9/8/98 AC 43.13.1B CHAPTER 2. FABRIC COVERING

SECTION 1. PRACTICES AND PRECAUTIONS

2-1. GENERAL. Cotton and Irish linen fab- 2-3. FABRIC-SYNTHETIC. rics were the coverings of choice from WWI through the 1950’s. However, in- a. STC-Approved Covering Materials. creases in cost and the short lifespan of natural There is a wide selection of STC-approved fabrics became the driving factors which re- covering materials available which utilize sulted in almost 100 percent replacement of synthetic fabric falling within the generic class original airframe fabrics by man-made, “Polyester” and may vary in characteristics. STC-approved, polyester, and glass filament Difference in the fabric may be denier, tenac- fabric. ity, thread count, weight, shrink, tension, and weave style. 2-2. PROBLEM AREAS. b. Polyester Filaments. Polyester Fila- a. Deterioration. Polyester fabric dete- ments are manufactured by polymerization of riorates only by exposure to ultraviolet radia- various select acids and alcohols, then extrud- tion as used in an aircraft covering environ- ing the resulting molten polymers through ment. When coatings completely protect the spinnerets to form filaments. The filaments fabric its service life is infinite. Therefore, it is are heat stretched to reduce to the desired very important to thoroughly protect the denier or size. It is the heat stretching that im- structure from deterioration before covering parts a memory in the filaments causing them and provide adequate inspection access to all to try and return to their original shorter length areas of fabric-covered components to allow when reheated at a controlled temperature. inspection for corrosion, wood rot, and mice Overheating will cancel the memory and melt infestation. Multiple drain holes in the lower the filaments. ends of all fabric-covered sections also provide needed ventilation to remove condensation. c. Covering Procedures. Coating types, covering accessories, and covering procedures b. Tension. Polyester fabric obtains also may vary; therefore, the covering proce- maximum tension on an airframe at 350 ºF, dures given in the pertinent manuals must be and will not be excessive on aircraft originally followed to comply with the STC. The FAA covered with natural fabric and 12 coats of STC-approved installation takes precedence Nitrate or Butyrate Dope. However, dope ap- over instructions in this advisory circular. plied over full heat-tauted fabric can develop excess tension after aging and damage light d. Installation. Initial installation of aircraft structures. Coatings other than dope polyester fabric is similar to natural fabric. will not increase fabric tension after aging. The fabric is installed with as little slack as The heat-tauting instructions given in the possible, considering fittings and other protru- manual of each STC-approved covering proc- sions. It may be sewn into an envelope, in- ess should be followed. stalled 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.

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2-4. AIRCRAFT FABRIC-NATURAL. attachment. Reinforcing tape is used over the Physical specifications and minimum strength cab on top of the fabric and for inter-rib requirements for natural fiber fabric, cotton . and linen, used to recover or repair compo- nents of an aircraft, are listed in table 2-1. b. Finishing Tape, sometimes referred to Tear resistance is an important factor when as surface tape, should have the same proper- considering aircraft fabric. A test method such ties as the fabric used to cover the aircraft. as ASTM D 1424 is recommended. Technical Standard Order TSO-C15d, entitled Aircraft c. Lacing Cord shall have a minimum Fabric, Grade A (AMS 3806D); and breaking strength of 40 lb. Lacing cord meet- TSO-C14b, Aircraft Fabric, Intermediate ing the specifications listed in table 2-2 is ac- Grade (AMS 3804C) current edition, respec- ceptable. Rib lace cord should have a micro- tively, describe the minimum standards that all crystalline fungicidal wax, paraffin-free wax, fabric must meet to qualify as aircraft covering or beeswax coating, or other approved treat- material. ment to prevent wearing and fraying when pulling through the structure. 2-5. RECOVERING AIRCRAFT. Re- cover or repair aircraft with a fabric of equal d. Machine Thread shall have a mini- quality and strength to that used by the original mum breaking strength of 5 lb. Thread meet- aircraft manufacturer. It is recommended that ing the specifications listed in table 2-2 is ac- fabric conforming to TSO-C15d or TSO-C14b ceptable. be used to recover aircraft originally covered with lower strength fabric conforming to e. Hand-Sewing Thread shall have a AMS 3802, current edition. minimum breaking strength of 14 lb. Thread meeting the specifications listed in table 2-2, is NOTE: Recovering or repairing air- acceptable. When covering with STC- craft with any type fabric and/or approved fabric covering material, use the type coating other than the type used by of sewing thread approved by the STC and the original aircraft manufacturer is manufactured under the specific PMA. considered a major alteration. Obtain approval from the FAA on fabric and f. Flutter Precautions. When re-covering installation data. Cotton and linen rib or repairing control surfaces, especially on lacing cord, machine and hand-sewing high performance , make sure that thread, and finishing tapes should not dynamic and static balances are not adversely be used with polyester and glass fabric affected. Weight distribution and mass bal- covering. ance must be considered to preclude to possi- bility of induced flutter. a. Reinforcing tape minimum tensile strength is listed in table 2-2. Reinforcing tape 2-6. PREPARATION OF THE STRUC- meeting specification MIL-T-5661, Type I, TURE FOR COVERING. One of the most current edition, is acceptable. Reinforcing tape important items when covering aircraft is the should have a minimum 40 lb. resistance with- proper preparation of the structure. Before out failure when static tested in shear against a covering, the airframe must be inspected and single rib lace, or a pull-through resistance approved by a FAA-certified mechanic or re- when tested against a single-wire clip, , pair station. screw, or any other type of fabric-to-rib

