Par 1-44 Page 1-25 contacted for approval before making repairs not in accordance with the manufacturer-approved instructions or the recommendations of this AC. Longitudinal cracking or the recurrence of cracking 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 plywood repair stock is procured from another climatic region, it should be allowed to season, in the same storage area as the part to be repaired, for no less than 2 weeks.
c. Preventing Cracks. An important step in the prevention of longitudinal cracking, particularly in spar butts, is to ensure that the wood is thoroughly sealed with a penetrating and highly moisture-resistant finish. Application of a thin, slow-curing epoxy adhesive or sealer can be very effective in slowing or preventing moisture changes in spar butts.
1-45. ELONGATED HOLES IN SPARS. In cases of elongated bolt holes in a spar, or cracks in the vicinity of bolt holes, splice in a new section of spar, or replace the spar entirely. If hole elongation or cracking is minimal and the bolt holes are for noncritical fittings, repair (rather than replacement) may be feasible. Obtain approval for any such repair from the manufacturer or a representative of the FAA. In many cases, it has been found advantageous to laminate the new section of the spar, particularly if the spar butts are being replaced.
1-46. RIB REPAIRS. Ribs may be replaced by new parts made by the manufacturer or the holder of a PMA for that part. Owner-produced ribs may be installed providing they are made from a manufacturer-approved drawing or by reference to an existing original rib. A rib may be made by reference to an existing rib providing sufficient evidence is presented to verify that the existing rib is an original part and that all materials and dimensions can be determined. The contour of the rib is important to the safe flying qualities of the aircraft, and care should be taken that any replacement ribs accurately match the manufacturer’s original design.
a. Rib Repair Methods. Acceptable methods of repairing damaged ribs are shown in Figure 1-11. Wood ribs should not be nailed to wood spars by driving nails through the rib cap strips, as this weakens the rib materially. The attachment should be by means of adhesive with cement coated, barbed, or spiraled nails driven through the vertical rib members on each face of the spar.
FIGURE 1-11. REPAIR OF WOOD RIBS 12F RECOMMENDED * 10F MINIMUM
DAMAGED AREA
ORIGINAL GUSSET
PLYWOOD- 'SPRUCE OR
BASSWOOD AT A SPAR 4A 5A
AT TRAILING EDGE A,B,C,D,E,F,G = ORIGINAL DIMENSIONS.
REINFORCEMENT PLATES
SHALL BE PLYWOOD AT A JOINT TO TRUSS MEMBERS GLUED AND NAILED. THE DIRECTION OF THE
FACE GRAIN MUST BE
THE SAME AS ORIGINAL GUSSET.
* ЗА → 10A
BETWEEN JOINTS
b. Compression Rib Repair. Acceptable methods of repairing damaged compression ribs are shown in Figure 1-12.
(1) Figure 1-12(A) illustrates the repair of a compression rib of the "" 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 is assumed to be cracked through the cap strips, web member, and compression member in the illustration. Cut the compression member as shown in Par 1-46 Page 1-26
Par 1-46 Page 1-27 Figure 1-12(D). Cut and replace the aft portion of the cap strips, and reinforce as shown in Figure 1-11. The plywood side plates are bonded on, as indicated in Figure 1-12(A). These plates are added to reinforce the damaged web.
(2) Figure 1-12(B) illustrates a compression rib of the type that is basically a standard rib with rectangular compression members added to one side and plywood web to the other side.
The method used in this repair is essentially the same as in Figure 1-12(A) except that the plywood reinforcement plate, shown in section B-B, is continued the full distance between spars.
(3) Figure 1-12(C) illustrates a compression rib of the “I” type with a rectangular vertical member on each side of the web. The method of repair is essentially the same as in Figure 1-12(A) except the plywood reinforcement plates on each side, shown as striped blocks in section C-C, are continued the full distance between spars.
FIGURE 1-12. TYPICAL WING COMPRESSION RIB REPAIRS SEE FIGURE 1-11 FOR REPAIR REPAIR OF CAP STRIPS. DAMAGE
SEE (D) (A) ЗА - SEE FIGURE 1-11 FOR REPAIR DAMAGE REPAIR OF CAP STRIPS.
(B) SEE FIGURE 1-11 FOR
REPAIR OF CAP STRIPS DAMAGE DED A-A 8 - B c-c
PLAND PACE CHEON DEMEET ON AS THICKNESS
1-47. PLYWOOD SKIN REPAIR. Make extensive repairs to damaged stressed skin plywood structures in accordance with specific recommendations from the aircraft manufacturer. It is recommended that repairs be made by replacing the entire panel, from one structural member to the next, if damage is very extensive. When damaged plywood skin is repaired, carefully inspect the adjacent internal structure for possible hidden damage. Repair any defective frame members prior to making skin repairs.
Par 1-46 Page 1-28
1-48. DETERMINATION OF SINGLE OR DOUBLE CURVATURE. Much of the outside surface of plywood aircraft is curved. On such areas, plywood used for repairs to the skin must be similarly curved. Curved skins are either of single curvature or of double (compound) curvature. A simple test to determine which type of curvature exists may be made by laying a sheet of heavy paper on the surface in question. If the sheet can be made to conform to the surface without wrinkling, the surface is either flat or single curvature. If the sheet cannot be made to conform to the surface without wrinkling, the surface is of double curvature.
1-49. REPAIRS TO SINGLE CURVATURE PLYWOOD SKIN. Repairs to single curvature plywood skin may usually be formed from flat plywood, either by bending it dry or after soaking it in hot water. The degree of curvature to which a piece of plywood can be bent will depend upon the direction of the grain and the thickness. Table 1-2 is a guide for determining which process of bending should be used for the curvature being considered.
TABLE 1-2. MINIMUM RECOMMENDED BEND RADIT FOR AIRCRAFT PLYWOOD 10 PERCENT MOISTURE
CONTENT, BENT ON COLD
MANDRELS
THOROUGHLY SOAKED IN
HOT WATER AND BENT ON
COLD MANDRELS AT 90° TO AT 0° OR 45° AT 90° TO AT 0° OR 45°
FACE GRAIN TO FACE FACE GRAIN TO FACE
PLYWOOD GRAIN GRAIN
CHARACTERISTICS
THICKNESS NUMBER
MINIMUM BEND RADIUS (INCHES) (INCHES) OF PLIES .035 2.0 1.1 0.5 O.1 .070 5.2 3.2 1.5 0.4 .100 8.6 5.2 2.6 0.8
• 125 12 7.1 3.8 1.2
• 155 16 10 5.3 1.8 .185 20 13 7.1 2.6
• 160 5 17 6 2
• 190 5 21 14 7 3 .225 27 17 10 4 .250 31 20 12 5 .315 43 28 16 7 375 5 54 36 21 10
a. Plywood, after softening, may be bent on a cold ventilated form, or it may be bent over the leading edge near the area being patched if space permits. In either method the repair part Par 1-48 Page 1-29
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