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or at spars. Acceptable methods of repairing damaged ribs are shown in figure 2-9.

(3) Compression ribs. Acceptable methods of repairing damaged compression ribs are shown in figure 2-10. (A) illustrates the repair of a compression rib of the "I" section type, i. e., wide, shallow cap strips, a center plywood web, with a rectangular compression member on each side of the web. The rib is assumed to be cracked through cap strips, web member, and compression member. Cut the compression member as shown in (D), remove, and replace the shortest section, adding the reinforcing blocks as also shown in (D). Cut and replace the aft portion of the cap strips, reinforcing as shown in figure 2-9, except that the reinforcing blocks are split in the vertical direction to straddle the center web. The plywood side plates, as indicated in (A), are glued on. These plates are added to reinforce the damaged web. (B) illustrates a compression rib of the type that is basically a standard rib with rectangular compression members added to one side and a plywood web to the other side. The method used in this repair is essentially the same as in (A) except that the plywood reinforcing plate shown solid black in section B-B is continued the full distance between spars. (C) illustrates a compression rib of the "I" type with a rectangular vertical member each side of the web. The method of repair is essentially the same as in (A) except that the plywood reinforcing plates on each side shown in solid black in section C-C are continued, as in (C), the full distance between spars.

(e) Plywood skin-(1) General. Extensive repairs to damaged stressed skin plywood structures should be made in accordance with specific recommendations from the 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, the adjacent internal structure should be carefully inspected for possible hidden damage. Any defective frame members should be repaired prior to making skin repairs.

(i) Types of patches. Four types of patches-the surface or overlay patch. the splayed patch, the plug patch, and the scarf patch-are acceptable for repairing plywood skins. Surface patches should not be used on skins over one

eighth-inch thick. Splayed patches should not be used on skins over onetenth-inch thick. There are no skin thickness limitations for the use of scarf patches and plug patches.

(ii) 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 fit the surface without wrinkling, the surface is either flat or has single curvature. If, however, the sheet cannot be made to fit the surface without wrinkling, the surface is of double curvature.

(iii) Repairs to single curvature skin. (a) Repairs to skins of single curvature 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 2-3 is presented as a guide in determining which process of bending should be used for the curvature being considered.

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(b) Plywood after softening may be bent on a cold ventilated form or it may be bent over the leading edge near the part being patched if space permits. In P either method it 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. A fan to circulate the air over the bent piece will speed the drying. 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 & 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 form, however, only long enough to dry to room conditions.

(iv) Repairs to double curvature 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 following the recommended procedures.

(2) Splayed patch. Small holes with largest dimensions not over 15 times the skin thickness, in skins not more than one-tenth-inch in thickness, may be repaired by using a circular splayed patch as illustrated in figure 2-11. 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. The following steps should be taken in making a splayed patch:

(i) Lay out the patch according to figure 2-11. Center the dividers as near to the damage as is possible or tack a small piece of plywood over the hole for a center point and draw two circles, the inner one to be the size of the hole and the outer one 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 completely through.

(ii) Taper the hole evenly to the outer circle with a chisel, knife, or rasp.

(iii) Prepare a circular tapered patch to fit the prepared hole, and glue the patch into place with face grain direction matching that of the original surface.

(iv) Use waxed paper between the patch and a plywood pressure plate cut to the exact size of the patch. This prevents extruded glue from binding patch and plate together. Center the plate carefully over the patch.

(v) Apply pressure. As there is no reinforcing 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 vertical surfaces apply hand clamps lightly but snugly. On patches too far in for the use of standard hand clamps, jaws of greater length may be improvised.

(vi) Fill, sand, and refinish the patch. (3) Surface patch. Plywood skins that are damaged between or along framing members may be repaired by surface of overlay patches as shown in figure 2-13. The damaged skin should be trimmed to

a rectangular or triangular shape and the corners rounded. The radius of rounded corners should be at least five times the skin thickness. Surface patches should be covered with fabric before finishing. Fabric should overlap the original skin at least 2 inches. 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. Patches located entirely aft of the 10 percent chord line should have their forward edges beveled to 4 times the skin thickness. Surface patches may have as much as a 50-inch perimeter and may cover as much as 1 frame (or rib) space. The face grain direction should be the same as the original skin.

(4) Scarf patch-(i) General. (a) A properly prepared and inserted scarf patch is the best repair for damaged plywood skins. It is the preferred type for most skin repairs. Figure 2-14 shows the details and dimensions to be used when installing typical scarf skin patches when the back of the skin is accessible. Figure 2-15 should be followed 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 species 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.

(b) 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 sawed surfaces are not recommended as they are likely to be rough or inaccurate.

(c) Nail strip gluing is often the only method available for gluing scarf joints in plywood when used in repair work; therefore 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 should be the same as that of the original skin.