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TABLE 2-1. Cotton and linen fabrics. Minimum Minimum Minimum Materials Specification Tensile Tearing Tensile Thread Use and Strength New Strength New Strength Count Per Remarks (undoped) (undoped) Deteriorated Inch (ASTM D 1424) (undoped) For use on all cloth TSO-C15d, as 80 pounds 5 pounds 56 pounds 80 min., aircraft. mercerized amended, per inch warp and fill. per inch. 84 max. Required on cotton references warp and fill. warp and fill. aircraft with (Grade “A”). Society loading Automotive of 9 p.s.f. or Engineers greater or AMS 3806d, placard never as amended or exceed speed MIL-C-5646 of 160 m.p.h. or greater. For use on Airplane cloth TSO-C14b, as 65 pounds 4 pounds warp 46 pounds per 80 min., aircraft with mercerized amended, per inch warp and fill. inch. 94 max. wing loading cotton. references and fill. warp and fill. less than Society 9 p.s.f. and Automotive never exceed Engineers speed of less AMS-3804c, than as amended. 160 m.p.h. For use on Airplane cloth Society 50 pounds per 3 pounds warp 35 pounds per 110 max. gliders with mercerized Automotive inch warp and and fill. inch. warp and fill. wing loading cotton. Engineers fill. of 8 p.s.f. or AMS 3802, as less, provided amended. the placarded never-exceed speed is 135 m.p.h. or less. This material Aircraft linen. British 7F1. meets the minimum strength Requirements of TSO-C15.

a. Battery Box Treatment. An asphaltic, with a # 44 (0.086) drill bit. Size 6 screws, rubber-based acid-proof coating should be ap- drill bit size # 36 (0.1065), may be installed in plied to the structure in the area of a battery stripped or worn holes drilled for box, by brush, for additional protection from size 4 screws, usually without redrilling. Worn battery acid. Control cables routed in the area holes for wire clips and wire barbs should be of the battery box should be coated with redrilled a minimum 1-1/2 hole distance from paralketone. the original locations using a drill jig to ensure correct spacing, with the appropriate size drill b. Worn Holes. Oversized screw holes or bit. Drill bit size # 30 (0.128) may be used to worn size 4 self-tapping screw holes through redrill oversize holes for 1/8-inch diameter ribs and other structures used to attach fabric blind a minimum 1-1/2 hole diameter may be redrilled a minimum 1-1/2 hole di- distance from the original location. ameter distance from the original hole location

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TABLE 2-2. Cotton and Linen, Tapes and Threads.

Minimum Yards Per Materials Specification Yarn Size Tensile Strength Pound Use and Remarks Used as reinforcing tape on fabric and Reinforcing MIL-T-566 1 E, 150 pounds per under rib lacing cord. Strength of other tape, cotton. Type 1 1/2 inch width. widths approx. in proportion. MIL-Y-1140H Lacing cord, Lacing fabric to structures. Unless Federal T-C-57 40 pounds. 310 minimum. prewaxed already waxed, must be lightly waxed 1F braided cotton. before using. Lacing cord, Lacing fabric to structures. Unless MIL-C-5648A 80 pounds. 170 minimum. braided cotton. already waxed, must be lightly waxed before using. Lacing cord Lacing fabric to structures. Unless thread, high MIL-T-5660B Ticket No. 62 pounds. 480 minimum. already waxed, must be lightly waxed tenacity cotton. 10. before using. Machine Federal V-T- thread cotton 276H 20/4 ply 5 pounds. 5,000 nominal. Use for all machine sewing. Hand-Sewing Federal V-T- Use for all hand-sewing. thread cotton. 276H Type III B 8/4 ply 14 pounds. 1,650 nominal. Use fully waxed thread. Finishing Use over seams, leading edges, trailing (Surface) tape Same as fabric Same as fabric edges, outer edges and ribs, pinked, cotton. used. used. raveled or straight edges.

c. Fairing Precautions. Aluminum and stainless parts need not be dope- replacement fairings installed in proofed. short sections may telescope during normal bending loads or from thermal expansion (1) A solvent-sensitive primer on fer- and contraction. This action may cause a rous and aluminum components wrinkle to form in the fabric, at the edge of the which will be in contact with fabric may be lap joint. Leading edge fairing sections may protected from solvent damage by overcoating be fastened together with rivets or screws to with a two-part epoxy primer. Epoxy primer prevent telescoping after installation. Trailing meeting MIL-P-53022B is acceptable. edges should be adequately secured to prevent movement and wrinkles. (2) Small metal or wood surfaces, such as rib caps, to which fabric will not be dope d. Dope Protection. Solvents found in bonded as a part of the particular fabric at- nitrate and butyrate dope will penetrate, wrin- tachment procedure may be protected from kle, lift, or dissolve most one-part wood var- dope damage by cellophane tape or aluminum nishes and one-part metal primers. All wood foil. surfaces that come in contact with doped fabric should be treated with a protective coating e. Chafe Protection. Fabric and finishing such as aluminum foil, cellulose tape, or dope- tape is often cut through with sandpaper over proof paint to protect them against the action sharp edges during the coating and finishing of the solvents in the dope. This can also be procedure and later polishing. All sharp metal accomplished by recoating with a suitable, edges or protruding screws, nails, rivets, and solvent resistant two-part epoxy varnish, which bolt heads should be covered with an anti- will be impervious to solvent penetration and chafe tape to prevent cutting and wearing damage after curing. Clad aluminum through the fabric after installation. Use

Page 2-4 Par 2-6 9/8/98 AC 43.13-1B appropriate non-bleeding cotton adhesive glass fiber deck cloth instead of fabric, no coated tape, finishing tape, or strips of fabric, sealer or dope should be applied to the ply- cut from the fabric being used to cover the air- wood as it would inhibit penetration of epoxy craft, doped in place. resin.

