If the back of a damaged plywood skin is accessible (such as a fuselage skin), it should be repaired with a scarf patch, following the details shown in Figure 1-15. Whenever possible, the edges of the patch should be supported as shown in Figure 1-15, section C-C. When the damage follows or extends to a framing member, the scarf may be supported as shown in
Par 1-51 Page 1-35 Figure 1-15, section B-B. Damages that do not exceed 25 times the skin thickness in diameter after being trimmed to a circular shape and are not less than 15 times the skin thickness to a framing member, may be repaired as shown in Figure 1-15, section D-D.
(a) The backing block is carefully shaped from solid wood and fitted to the inside surface of the skin, and is temporarily held in place with nails.
(b) Use waxed paper or plastic wrap to prevent bonding of the backing block to the skin.
(c) A hole, the exact size of the inside circle of the scarf patch, is made in the block, and is centered over the trimmed area of damage.
(d) The block is removed, after the adhesive on the patch has set, leaving a flush surface to the repaired skin.
(4) Steps in making a scarf patch when the back of the skin is not accessible are as follows:
(a) After removing damaged sections, install backing strips, as shown in Figure 1-16, along all edges that are not fully backed by a rib or a spar. To prevent warping of the skin, backing strips should be made of a soft-textured plywood, such as yellow poplar or spruce, rather than solid wood. All junctions between backing strips and ribs or spars should have the end of the backing strip supported by a saddle gusset of plywood.
(b) If needed, nail and bond the new gusset plate to rib. It may be necessary to remove and replace the old gusset plate with a new saddle gusset, or it may be necessary to nail a saddle gusset over the original gusset.
(c) Attach nailing strips to hold backing strips in place while the adhesive sets. Use a bucking bar, where necessary, to provide support for nailing. After the backing strips are fully bonded, install the patch.
d. Plug Patch. Either oval or round plug patches may be used on plywood skins provided the damage can be covered by the patches whose dimensions are given in Figure 1-17 and Figure 1-18. The plug patch is strictly a skin repair, and should be used only for damage that does not involve the supporting structure under the skin. The face-grain direction of the finished patch must match the surrounding skin.
FIGURE 1-17. OVAL PLUG PATCH ASSEMBLY
OUTER EDGE OF DOUBLER
BUTT JOINT OF PATCH TO SKIN
INNER EDGE OF DOUBLER
NAIL HOLES -
SCREW HOLES -
TO BE FILLED BEFORE
FINISHING
PATCH DIMENSIONS B F SMALL | 1-1/2 2-1/8 1-1/2 7/8 3 4-1/2 LARGE 2 3-3/8 2-1/2 1-5/8
TWO ROWS OF SCREWS AND
NAILS REQUIRED ON LARGE BUTT JOINT OF PATCH TO SKIN PATCH.
S PLYWOOD DOUBLER (GRAIN PARALLEL TO SKIN) (LAMINATE DOUBLER FROM TWO PIECES OF 1/8" PLY IN AREAS OF
- PLUG PATCH (GRAIN PARALLEL TO SKIN) SKIN CURVATURE.)
- PLYWOOD SKIN Par 1-51 Page 1-36
FIGURE 1-18. ROUND PLUG PATCH ASSEMBLY
GRAIN DIRECTION OF SKIN, PATCH, AND DOUBLER.
- SAW CUT IN DOUBLER.
BUTT JOINT OF PATCH TO SKIN.
INNER EDGE OF DOUBLER.
- OUTER EDGE OF DOUBLER.
NAIL HOLES -
SCREW HOLES - TO BE FILLED BEFORE FINISHING.
- BUTT JOINT OF PATCH TO SKIN F PLUG PATCH
PLYWOOD SKIN SAW CUT IN DOUBLER PLYWOOD DOUBLE
LAMINATE DOUBLER FROM TWO PIECES OF 1/8" PLY IN AREAS OF SKIN CURVATURE.)
DIMENSIONS A C SMALL CIRCULAR PLUG PATCH 2 5/8 2 1 3/8 LARGE CIRCULAR PLUG PATCH 3 7/8 3 2 1/8 (TWO ROWS OF SCREWS AND NAILS REQUIRED
FOR LARGE PATCH)
(1) Steps in making an oval plug patch are as follows:
(a) Explore the area about the hole to be sure it lies at least the width of the oval doubler from a rib or a spar. Refer to Figure 1-17 for repair details.
(b) Prepare a patch and a doubler of the same species plywood as the surrounding skin using the dimensions shown in Figure 1-17.
Par 1-51 Page 1-37
Par 1-51 Page 1-38
(c) Lay the oval plug patch over the damage and trace the patch onto the skin. Saw to the line, and trim the hole edges with a knife and sandpaper.
