Par 1-46 Page 1-83
Ends of reinforcement plates
should be feathered out Section A-A with a 5:1 slope.
FIGURE 1-7. Repairs to built-up “I” spar.
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.
Compression Rib Repair. Acceptable methods of repairing damaged compression ribs are shown in figure 1-12.
Figure 1-12(A) illustrates the repair of a compression rib of 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 is assumed to be cracked through the cap strips, web member, and compression member in the illustration. Cut the compression member as shown in 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.
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
Par 1-46 Page 1-84 in figure 1-12(A) except that the plywood reinforcement plate,
A, B, C, D = ORIGINAL DIMENSIONS
FIGURE 1-8. Method of splicing box spar flanges (plate method).
shown in section B-B, is continued the full distance between spars.
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.
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.
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.
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
Par 1-49 Page 1-85
1. AFTER INSERTED WEB HAS BEEN BONDED IN PLACE, BOND COVER STRIPS OVER ENTIRE
LENGTH OF WEB SPLICE JOINTS.
2. SECTIONAL SHAPE OF FILLER BLOCKS MUST CONFORM EXACTLY TO TAPER OF SPAR. THEY MUST
NOT BE TOO TIGHTLY FITTED, FOR WEDGING ACTION WILL LOOSEN EXISTING BONDS OF WEBS TO FLANGES. IF TOO LOOSELY FITTED, CRUSHING OF WEB WILL OCCUR WHEN CLAMPING.
3. DIRECTION OF FACE GRAIN OF INSERTED WEB SECTION AND COVER STRIPS TO BE SAME AS ORIGINAL WEB.
FIGURE 1-9. Method of splicing box spar webs.
Par 1-49 Page 1-86
FIGURE 1-10. Method of reinforcing a longitudinal crack and/or local damage in a solid spar.
for determining which process of bending should be used for the curvature being considered.
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 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.
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 ºF), 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.
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 onhand 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.
12F RECOMMENDED 10F MINIMUM
DAMAGED AREA
ORIGINAL GUSSET
PLYWOOD-
SPRUCE OR
BASSWOOD AT A SPAR * 4A → ЗА
- 5A - →C→ • 5A ~ AT TRAILING EDGE 10A A,B,C,D,E,F,G = ORIGINAL DIMENSIONS.
REINFORCEMENT PLATES
SHALL BE PLYWOOD AT A JOINT TO TRUSS MEMBERS THE DIRECTION OF THE GLUED AND NAILED.
FACE GRAIN MUST BE
THE SAME AS ORIGINAL GUSSET.
— ЗА • 10A - ЗА - з‡
BETWEEN JOINTS FIGURE 1-11. Repair of wood ribs.
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SEE FIGURE 1-11 FOR - REPAIR REPAIR OF CAP STRIPS. DAMAGE
SEE (D) (А) ЗА - ЗА SEE FIGURE 1-11 FOR DAMAGE REPAIR OF CAP STRIPS. REPAIR (B) SEE FIGURE 1-11 FOR
REPAIR OF CAP STRIPS REPAIR DAMAGE - (C)
•12A, BA COINIMENDED 6A 6 A (D)
A-A B-B
PLAND PACE GRAIN DERECTTON AS THIGINESS FIGURE 1-12. Typical wing compression rib repairs.
Par 1-50 Page 1-88
Par 1-50 Page 1-89 TABLE 1-2. Minimum recommended bend radii for aircraft plywood.
10 PERCENT MOISTURE
CONTENT, BENT ON COLD
MANDRELS
THOROUGHLY SOAKED IN HOT
WATER AND BENT ON COLD
MANDRELS
PLYWOOD
CHARACTERISTICS AT 90 º TO A T 0 º OR 45º
FACE GRAIN
T O F A C E
GRAIN AT 90 º TO A T 0º0R 45°
FACE GRAIN
TO FACE
GRAIN
THICKNESS (INCHES) NUMBER
OF PLIES MINIMUM BEND RADIUS (INCHES) .035 3 2.0 1.1 0.5 0.1 .070 3 5.2 3.2 1.5 0.4 .100 3 8.6 5.2 2.6 0.8 .125 3 12 7.1 3.8 1.2 .155 3 16 10 5.3 1.8 .185 3 20 13 7.1 2.6 .160 5 17 11 6 2 .190 5 21 14 7 3 .225 5 27 17 10 4 .250 5 31 20 12 5 .315 5 43 28 16 7 .375 5 54 36 21 10
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.
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.
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.
Taper the hole evenly to the outer mark with a chisel, knife, or rasp.
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.
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.
AC 43.13-1B
Par 1-36 Page 1-90 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.
Fill, sand, and refinish the patch.
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 entir ely 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 damage d 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-gr ain 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.
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, wh en 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.
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.
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.
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 section C-C of figure 1-15. When the damage follows or extends to a framing member, the scarf may be supported as shown in section B-B of figure 1-15. 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.
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.
Use waxed paper or plastic wrap to prevent bonding of the backing block to the skin.
Par 1-51 Page 1-27
FIGURE 1-13. Splayed patch.
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.
The block is removed, after the adhesive on the patch has set, leaving a flush surface to the repaired skin.
Steps in making a scarf patch when the back of the skin is not accessible are as follows:
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.
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.
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.
Plug Patch. Either oval or round plug patches may be used on plywood skins provided the damage can be covered by thepatches 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.
Steps in making an oval plug patch are as follows:
Explore the area about the hole to be sure it lies at least the width of the oval
J!
FRONT SPAR
TRIMMED OPENING
- MINIMUM RADIUS 5T
- RIBS B 8 DAMAGE →A
SADDLE GUSSET
REAR SPAR C→ 30T PATCHI — 12T → • PLYWOOD SKIN 8T ЗT (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—→ PATCH -4T
UNSUPPORTED RIB CAP LAP
SECTION B-B SECTION C-C FIGURE 1-14. Surface patches.
Par 1-54 Sec 4 Page 28
SADDLE GUSSET
NAILING STRIPS
MAXIMUM DIAMETER 25T
• MINIMUM DISTANCE 15T
TEMPORARY
BACKING 30T PATCH 12T
BACKING
PLYWOOD SADDLE GUSSET MINIMUM THICKNESS • T
BONDED IN PLACE Section A - (1/4" min.)
12 T - PATCH PATCH 12T
FRAMING FRAMING
MEMBER MEMBER BACKING BACKING •
CLAMP AND BOND BACKING CLAMP AND BOND BACKING
TO FRAME AND SKIN TO FRAME AND SKIN Section B - B NAILING STRIPS T Section C - C 12T WAXED PAPER OR PLASTIC WRAP TEMPORARY BACKING 1
BLOCK-SHAPE TO FIT SKIN Section D - D FIGURE 1-15. Scarf patches (back of skin accessible).
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FRONT SPAR →C RIBS SADDLE GUSSET +
REAR SPAR
- 30 T -12T= PLYWOOD SKIN 8T (1" MINIMUM) ЗT (¼" MINIMUM) - RIB CAP
PLYWOOD SADDLE GUSSET
SECTION A-A NAIL AND GLUE IN PLACE PLYWOOD (MINIMUM THICKNESS "T") SKIN -12T→ PATCH *12 T. - PATCH
T SPAR RIB CAP - ЗТ (¼" MINIMUM)
PLYWOOD OR SPRUCE PLYWOOD SKIN
SECTION B-B SECTION C-C FIGURE 1-16. Scarf patches (back of skin not accessible).
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