(1) Small holes cut through the fabric to (1) Sand surfaces as needed to accommodate flying wires, control cables, and remove old loose dope or varnish residue to fittings, must be reinforced with finishing tape provide a clean bonding surface. Remove any or fabric patches cut from the same fabric used oil, grease, or other contamination with a suit- for the covering. able solvent such as naphtha. Small, rough ar- eas and irregularities in the plywood surface (2) Areas needing additional chafe pro- and around any plywood repairs may be filled tection such as control cables routed firm and smoothed with an appropriate commercial against the fabric surface should be protected grade wood filler. Filling large warp depres- with patches cut from cotton duck, leather, or sions on plywood surfaces with a wood filler plastic. These patches may be sewn, doped, or for cosmetic purposes is not acceptable. cemented in place, as appropriate. (2) After cleaning and sanding all ply- (3) Any drag and anti-drag wires in the wood surfaces, seal the wood grain with a suit- should be protected from chafing at able solvent resistant two-part epoxy varnish. cross points. After the varnish has thoroughly dried, apply two brush or spray coats of clear dope, allow- f. Inter-Rib Bracing. Use a woven fabric ing sufficient drying time between coats. tape of the same quality and width as that used for the rib lace reinforcing, where so incorpo- 2-7. FABRIC SEAMS. Seams parallel to rated in the wing design by the original aircraft the line of flight are preferable; however, manufacturer. When the original routing for spanwise seams are acceptable. the inter rib bracing is not known, the tape will be routed diagonally, alternating between the a. Sewn Seams. top and bottom of each rib cap on each succes- sive rib, if a single pair, half way between the (1) Machine-sewn seams should be front and rear spars. The number of tape pairs double stitched using any of the styles illus- will duplicate the original aircraft manufac- trated in figure 2-1 A, B, C, or D. A machine- turer’s installation. Tapes will be routed con- sewn seam used to close an envelope at a tinuously from the wing butt to the wingtip wingtip, wing , and bow, with one turn of tape around each inter- control surface trailing edge, and a mediate rib cap strip. Care should be given to may be made with a single stitch position the tape so as not to interfere with when the seam will be positioned over a control cables, bellcranks or push-pull rods. structure. (See figure 2-1 E.) The envelope size should accommodate fittings or other g. Preparation of Plywood Surfaces for small protrusions with minimum excess for in- Covering. Prior to covering plywood sur- stallation. Thick or protruding leading edge faces, prepare the surface by sanding, cleaning, sewn seams should be avoided on thin airfoils and applying sealer and dope. When plywood with a sharp leading edge radius because they surfaces are to be covered with light weight may act as a stall strip.

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(2) Hand sew, with plain overthrow or that will minimize any protrusions. Cover the baseball stitches at a minimum of four stitches seams with finishing tape at least 3 inches per inch, or permanent tacking, to the point wide, centering the tape on the seam. where uncut fabric or a machine-sewn seam is reached. Lock hand sewing at a maximum (7) Sewn seams parallel to the line of of 10 stitch intervals with a double half hitch, flight (chordwise) may be located over ribs. and tie off the end stitch with a double half However, careful attention must be given to hitch. At the point where the hand-sewing or avoid damage to the seam threads by rib lace permanent tacking is necessary, cut the fabric needles, screws, rivets, or wire clips that are so that it can be doubled under a minimum of used to attach the fabric to the rib. Cover 3/8 inch before sewing or permanent tacking is chordwise seams with a finishing tape at least performed. (See figure 2-2.) 3 inches wide with the tape centered on the seam. (3) After hand sewing is complete, any temporary tacks used to secure the fabric over b. Doped Seams. wood structures may be removed. (1) For an overlapped and doped span- (4) Cover a sewn spanwise seam on a wise seam on a wing’s leading edge, overlap wing’s leading edge with a minimum 4-inch the fabric at least 4 inches and cover with fin- wide pinked-edged surface tape with the tape ishing tape at least 4 inches wide, with the tape centered on the seam. centered at the outside edge of the overlap seam. (5) Cover a spanwise-sewn seam at the wing trailing edge with pinked-edge surface (2) For an overlapped and doped span- tape that is at least 3 inches wide. For aircraft wise seam at the trailing edge, lap the fabric at with never-exceed speeds in excess of least 3 inches and cover with pinked-edge sur- 200 mph, cut V notches at least 1 inch in depth face tape at least 4 inches wide, with the tape and 1/4 inch in width in both edges of the sur- centered on the outside edge of the overlap face tape when used to cover spanwise seams seam. on trailing edges of control surfaces. Space notches at intervals not exceeding 6 inches. (3) For an overlapped and doped seam On tape less than 3 inches wide, the notches on wingtips, wing butts, perimeters of wing should be 1/3 the tape width. In the event the control surfaces, perimeters of empennage sur- surface tape begins to separate because of poor faces, and all fuselage areas, overlap the fabric adhesion or other causes, the tape will tear at a 2 inches and cover with a finishing tape that is notched section, thus preventing progressive at least 3 inches wide, centered on the outside loosening of the entire length of the tape which edge of the overlap seam. could seriously affect the controllability of the aircraft. A loose tape acts as a only on (4) For an overlapped and doped seam a movable surface. It becomes a on a on a wing’s leading edge, on aircraft with a fixed surface and has no effect at the trailing velocity never exceed (Vne) speed up to and edge other than drag. including 150 mph, overlap the fabric 2 inches and cover with a finishing tape that is at least (6) Make spanwise-sewn seams on the 3 inches wide, with the tape centered on the wing’s upper or lower surfaces in a manner outside edge of the overlap seam.

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FIGURE 2-1. Fabric seams.

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TEMPORARY TACKS MAY BE USED TO SECURE FABRIC IN POSITION FOR HAND- SEWN SEAMS OVER A WOOD STRUCTURE. PERMANENT TACKS MAY BE USED TO SECURE FABRIC AT WING BUTT STRUCTURES IN LIEU OF A HAND-SEWN SEAM AND SHOULD BE OF THE APPROPRIATE LENGTH AND GAUGE, AND MADE OF CORROSION RESISTANT STEEL OR BRASS.

FIGURE 2-2. Typical methods of attaching fabric.