(d) Mark the exact size of the patch on one surface of the oval doubler and apply adhesive to the area outside the line. Insert doubler through the hole and bring it, adhesive side up, to the underside of the skin with the pencil outline of the patch matching the edges of the hole. If the curvature of the surface to be repaired is greater than a rise of 1/8 inch in 6 inches, the doubler should be pre-formed by hot water or steam bending to the approximate curvature.
As an alternative to preforming of the 1/4 inch stock, the doubler may be laminated from two thicknesses of 1/8 inch ply.
(e) Apply nailing strips outlining the hole to apply bonding pressure between doubler and skin. Use a bucking bar to provide support for nailing. When two rows of nails are used, stagger nail spacing. Allow adhesive to cure.
(f) Apply adhesive to remaining surface of the doubler and to the mating surface on the patch. Lay the patch in position over the doubler, and screw the pressure plate to the patch assembly using a small nail to line up the holes that have been previously made with patch and plate matching. No. 4 round head screws are used. Lead holes in the plywood doubler are not necessary. Waxed paper or plastic wrap between the plate and patch prevents adhesive from bonding the plate to the patch. No clamps or further pressure need be applied, as the nailing strips and screws exert ample pressure.
(2) Round plug patches may be made by following the steps in Figure 1-18. The steps are identical to those for making the oval patch except for the insertion of the doubler. In using the round patch, where access is from only one side, the round doubler cannot be inserted unless it has been split.
1-52. FABRIC PATCH. Small holes not exceeding 1 inch in diameter, after being trimmed to a smooth outline, may be repaired by doping a fabric patch on the outside of the plywood skin.
The edges of the trimmed hole should first be sealed, and the fabric patch should overlap the plywood skin by at least 1 inch. Holes nearer than 1 inch to any frame member, or in the leading edge or frontal area of the fuselage, should not be repaired with fabric patches.
1-53.–1-63. [RESERVED.]
Par 1-64 Page 1-39 SECTION 5. FINISHING WOOD STRUCTURES
1-64. GENERAL. Any repair to spars, ribs, skin surfaces, or other structural parts of the airframe involves finishing as the final step of the job. The surface finish is the final line of defense to prevent the destructive effects of moisture entry into the structure. The time and effort spent during the preparatory phase of the refinishing process will be reflected in the appearance and longevity of the finished surface. Adherence to the instructions issued by the finish manufacturer is necessary to obtain the appearance desired and protective characteristics for the product used. The primary objective of interior finishes is to afford protection of the wood against serious change in moisture content when exposed to damp air or to water that gains entrance to closed spaces by condensation or by penetration of rain, mist, or fog. Coatings on contact areas between wood and metal protect the metal against corrosion from moisture in the wood. The primary objectives of the exterior finish are to protect the wood against weathering, provide a suitable appearance, and present a smooth surface in flight.
1-65. ACCEPTABLE FINISHES. Any varnish conforming to A-A-1788, Varnish, Oil:
Interior, as amended, or other coating approved by the airframe manufacturer or the FAA is acceptable. Exterior surfaces must be further protected from the effects of abrasion, weather, and sunlight. A number of systems for exterior finishing have Supplemental Type Certificate (STC) approval and are manufactured under a PMA. (See Chapter 2, Fabric Covering.) Low viscosity epoxy adhesive (meeting the requirements of paragraph 1-4 for that purpose) may be used as an internal surface finish when subsequent bonding is necessary.
1-66. PRECAUTIONS.
a. When making repairs, avoid excessive contamination of surfaces with adhesive squeeze-out at joints and on all surfaces. Excess adhesive should always be removed before applying finish. Because many paints and adhesives are incompatible, even a slight amount of adhesive underneath the finish may cause premature deterioration of the finish.
b. Soiling substances, such as oil and grease, should be removed as completely as possible. Naphtha may be used to remove surface deposits of oil and grease; however, thinned residue may penetrate into any unprotected wood. In areas where minor amounts of oil or grease have penetrated the wood surface, removal may be accomplished by use of an absorbent type of cleaner such as gunsmith’s whiting or a clothing spot lifter such as K2r™. Marks that are made by grease pencils or lumber crayons containing wax are harmful and should be removed, but marks made by ordinary soft graphite pencils and nonoily stamp pad inks may be safely finished over. All dust, dirt, and other solid particles should be removed.
c. Sawdust, shavings, and chips must be removed from enclosed spaces before they are sealed off by replacement of skin. A vacuum cleaner is useful for such cleaning.
