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.]
Par 2-1 Page 2-1 CHAPTER 2. FABRIC COVERING SECTION 1. PRACTICES AND PRECAUTIONS
2-1. GENERAL. Cotton and Irish linen fabrics were the airframe coverings of choice from WWI through the 1950s. However, increases in cost and the short lifespan of natural fabrics became the driving factors which resulted in almost 100 percent replacement of original airframe fabrics by man-made, Supplemental Type Certificate (STC)-approved polyester, and glass filament fabric.
2-2. PROBLEM AREAS.
a. Deterioration. Polyester fabric deteriorates only by exposure to ultraviolet (UV) radiation as used in an aircraft covering environment. When coatings completely protect the fabric its service life is infinite. Therefore, it is very important to thoroughly protect the structure from deterioration before covering and provide adequate inspection access to all areas of fabric-covered components to allow inspection for corrosion, wood rot, and mice infestation.
Multiple drain holes in the lower ends of all fabric-covered sections also provide needed ventilation to remove condensation.
b. Tension. Polyester fabric obtains maximum tension on an airframe at 350 ºF, and will not be excessive on aircraft originally covered with natural fabric and 12 coats of Nitrate or Butyrate Dope. However, dope applied over full heat-tauted fabric can develop excess tension after aging and damage light aircraft structures. Coatings other than dope will not increase fabric tension after aging. The heat-tauting instructions given in the manual of each STC-approved covering process should be followed.
2-3. AIRCRAFT FABRIC-SYNTHETIC.
a. STC-Approved Covering Materials. There is a wide selection of STC-approved covering materials available which utilize synthetic fabric falling within the generic class “Polyester” and may vary in characteristics. Difference in the fabric may be denier, tenacity, thread count, weight, shrink, tension, and weave style.
b. Polyester Filaments. Polyester filaments are manufactured by polymerization of various select acids and alcohols, then extruding the resulting molten polymers through spinnerets to form filaments. The filaments are heat stretched to reduce to the desired denier or size. It is the heat stretching that imparts a memory in the filaments causing them to try and return to their original shorter length when reheated at a controlled temperature. Overheating will cancel the memory and melt the filaments.
c. Covering Procedures. Coating types, covering accessories, and covering procedures also may vary; therefore, the covering procedures given in the pertinent manuals must be followed to comply with the STC. The Federal Aviation Administration (FAA) STC-approved installation takes precedence over instructions in this advisory circular.
d. Installation. Initial installation of polyester fabric is similar to natural fabric. The fabric is installed with as little slack as possible, considering fittings and other protrusions. It may be
Par 2-3 Page 2-2 sewn into an envelope, installed as a blanket, or installed by cementing to the airframe with a fabric cement. Each STC may differ in the cement seam overlap, type of sewn seam, heat-shrinking procedures, and temperature.
2-4. AIRCRAFT FABRIC-NATURAL. Physical specifications and minimum strength requirements for natural fiber fabric, cotton and linen, used to recover or repair components of an aircraft, are listed in Table 2-1. Tear resistance is an important factor when considering aircraft fabric. A test method such as ASTM D1424 is recommended. Technical Standard Order (TSO)-C15d, Aircraft Fabric, Grade A (Aerospace Material Specification (AMS) 3806G); and TSO-C14b, Aircraft Fabric, Intermediate Grade (AMS 3804E), respectively, describe the minimum standards that all fabric must meet to qualify as aircraft covering material.
TABLE 2-1. COTTON AND LINEN FABRICS Materials Specification Minimum Tensile Strength New (undoped) Minimum Tearing Strength New (undoped) (ASTM D1424) Minimum Tensile Strength Deteriorated (undoped) Thread Count Per Inch Use and Remarks Airplane cloth mercerized cotton (Grade “A”).
TSO-C15d, as amended, references SAE AMS 3806G, as amended or MIL-C-5646 80 lb.
per inch warp and fill.
5 lb. warp and fill.
56 lb. per inch. 80 min., 84 max.
warp and fill.
For use on all aircraft.
Required on aircraft with wing loading of 9 psf or greater or placard never exceed speed of 160 mph or greater.
Airplane cloth mercerized cotton.
TSO-C14b, as amended, references SAE AMS 3804E.
65 lb.
per inch warp and fill.
4 lb. warp and fill.
46 lb. per inch. 80 min., 94 max.
warp and fill.
For use on aircraft with wing loading less than 9 psf and never exceed speed of less than 160 mph.
Airplane cloth mercerized cotton.
SAE AMS 3802, as amended.
50 lb.
per inch warp and fill 3 lb. warp and fill.
35 lb. per inch. 110 max.
warp and fill For use on gliders with wing loading of 8 psf or less, provided the placarded never exceed speed is 135 mph. or less.
Aircraft linen.
British 7F1. This material meets the minimum strength requirements of TSO-C15.
Par 2-5 Page 2-3
2-5. RECOVERING AIRCRAFT. Recover or repair aircraft with a fabric of equal quality and strength to that used by the original aircraft manufacturer. It is recommended that fabric conforming to TSO-C15d or TSO-C14b be used to recover aircraft originally covered with lower strength fabric conforming to AMS 3802.
NOTE: Recovering or repairing aircraft with any type fabric and/or coating other than the type used by the original aircraft manufacturer is considered a major alteration. Obtain approval from the FAA on fabric and installation data. Cotton and linen rib lacing cord, machine and hand-sewing thread, and finishing tapes should not be used with polyester and glass fabric covering.
a. Reinforcing tape minimum tensile strength is listed in Table 2-2. Reinforcing tape meeting specification MIL-T-5661 Type I, is acceptable. Reinforcing tape should have a minimum 40 lb. resistance without failure when static tested in shear against a single rib lace, or a pull-through resistance when tested against a single-wire clip, rivet, screw, or any other type of fabric-to-rib attachment. Reinforcing tape is used over the rib cab on top of the fabric and for inter-rib bracing.
b. Finishing Tape, sometimes referred to as surface tape, should have the same properties as the fabric used to cover the aircraft.
c. Lacing Cord shall have a minimum breaking strength of 40 lb. Lacing cord meeting the specifications listed in Table 2-2 is acceptable. Rib lace cord should have a microcrystalline fungicidal wax, paraffin-free wax, or beeswax coating, or other approved treatment to prevent wearing and fraying when pulling through the structure.
d. Machine Thread shall have a minimum breaking strength of 5 lb. Thread meeting the specifications listed in Table 2-2 is acceptable.
e. Hand-Sewing Thread shall have a minimum breaking strength of 14 lb. Thread meeting the specifications listed in Table 2-2, is acceptable. When covering with STC-approved fabric covering material, use the type of sewing thread approved by the STC and manufactured under the specific Parts Manufacturer Approval (PMA).
f. Flutter Precautions. When re-covering or repairing control surfaces, especially on high performance airplanes, make sure that dynamic and static balances are not adversely affected.
Weight distribution and mass balance must be considered to preclude to possibility of induced flutter.