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(5) For an overlapped and doped seam (1) An alternative method, when fabric on the perimeter of a wing (except a leading of sufficient width is available, is to sew to- edge), perimeters of wing control surfaces, pe- gether, side-by-side, two sections of fabric, rimeters of empennage surfaces, and all areas placing the seams spanwise on the leading of a fuselage, on aircraft with a Vne speed up edge, then fit and sew the wingtip and trailing to and including 150 mph, overlap the fabric edge in the same manner as the multiple piece 1 inch and cover with a finishing tape that is at chordwise envelope. least 3 inches wide, centered on the outside edge of the overlap seam. (2) An envelope may be developed for the fuselage in the same manner, with a final 2-8. COVERING METHODS. The closing along a longeron by hand-sewn or method of fabric attachment should be identi- overlapped and doped seams in accordance cal, as far as strength and reliability are con- with the aircraft Vne speed. cerned, to the method used by the manufac- turer of the airplane being recovered or re- b. The Blanket Method. A blanket is paired. Carefully remove the old fabric from developed by sewing together, side-by-side, the airframe, noting the location of inspection multiple sections of fabric with the seams covers, drain grommets, and method of at- chordwise or two wide sections of fabric, tachment. Cotton or linen fabric may be ap- side-by-side, placing the seam spanwise on the plied so that either the warp or fill-threads are leading edge, the same as an envelope. Close parallel to the line of flight. Either the enve- the three remaining sides with a hand-sewn lope method or blanket method of covering is seam or overlapped and doped seams in accor- acceptable. dance with the aircraft Vne speed. Small components may be covered by wrapping one a. The Envelope Method. A wing enve- piece of fabric over a straight leading or trail- lope may be developed by two methods. Ma- ing edge, then closing three sides with hand- chine sew together, side by side multiple fabric stitched or overlapped and doped seams in ac- sections, cut to reach chordwise around the cordance with the aircraft Vne speed. wing, starting and ending at the trailing edge with a minimum of 1 inch excess length. The NOTE: All overlapped and doped sewn envelope is then positioned around the seams will be made only over under- wing and secured with closely spaced T-Head lying supporting structures extending pins at the wingtip and trailing edge. Excess the full width of the seam. material may then be trimmed. Carefully re- move the envelope and complete by machine c. Machine-sewn alternate. An alternate sewing at the wingtip and along the trailing to machine-sewn seams on a wing envelope or edge, except where the geometry of the wing blanket is to use two sections of wide fabric ( and cut out) would prevent the spanwise. Attach the fabric with overlapped sewn envelope from being reinstalled. After and doped seams at the leading and trailing reinstalling the envelope, the un-sewn sections edge, wingtip and wing butt, in accordance and butt end are then closed by hand-sewn or with the aircraft Vne speeds. (Reference para- overlapped and doped seams in accordance graph 2-7 b.) Smaller components may be with the aircraft Vne speed. (Refer to para- covered in the same manner. The fuselage may graph 2-7 b.) be covered with multiple fabric sections with

Par 2-7 Page 2-9 AC 43.13-1B 9/8/98 overlapped and doped seams on the b. Reinforcing tapes should be of the or other wide fabric-forming structures in ac- appropriate width for hardware attachment cordance with the aircraft Vne speed. (Refer- such as screws, rivets, wire clips, etc., which ence paragraph 2-7 b.) pierce the center of the tape. Reinforcing tape under cord lacing should be the same width as d. Holes in Fabric. Never cut any holes the rib to which the fabric is laced and may be in the fabric for inspection panels, spar fittings, comprised of multiple widths positioned or drain grommets; or attach the fabric to the side-by-side to achieve the required width. airframe with rib lacing screws, rivets, clips, or rib stitch cord until the fabric has been semi- c. When the aircraft Vne speed is over tauted and stabilized with several coats of 250 mph, anti-tear strips, cut from the same dope. quality fabric used to cover the aircraft, are recommended for use under reinforcing tape 2-9. REINFORCING TAPE. on the entire top surface of the wing and on the portion of the wing’s bottom surface in the a. Reinforcing tape should be securely propeller slipstream. The propeller slipstream bonded to the fabric surface with dope before is considered to be the propeller diameter plus cord lacing or installation of hardware. Where one outboard rib. The anti-tear strip should be multiple attachments are in close proximity, installed completely around the wing, begin- such as on a wing rib, continuous reinforcing ning and ending at the trailing edge in the pro- tape should be installed, extending at least peller slipstream, and installed from the trail- 1 inch past the last attachment at each end. ing edge over the leading edge and back under Random or wide spacing, such as on fuselage to the front spar on the balance of the ribs. stringers or empennage surfaces, may be rein- Anti-tear strips should extend 1/2 inch past the forced with 2-inch lengths of reinforcing tape wing rib cap edges and be thoroughly bonded centered on the attachment location. to the fabric with dope before the reinforcing tape is installed. (See figure 2-3.)

FIGURE 2-3. Exploded side view of rib.

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2-10. LACING. cial attention should be given to avoid interfer- ence with the routing of any control cable, a. Fabric should be attached to aircraft bellcrank, or any other movable item. To pre- components to prevent ballooning due to vent chafing and cutting of the lacing cord, aerodynamic forces, in the identical manner control cables or any other movable items and locations as used by the original aircraft should be tensioned or positioned to their manufacturer. Any deviation from the original normal alignment before rib lacing and method(s) of attachment, such as screws, riv- checked afterwards to ensure adequate clear- ets, wire clips, lacing cord, etc., are considered ance. When a lace cord will be chafed by a a major alteration and in conflict with the air- moving component, a blindstitch may be made craft type design data. Obtain FAA approval around the top and bottom rib caps as illus- on any deviation. trated in figure 2-11.

NOTE: When the type of rib lace c. Stationary structures interfering with knot used by the original aircraft needle routing may be circumvented by align- manufacturer is not known, the modi- ing the needle forward or aft adjacent to the rib fied seine knot shown in figure 2-4 cap. Pull the needle through the wing and then through figure 2-9c will be used. return through the same hole and exit at the desired adjacent location.

NOTE: The first lace on a wing rib should be spaced from the leading edge fairing no more than 1/2 the re- quired lace spacing for the balance of the rib.

d. Both surfaces of fabric covering on wings and control surfaces must be securely fastened to the ribs by lacing cord or any other method originally approved for the aircraft. Care must be taken to insure that all sharp edges against which the lacing cord may bear are protected by tape in order to prevent abra- sion of the cord. Separate lengths of lacing cord may be joined by the splice knot shown in figure 2-10 or tied off. The first loop is tied with a square knot as illustrated in figure 2-5 and figure 2-9a, and the knot secured with a half hitch on each side after the lacing is pulled tight around the rib. The needle is then routed through the wing and around the rib cap at the

FIGURE 2-4. Standard external modified seine knot used for single and double rib lacing.

b. During the installation of lacing cord through a wing or any other component, spe-

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FIGURE 2-5. Starting stitch for rib lacing.

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FIGURE 2-6. Standard single-loop lacing.

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FIGURE 2-7. Standard knot for double-loop lacing.

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FIGURE 2-8. Standard double-loop lacing (optional).

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FIGURE 2-9a. Alternate sequence to tie a modified seine knot for rib lacing.