(ii) Scarf patches (back of skin accessible). (a) When the back of a damaged plywood skin is accessible (such as a fuselage skin), it should be repaired with scarf patches following the details shown in figure 2-14. Whenever possible the edge of the patch should be supported as

shown in section C-C. When the damage follows or extends to a framing member, however, the scarf may be supported as shown in section B-B.

(b) Damages that do not exceed 25 times the skin thickness in diameter after being trimmed to a circular shape, and if the trimmed opening is not nearer than 15 times the skin thickness to a framing member, may be repaired as shown in figure 2-14, section D-D. The backing block is especially shaped from solid wood and fitted to the inside surface of the skin, and is temporarily held in place with nails. 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 glue on the patch has set, and leaves a flush surface to the repaired skin.

(iii) Steps in making scarf patch (back of skin not accessible). (a) After removing damaged sections, install backing strips, as shown in figure 2-15, 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 glue new gusset plate to rib. It may be necessary to remove and replace the old gusset plate by a new saddle gusset or it may be necessary to nail a saddle gusset over the original.

(c) Attach nailing strips to hold backing strips in place while the glue sets. Use bucking bar where necessary to provide support for nailing. Unlike the smaller patches made in a continuous process, work on the airplane must wait while the glue holding the backing strips sets. After setting, complete finishing in usual manner.

(5) Plug patches-(1) General. Two types of plug patches, oval and round, may be used on plywood skins provided the damage can be covered by the patches whose dimensions are given in figures 2-12 and 2-16. As the plug patch is strictly a skin repair it should be used only for damage that does not involve the supporting structure under the skin. Oval patches must be prepared with the face grain carefully oriented to the same direction as the original skin. Orienta

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(b) Lay a previously prepared oval Ho plug patch over the damage and trace the patch. Saw to the line and trim the hole edges with a knife and sandpaper. rep (c) Mark the exact size of the patch on one surface of the oval doubler and Ge apply glue to the area outside the line. The oval doubler should be made of some soft-textured plywood, such as yellow poplar or spruce. Insert doubler through the hole and bring it, glue side up, to the underside of the skin with its pencil outline of the patch matching the edges of the hole. If the curvature of the surface to be repaired is greated than a rise of one-eighth inch in 6 inches, the doubler should be preformed, by hot water or steam bending, to the approximate curvature.

(d) Apply nailing strips, outlining the hole, to apply glue pressure between doubler and skin. Use bucking bar to provide support for nailing. When two rows of nails are used, stagger nail spacing.

(e) Apply glue to remaining surface and to an equivalent surface on the patch.

(f) 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 cellophane between the plate and patch prevents glue from sealing the plate to the patch. No clamps or further pressure need be applied as the nailing strips and screws exert ample pressure. Hot sandbags, however, may be laid over the patch to speed the setting of the glue. Finish in the usual manner.

(iii) Round plug patch. The steps in making a round plug patch shown in figure 2-16 are identical with those for making the oval patch except the insertion of the doubler. In using the round patch, where access is from only one

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side, the round doubler cannot be inEserted unless it has been split.

(6) 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 Eof the trimmed hole should first be = sealed, and the fabric patch should over

lap 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.

(f) Finishing structural repairs—(1) General. Any repair to spars, ribs, skin surfaces, or other structural parts of the airframe involves finishing as the final step in the job.

(2) Precautions to be observed. (i) When making repairs, avoid excessive contamination of surfaces with glue squeeze-out at joints and on all surfaces.

Excess glue should always be removed before applying finish. Because paints and glues are incompatible, even a slight amount of glue underneath the finish may cause premature deterioration.

(ii) Soiling substances, such as oil and -grease, should be removed as completely as possible. Naphtha may be used to sponge off oil and grease. Markings 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 nonblotting stamp-pad inks may be safely finished over. All dust, sander dust, dirt, and other solid particles should be cleaned off.

(iii) Sawdust, shavings, and chips should be removed from enclosed spaces before they are sealed off by replacement of skin. A vacuum cleaner is useful for such cleaning.

(iv) Since no satisfactory gluable sealer has yet been developed, it is necessary to avoid applying sealer over the areas where glue will be applied. Areas to receive glue should be marked off with pencil, allowing an additional one-fourth inch on each side of the glue area to provide for misalinement when mating the parts. It is preferable to leave some unsealed areas rather than risk weakening the glue joint by accidental overlap of the sealer into the glued areas.

(v) Finish is likely to crack when applied over flush-driven nails and screws.

To avoid this a strip of tape may be applied over the heads after application of sealer and before the final finish is applied.

(vi) Fill all holes left from nail-strip gluing or countersunk nails and screws with a wood filler before finishing the surface. It may be necessary to cover with a patching putty the slight depressions left after applying filler if a completely smooth surface is desired, but as a rule patching putty may be dispensed with safety.

(vii) Surfaces which are likely to come in contact with fabric during the doping process should be treated with a dope-proof paint, cellophane tape, etc., to protect them against the action of the solvents in the dope.

surfaces.