d. Since most adhesives will not bond satisfactorily to sealers, it is necessary to avoid applying sealer over the areas where adhesive will be applied. Mark off areas to receive adhesive, and allow an additional 1/4 inch on each side of the adhesive area to provide for misalignment when mating the parts. It is preferable to leave some unsealed areas rather than risk weakening the joint by accidental overlap of the sealer into the bonded areas. Wherever possible,
Par 1-66 Page 1-40 apply sealer to the margins after the adhesive has cured. As an example, the lower skin of a wing bay would be installed first, leaving access from above to apply sealer. All low spots (where moisture would collect) are well sealed. The top skin would be installed last, so that the only unsealed margins would be on upper surfaces where moisture is least likely to collect.
e. An alternative to the previous paragraph is to use an approved epoxy coating and compatible epoxy adhesive. Apply the coating, allowing 1/4 inch margins as in the previous paragraph. After the coating has cured, apply epoxy adhesive to joint surfaces, and overlap the adhesive onto the sealer. Close joint and clamp. The epoxy adhesive will bond satisfactorily to the coating and ensure a complete coverage of the wood surfaces. Use only approved and compatible adhesives and coatings for this method.
1-67. FINISHING OF INTERIOR SURFACES. Finish repaired ribs, spars, interior of plywood skin, and other internal members, including areas of contact between metal and wood, by applying one thinned coat (for penetration into wood grain) of varnish or other acceptable finish, followed by two full coats. Protect built-up box spars and similar closed structures on the interior in the same way. Where better protection is required, as on the surfaces of wheel wells and the bottoms of hulls below the floor boards, an additional coat of aluminized sealer consisting of 12 to 16 ounces of aluminum paste per gallon of sealer, may be applied.
1-68. FINISHING OF EXTERIOR SURFACES. Exterior surfaces require more protection than interior areas due to the effects of abrasion, weather, and sunlight. (See Chapter 2.) Tests have shown that the interior temperature of wooden aircraft structures can reach 185° F when the aircraft is finished in a dark color and parked outdoors on a hot, still day. Exposure to prolonged high temperature is detrimental to wood, adhesives, and finishes. Wood loses approximately 25 percent of its strength at 125° F. For this reason, the mechanic should consider temperature effects when selecting finish colors or looking for areas of likely deterioration. The lowest temperatures are found when the aircraft is finished in white or very light colors, while darker colors produce higher temperatures. A general trend toward higher temperatures may be seen when exterior colors are yellow, pink, light blue, aluminum, purple, blue, light green, orange, tan, red, green, brown, and black. A lighter shade of a particular color helps to reduce temperatures.
1-69. FINISHING OF END GRAIN SURFACES. End grain portions of wooden members are much more absorbent than side grain. Because of this extreme vulnerability to moisture entry, it is necessary to take extra precautions to seal end grain.
a. Apply at least one thinned coat of acceptable sealer to ensure maximum penetration, and then follow with as many full strength coats as necessary to achieve a smooth, glossy coating. Depending on the type wood to be finished, two to four full coats will be required. A final coat of aluminized varnish may be applied to end grain surfaces. If the surfaces are to be finished with dope or lacquer, a dope-proof sealer, similar to Federal Specification A-A-1788, or epoxy sealer should be used.
b. Exposed end grain includes such surfaces as spar butts, skin edges, areas around vent holes, inspection holes, fittings, and exposed scarfed or tapered surfaces.
Par 1-70 Page 1-41
1-70. FINISHING WITH FABRIC OR TAPE. A number of systems for exterior finishing have STC approval and are manufactured under a PMA. Follow the product manufacturer’s instructions for the system used.
a. If the finish surrounding the repair is a traditional dope system, seal the wood grain with a suitable solvent resistant one-part varnish, commonly described as “dope proof,” or a twopart epoxy varnish. Follow with two coats of clear dope, and allow sufficient drying time between coats.
b. Apply a third coat of clear dope and lay a piece of pinked-edge airplane cloth into the wet film. All air bubbles should be worked out by brushing to ensure maximum adhesion. When dry, apply one brush coat, to ensure proper penetration, and at least one spray coat of clear dope.
The dried spray coat may be sanded with fine sandpaper to obtain a smoother finish. Complete the refinishing of the surface by application of a topcoat as required to match the adjacent area.
1-71. SEALING OF BOLT HOLES. Bolt holes in wooden structure provide a vulnerable entry point for moisture. Variations in moisture content around bolt holes can lead to decay or splitting. In addition, excessive moisture at bolt holes promotes corrosion of the bolts. Sealing of the wood surfaces in bolt holes 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 bolt installation.
1-72.–1-79. [RESERVED.]