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FIGURE 2-9b. Alternate sequence to tie a modified seine knot for rib lacing.

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Step 9. Pull cord section “D” perpendicular to the fabric sur- Step 10. After all slack is removed by pulling cord section “D,” face to remove all slack in the cord back to the last switch hands and place a thumbnail on the loose rib lacing knot while working the loose knot to the knot formed on the right-hand side, then secure the right side. Do not pull cord section “E.” knot by pulling firmly perpendicular to the fabric surface on cord section “E.”

Step 11. After completing all lacing in the same sequence, the end is secured with a half hitch after the modi- fied seine knot. The knot is pulled to the inside by routing the needle thru the wing before cutting the cord to leave the end inside.

FIGURE 2-9c. Alternate sequence to tie a modified seine knot for rib lacing.

Page 2-18 Par 2-10 9/8/98 AC 43.13-1B next rib lace location with the cord and knot aircraft manufacturer, should be reinstalled in remaining on top of the fabric surface as illus- the same location and spacing as installed by trated in figure 2-5, figure 2-6, and figure 2-8. the original aircraft manufacturer. The lace An alternate method is to route the needle un- cord is routed around the stringer, rib cap, or der the fabric and out through the next lace lo- other structure using an appropriate length, cation, then back down through the wing as single or double pointed, curved needle as il- illustrated in figure 2-9a through figure 2-9c. lustrated in figure 2-11. Blind laces are tied A modified seine knot is then tied as illustrated with a square knot, then pulled tight and se- in figure 2-4 through figure 2-9c. cured with a half hitch at each side. The lace may be rotated to place the knot under the fab- (1) Rotate each lace loop to place the ric surface before cutting the cord. knot at the side of the rib cap to reduce the protrusion and aerodynamic interference be- fore moving to the next lace location, or the cord routed under the fabric to the next lace location as illustrated in figure 2-9a through figure 2-9c. The end cord is then cut off leav- ing a minimum of 1/4 inch stub. Lacing ten- sion should be uniform.

(2) Repeated pulling of long lengths of lacing cord may remove wax coating from the cord and cause fraying. Convenient lengths of rib lacing cord may be used to lace long or The splice knot is made by crossing the thick ribs. The end of each length is tied off ends of the cord, and making four complete with a half hitch as illustrated in figure 2-9c, or wraps with the small end of the free piece around the end of the standing piece. The end is then if needed, separate lengths of lacing cord may doubled back through the formed loop. The be joined by using the splice knot illustrated in other free end is wrapped and doubled back. figure 2-10. The long ends of the cords are then pulled until the knot is tight. The short ends are cut close to the knot. This finishes the splice knot. (3) Lacing is installed through other components, where applicable, in the same FIGURE 2-10. Splice knot. manner as a wing. Single, wide space lace at- tachments, usually used on empennage sur- 2-11. STITCH SPACING. faces, are tied with a square knot and half hitch on each side, the same as a starting wing rib a. Rib lace spacing on wings, formerly lace illustrated in figure 2-9a, steps 1 and 2. referred to as stitch spacing, should be no The lace may be rotated to place the knot un- greater than the spacing used by the original der the fabric before cutting the cord. aircraft manufacturer. When the original spacing cannot be determined the maximum e. Blind lacing on a fuselage, wing rib spacing illustrated in figure 2-12 should be caps above and below a , and any used on the wings and wing control surfaces. other component, when used by the original

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FIGURE 2-11. Blindstitch lacing - square knot secured with half hitches.

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FIGURE 2-12. Fabric attachment spacing

b. When the original lace spacing on the affected by solvents found in adhesives or empennage surfaces and fuselage, is not dopes, nor do they become brittle because of known, a maximum spacing of two times the age or cold weather. spacing shown in figure 2-12 for the slipstream area (prop wash) on the wings may be used. 2-12. FASTENERS. Several designs employ screws, rivets, or single-wire c. The installations of fabric attach- metal clips to secure the fabric to the wing. ments such as screws, rivets, wire clips, and rib lacing should be delayed until the fabric is a. Screws holding the old fabric can be stabilized and pulled taut with dope. This ac- removed after spinning a small sharpened tube tion is delayed to avoid pulling wing ribs and around each screw or using a razor blade to cut other structures out of alignment or tearing the and peel away the finishing tape. Care must be fabric at attachment points as the fabric be- taken not to mark or scribe the underlying comes taut. All lacing should be installed ad- metal or wood structure. Blind rivets through jacent to the structure to which the fabric is ribs can be removed by drilling in the center to being laced, to avoid tearing the fabric and/or undercut the head. creating slack in the cord loop when a load is applied. Where plastic washers were used by b. Single-wire clips may be removed the aircraft manufacturer to provide increased without damage to the rib by inserting a wide, pull-through resistance, under the heads of riv- thin screwdriver blade under the clip and care- ets or screws, the same diameter aluminum fully twisting. Apply a lifting force at the clip washer may be used as replacement. Alumi- end to pull it up through the hole. num washers are used because they are not

Par 2-11 Page 2-21 AC 43.13-1B 9/8/98

NOTE: It is important that any dam- width should be sufficient to bond the fabric a age found to ribs, such as oversize minimum of 3/8 inch on each side of all fabric rivet or screw holes, and cracks or attachments. Two inch width tape is normally breaks in the rib cap, should be tagged used. Tapes over wing rib lacing should ex- immediately for easy location and re- tend a minimum of 1/2 inch past each end of pair later. any reinforcing tapes. Random or widely- spaced attachments may be covered by indi- c. When repairs are made to fabric sur- vidual sections of fabric or finishing tape. faces attached by special mechanical methods, duplicate the original type of fastener. When c. Installation of finishing tapes for ad- self-tapping screws are used for the attachment ditional wear resistance is recommended over of fabric to the rib structure, observe the fol- the edges of all fabric-forming structures. This lowing procedure: includes fuselage stringers, longerons, leading and trailing edges, false or nose ribs, control (1) Redrill the holes where necessary surfaces, and empennage ribs not already cov- due to wear, distortion, etc., and in such cases, ered and protected by a finishing tape that is use a screw one size larger as a replacement. required to be on a fabric seam or fabric at- tached to the structure. Compound surfaces, (2) Extend the length of the screw be- such as wingtip bow and empennage surfaces, yond the rib capstrip at least two threads. are more conveniently taped using bias cut finishing tape, which easily conforms to the (3) Install a thin washer, preferably compound contour, rather than notching linear aluminum, under the heads of screws and dope cut tape to fit the surface. Bias cut tape will be pinked-edge tape over each screw head. reduced to approximately two thirds the origi- nal cut width when pulled tight around a 2-13. FINISHING TAPE. wingtip bow and should be considered when selecting the width of tape for the various lo- a. Finishing tape (surface tape) is in- cations. stalled after the fabric has been pulled taut with the initial dope application. This proce- d. Finishing tapes are applied by coating dure is performed to prevent ripples from the fabric surface over which the tape will be forming in fabric panels adjacent to newly ap- applied with dope, applying the tape over the plied tapes. Ripple formation is caused by the wet dope film, then brushing the tape firmly inability of the combined tape and fabric to onto the fabric surface. This action will assure tighten uniformly with adjacent fabric when a good bond by thoroughly saturating and wet- additional dope is applied. ting the finishing tape.