(3) Finishing of interior Repaired ribs, spars, interior of plywood skin, and other internal members, including areas of contact between metal and wood, should be finished by applying at least two coats of spar varnish. Built-up box spars and similar closed structures should be protected on the interior by at least one heavy coat of spar varnish or lionoil. 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.

(4) Finishing of exterior surfaces. Exterior surfaces should first be sealed with at least two coats of sealer or spar varnish. The surface finish should then be completed by the application of enamel, aluminized varnish or other special finish as required to duplicate the original finish. If dope or lacquer is used to complete the finish, the sealer coats should be dope-proof. Spar varnish or sealer conforming to Specification MIL-V-6894 is satisfactory.

(5) Finishing of end-grain surfaces. (i) End-grain surfaces, such as edges of plywood skins and holes in spars and other primary structural members, require careful protection. Sand these surfaces smooth. Apply two coats of a highly pigmented sealer, or 1 coat of wood filler, and 1 coat of clear sealer to end-grain interior surfaces and cut holes. Exterior end-grain surfaces (except those covered with doped fabric) require an additional (third) coat of clear sealer. 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 Specification MIL-V-6894 should be used.

(ii) Exposed end-grain includes such surfaces as those around vent holes, inspection holes and fittings, and exposed scarfed or tapered surfaces such as those of tapered blocking.

(6) Finishing with fabric or tape. (i) To refinish with fabric or tape, it is first necessary to insure that paint has been removed from an area greater than that to be covered by the fabric.

(ii) Apply 2 brush coats of a dopeproof sealer similar to Specification MIL-V-6894, allowing the first coat to dry 2 hours and the second coat at least 6 hours. Follow with 1 coat of clear dope, and allow it to dry 45 minutes. Apply a second coat of clear dope and iay into the wet film a piece of pinked-edge airplane cloth. All air bubbles should be worked out by brushing to insure maximum adherence. Allow this to dry 45 minutes. Apply 1 brush coat to insure proper penetration and at least 1 spray coat of clear dope, allowing each to dry 45 minutes. The dried spray coat may be scuffed with fine sandpaper to obtain a smoother finish. Complete the refinishing of the surface by application of lacquer, enamel, or aluminized varnish as required to match the adjacent area.

(iii) The size of the fabric patch should be such as to extend at least one-halfinch on each side of any crack or group of cracks, at least 1 inch on each side of a scarfed joint glue line, and at least 2 inches beyond any edge of a skin patch, to insure proper adhesion.

[Supp. 1, 18 FR. 7391, Nov. 21, 1953, as amended by Supp. 7, 22 F.R. 600, Jan. 30, 1957]

§ 18.30-3 Fabric covering (FAA policies which apply to § 18.30).

(a) Textile materials. All fabric, surface tape, reinforcing tape, machine thread, lacing cord, etc., used for recovering or repairing an aircraft structure should be of high-grade aircraft textile material of at least as good quality and equivalent strength as those described in subparagraphs (1) through (7) of this paragraph.

(1) Aircraft fabric. Acceptable fabric such as cotton and linen for covering wings, control surfaces, and fuselages are listed in table 3-1. Fabrics conforming

to the Automotive Material Specifications incorporate a continuous marking showWing the specification number to permit identification of the fabric in the field. (See § 18.30-8 (e).)

(2) Re-covering aircraft with original type fabric. Aircraft should be recovered or repaired with fabric of at least as good quality and equivalent strength as that originally used on the aircraft. In re-covering aircraft which were originally covered with low strength or "so called" glider cloth, however, it is considered more desirable to use Grade A or intermediate fabric conforming to AMS 3806 or 3804, as amended, respectively.

(3) Reinforcing tape. Acceptable reinforcing tape is listed in table 3-2. Reinforcing tape should be of similar quality to the fabric and at least onehalf the strength of that conforming to specification MIL-T-5661.

(4) Surface tape. Surface tape (also finishing tape) should have approximately the same properties as the fabric used. See table 3-2.

(5) Lacing cord. Lacing cord should have the strength of at least 80 pounds double or 40 pounds single strand. Acceptable lacing cord is listed in table 3-2.

(6) Machine thread. Machine thread should have a strength of at least 5 pounds single strand (Table 3-2).

(7) Hand-sewing thread. Hand-sewing thread should have a strength of at least 14 pounds single strand (Table 3-2).

(b) Covering practices-(1) General. The method of fabric attachment should be identical, as far as strength and reliability are concerned, to the method used by the manufacturer of the airplane to be re-covered or repaired. Fabric may be applied so that either the warp or fill threads are parallel to the line of flight. Either the envelope method or blanket method of covering is acceptable. (See subparagraph (5) of this paragraph.)

(2) Flutter precautions. When repairing control surfaces, especially on high-performance airplanes, care should be exercised that the repairs do not involve the addition of weight aft of the hinge line. Such a procedure may adversely disturb the dynamic and static balance of the surface to a degree which would induce flutter. As a general rule, it will be required to repair control surfaces in such a manner that the struc

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