b. In addition to the tape widths re- 2-14. INSPECTION RINGS AND DRAIN quired to be installed over fabric seams speci- GROMMETS. fied in paragraph 2-7, finishing tape should be installed as weather protection over all rib a. Inspection Rings. Inspection access is lacing, screws, rivets, wire clips, or other de- provided adjacent to or over every control vices used to secure fabric. This includes bellcrank, drag-wire junction, cable guide, wings, control surface ribs, empennage surface pulley, wing fitting, or any other component ribs, and fuselage stringers, where so installed throughout the aircraft which will be inspected by the original aircraft manufacturer. Tape or serviced annually. They are installed only

Page 2-22 Par 2-12 9/27/01 AC 43.13-1B CHG 1 on the bottom side of the wings except where through openings in the sides and top, and installed on the top surface by the original when flying, everywhere throughout the manufacturer. structure. Taxiing on wet runways also splashes water up through any bottom holes. (1) Cutting the holes may be delayed Therefore, provisions must be made to drain until needed; however, all covers should be water from the lowest point in each fabric finished in matching colors with any trim lines panel or plywood component throughout the and stored until needed. Spraying matching airframe while in a stored attitude. Drain holes colors a year later is expensive and time con- also provide needed ventilation. suming. (1) Install drain grommets on the under (2) The 3-9/16 inch inside diameter side of all components, at the lowest point in cellulose acetate butyrate (CAB) plastic in- each fabric panel, when the aircraft is in stored spection access rings have become popular and attitude. Seaplane grommets, which feature a bond satisfactorily with Nitrate Dope or Fabric protruding lip to prevent water splashes Cement. Any metal inspection hole rein- through the drain hole, are recommended over forcements of a particular shape or special de- drain holes subject to water splashing on land sign or size, installed by the original manu- planes as well as seaplanes. The appropriate- facturer, should be reinstalled after cleaning. size holes must be cut through the fabric be- fore installing seaplane grommets. Plastic (3) Tapes or patches over aluminum drain grommets may be doped directly to the reinforcements are optional, but recommended fabric surface or mounted on fabric patches in the prop-wash areas on the wings and for- then doped to the covering. Installing a small ward fuselage bottom. fabric patch over flat grommets to ensure secu- rity is optional. Alternate brass grommets are (4) Fabric patches over plastic rings are mounted on fabric patches, then doped to the strongly recommended because plastic is not a fabric. stable material, becomes brittle at low tem- peratures, and fatigues and cracks from prop (2) After all coating applications and blast vibration. Plastic rings are often cracked sanding are completed, open all holes through during removal and installation of spring, clip- flat drain grommets by cutting through the fab- held covers. Patches with a minimum 1-inch ric with a small-blade knife. Do not attempt to overlap, should be installed with dope. open drain holes by punching with a sharp ob- ject because the drain hole will not remain b. Drain Grommets. Atmospheric tem- open. perature changes cause the humidity in the air to condense on the inside of aircraft surfaces 2-15.2-19. [RESERVED.] and pool in all low areas. Rainwater enters

Par 2-14 Page 2-23 (and 2-24) 9/8/98 AC 43.13-1B

SECTION 2. APPLICATION OF DOPE

2-20. GENERAL. Nitrate dope and bu- not necessary to use nitrate dope for the first tyrate dope are manufactured by treating cel- application and butyrate dope for all other ap- lulose, derived from wood pulp or cotton linter plications. The presence of naphtha in nitrate with select acids, then dissolving in a blend of dope, manufactured in accordance with can- solvents and adding plasticizers for flexibility. celed Mil Specs formulas, causes nitrate dope After a brush or spray application on fabric, to be incompatible with butyrate dope; there- the film develops tension and strength as the fore, nitrate dope should not be applied over solvents evaporate. The tension and strength butyrate dope for repairs or refinishing; how- will increase in proportion to the total film ever, butyrate dope may be applied over nitrate thickness. The fabric functions as a film for- dope. mer and carries no load until a crack develops in the dope film. An excessively-thick dope NOTE: Nitrate or butyrate dope film will develop too much tension and may thinners and retarders should not be warp or damage a light airframe. substituted for each other, nor should automotive coating-type thinners be a. Viscosity adjustments for brush or used. spray applications may also vary between brands. When the viscosity adjustment ratio is d. Clear dope produces the most tension not provided or is unclear, the product manu- and strength. Aluminum-pigmented dope will facturer should be contacted for detailed in- weigh slightly more than clear dope and de- structions. If instructions cannot be obtained velop less tension and strength for the same or the source of the dope is unknown, the dope film thickness. Pigmented color finishes will should be considered suspect and its use is not produce the least tension and strength due to advised. the higher ratio of plasticizers.

b. Dope, which has been stored for an e. During the coating-buildup procedure, extended period of time or under adverse con- solvents released from each succeeding coat ditions, should be suspected of becoming will penetrate and be absorbed into the previ- acidic and should be tested before being used ous dope film, temporarily releasing the ten- on cotton or linen fabric. In some cases, fresh sion and increasing the drying time between production dope has also been found to have a coats as the dope film becomes thicker. If high acid content and will begin to deteriorate elapsed time between coats exceeds several cotton or linen in a period of a few months. weeks at temperatures above 70 F, it is rec- The acid content of nitrate or butyrate dope ommended that several spray coats of an ap- should not exceed 0.06 percent, calculated as propriate dope thinner or dope with retarder acidic acid. An acidity test can be performed and/or rejuvenator added be applied to the by most testing laboratories if high acid con- lightly-sanded, dried dope film to open the sur- tent is suspected. face and provide cohesion for the next coat. This will reduce the possibility of surface c. Butyrate dope is superior to nitrate cracks caused by dissimilar tension between dope in weather exposure tests. However, ni- the old and new dope film. trate dope provides better adhesion to natural fiber than butyrate dope. The adhesion of bu- f. All dope coats through the final finish tyrate dope to natural fibers is adequate; it is may be applied with a brush; however, brush

Par 2-20 Page 2-25 AC 43.13-1B 9/8/98 marks will be noticeable in the finish. With 2-21. DOPE APPLICATION PROCE- increasing environmental concerns, high pres- DURE (Natural Fabrics). sure airless and high-volume low-pressure (HVLP) paint spray equipment is recom- a. Step 1. After the cotton or linen fabric mended over conventional siphon and pressure is installed in accordance with the procedures pot spraying equipment. High pressure airless specified in paragraphs 2-7 and 2-8, the fabric and HVLP paint spraying equipment will is wetted with distilled water to remove wrin- greatly reduce paint over-spray and fogging. A kles and fold creases, which will show in a spray gun, single coat is applied by overlap- gloss finish. Water may be applied by rubbing ping each consecutive pass 50 percent of the with a clean sponge or rag, or by using a paint fan width. A double coat is applied by re- spray gun. Do not use tap water. It may con- peating the coating application in the same di- tain minerals which will contaminate the fab- rection, or at a 90°€angle to the first coat ric. (cross coat) before the first coat has flashed off or dried dust free. For safety and helpful tips (1) As water is absorbed by the fibers, for doping, see tables 2-3 and 2-4. the threads swell, resulting in temporary tauting of the fabric panel. The fabric should TABLE 2-3. Safety tips for dope/paint. be allowed to dry before dope application, oth- SAFETY TIPS. erwise the water in the fibers will interfere Always ground the aircraft structure while sanding and with the dope penetration and adhesion. painting. Do not use an electric drill as a dope/paint mixer. Wear leather-soled shoes in the dope/painting area. (2) After the fabric has dried, the first Have an adequate, approved ventilation system. coat of dope is applied, brushing in one direc- Wear cotton clothes when doping or painting. tion to set the nap with a clean, non-shedding, Wear an approved face mask or respirator when 2-to 6-inch wide, semi-soft, long bristle paint spraying. Follow all the manufacturer’s instructions. brush.

TABLE 2-4. Tips for doping. (3) To offset the deteriorating effect of HELPFUL TIPS FOR DOPING. mildew or other fungus on natural fibers, espe- Limits for optimum application of dope: relative hu- cially in damp climates, it is recommended that midity 20 to 60%; temperature range 65° to 75 °F. a fungicide be added to the first coat of dope. Drying time will vary with temperature, humidity, amount of thinner used, and whether or not retarder The preferred fungicide is dimethyldithio- was added to the mixture. carbamate powder, which should be prepared Do not recoat until the surface is completely dry and per the manufacturer’s instructions. If no all active solvents have left the dope film. Spray all coats except the first three or four clear manufacturer’s instructions are available the coats, to avoid brush marks. zinc powder may be stirred in at a ratio of Over thinning is preferred to under thinning. 4 ounces, to one gallon of un-thinned nitrate or Addition of retarder will produce a smoother coat, but drying time between coats will be extended. butyrate dope, after the powder is wetted to a To get a clean line for the trim colors, apply a light paste with a 50/50 ratio of dope and thinner. coat of clear dope directly on the masking tape prior to painting. This will help eliminate the trim color from running under the masking tape. (4) Pre-mixed fungicidal dope, manu- Remember to always bring the dope to room factured in accordance with the formula speci- temperature before using. fied in MIL-D-7850, will have a transparent Rubbing compound and wax polish may be applied after all solvents have escaped (usually 2 weeks, purple tint to indicate the fungicide additive. depending upon the weather). Dope manufactured with other colors to

Page 2-26 Par 2-20 9/27/01 AC 43.13-1B CHG 1 identify the manufacturers’ products sold un- e. Step 4. After the covering is a dressed der proprietary trade names may or may not out, one or more coats of clear dope are have a fungicidal additive. brushed over all finishing tapes and fabric re- inforcing patches. This will balance the thick- (5) The viscosity of the dope should be ness of the dope film with the previously adjusted to uniformly wet the fabric, indicated coated areas of the fabric. It is very important by the fabric becoming translucent so that it that the porosity of the fabric be filled while penetrates through the fabric but does not drip brushing to avoid pinholes showing in the fin- or run down the opposite side. Any dope-runs ish. or pooling on the opposite side will shrink and distort the fabric, and may be visible on the f. Step 5. After drying at least 2 hours at finished surface. 70 °F, a third heavy coat of clear dope is ap- plied over the entire surface, preferably with a (6) The ideal temperature for application paint spray gun if brush marks are to be of dope or other coatings is 65 to 75 F and the avoided. After the third coat of dope has dried humidity should be less than 65 percent. As a at least 2 hours at 70 °F, the fabric should be general rule, each 10 F increase or decrease in taut and the dope film should show a gloss, ambient temperature will increase or decrease depending upon the dope quality and the ratio drying time by 100 percent. Dope should be of thinner added. If not, a fourth coat of clear allowed to warm to room temperature prior to dope may be applied, in the same manner as attempting to adjust the viscosity. the third coat.

b. Step 2. Depending upon the quality of NOTE: Three to four clear coats of the dope and the ratio of thinning, the fabric dope film showing a uniform gloss should start to become taut after the first combined with the aluminum- brushed coat of dope has dried approximately pigmented coats and finish coats is 1 hour at 70 °F. A second, heavier coat is ap- considered satisfactory for light air- plied by cross brushing at 90°F=to the first craft up to 9 lb. per square foot wing coat. Viscosity should be adjusted only as loading. Five to eight clear coats, de- necessary to brush out a heavy uniform coat. pending upon the quality of the dope If the fabric is not taut, with all sag removed, and resulting film thickness, are rec- after the second coat has dried approximately ommended for higher wing loading 2 hours, a third coat may be applied. aircraft to assure the covering does not stretch and lose tension. c. Step 3. After the fabric has become semi-taut and stabilized with the initial dope g. Step 6. After the clear coats are found application, and the rib lacing and other fabric to be satisfactory, two heavy cross-coats of attachments are completed as detailed in para- aluminum-pigmented dope are applied with a graphs 2-9 through 2-12, it is ready for spray gun to provide protection from ultravio- “dressing out” as described in paragraphs 2-13 let (UV) rays. Tests have shown that UV ra- and 2-14. diation will deteriorate cotton, linen, and poly- ester fabric; however, polyester fabric deterio- d. NOTE: “Dressing out” means applying rates at a rate half that of cotton or linen under all the finishing tapes, reinforcing patches, in- identical exposure conditions. UV radiation spection access ports, and drain grommets, etc.

Par 2-21 Page 2-27 AC 43.13-1B CHG 1 9/27/01 does not deteriorate glass fabric. Aluminum- protection along the edges of the finishing pigmented dope blocks UV radiation and pro- tapes and reinforcing patches is maintained. vides a sanding base. A gauge of ultraviolet protection in the field is to block all visible i. Step 8. Three coats of pigmented color light from penetrating through the fabric. finish are applied with a paint spray gun, al- Drying time between the two coats should be lowing adequate drying time between coats. at least 1 hour at 70 °F. The color finish may be wet sanded between coats, if desired, with a fine grit waterproof (1) An option to premixed aluminum sandpaper. Adding blush retarder to the final dope is to use aluminum-pigment paste. Alu- dope finish will improve the gloss. After dry- minum paste should be prepared per the manu- ing several weeks, a rubbing compound may facturer’s instructions. If no manufacturer’s be used to buff the finish and increase the instructions are available, mix 3 ounces (by gloss. A periodic application of a wax polish weight), of 325-mesh aluminum-pigment will help protect the finish from the weather paste, to 1 gallon of unthinned, clear dope. and environmental pollution. The aluminum paste should first be mixed to a cream consistency with a 50/50 ratio of dope NOTE: Drain holes should be opened and thinner before mixing into the unthinned soon after all finishing is complete to clear dope. A higher ratio of aluminum- insure drainage and to aid ventilation pigment added to the dope may cause a loss of of the structure. primer-coat and finish-coat adhesion, and peeling may occur especially when high tack (1) When exposed to the sun, dark col- tape is used to mask for the trim colors and ors absorb more sun energy and convert that registration numbers. energy to heat more easily than light colors. High temperatures dry out wood structures and (2) The viscosity of the mixed alumi- deteriorate organic materials in an aircraft num-pigmented dope should be adjusted for structure. Preferably the lighter color shades satisfactory spray gun application. are applied first and then overcoated with darker trim and registration number colors. h. Step 7. After two coats of aluminum- pigmented dope have dried at least 4 hours at (2) Only high-quality, solvent-resistant 70 °F, the surface may be wet sanded with crepe paper or polypropylene masking tape # 280 grit (or finer) waterproof sandpaper. The should be used to avoid finish bleed under the aluminum-pigmented dope should be sanded tape edge. Newspaper printing ink may trans- only to develop a smooth surface, not sanded fer to a fresh finish and should not be used for completely off to the clear dope undercoats. masking paper. Plastic sheeting should not be Do not sand over screwheads, rib lacing, or used as a dust cover on a fresh finish due to any structural sharp edges that will quickly cut possible bonding and damage. through fabric and require patching. Addi- tional coats of aluminum-pigmented dope may 2-22. COVERING OVER PLYWOOD. be applied and sanded, depending on the final Exposed, stressed plywood surfaces, such as finish desired. The last coat should not be wings, must be protected from weather sanded to assure ultraviolet

Page 2-28 Par 2-21 9/8/98 AC 43.13-1B deterioration with fabric at least equal to that b. After the first dope coat has dried at used by the original manufacturer. If the qual- least 1 hour at 70 °F, a second heavy coat of ity is not known, intermediate-grade fabric, clear dope is applied by brush to fill the fabric meeting TSO-C14b specification, is accept- weave and prevent pinholes. The installation able. Fabric may be installed in sections with of finish tape around the perimeter of the ply- a 1/2 inch edge overlap without covering the wood surfaces, leading edges, and other wear overlap with finishing tape. Fabric may also points, is optional but recommended for wear be installed with the edges butted together, and and chafe protection. The application of alu- the seam covered with a minimum 1-inch wide minum-pigmented dope coating, sanding, and finishing tape. The seams may be oriented in finish coats will be the same as that specified any direction, in reference to the line of flight. for fabric panel areas. Reinforcement grom- However, overlapped seams, not covered with mets are not required on drain holes through a finishing tape, should be oriented rearward. plywood surfaces. Fabric should be wrapped completely around a wing’s leading and trailing edges and other 2-23. COAT- components, where possible, to provide fabric- ING APPLICATION DEFECTS. to-fabric continuity around all edges to avoid a poorly-bonded fabric edge from peeling from a. Blushing. The appearance of light the plywood surface causing serious aerody- shaded dull areas on the surface as dope dries namic consequences. is the result of moisture in the atmosphere condensing on a surface due to the cooling ef- a. After the plywood surface is pre- fect of the fast-evaporating components of pared, and the two pre-coats of clear dope dope thinner escaping from the coating. have dried as recommended in para- Blushing can occur at any temperature when graph 2-6 g, the fabric is pulled snug and the humidity is above 65 percent. There are bonded with clear dope around the perimeter several ways to remedy this problem. The of the fabric section. The fabric is then wetted drying time may be slowed by adding up to with distilled water to remove fold creases, in 1 quart of blush retarder to 1 gallon of dope or the same manner described for fabric panel ar- by increasing the temperature of the dope room eas. After the water has evaporated, a heavy and eliminating any cooling draft from blow- coat of low-viscosity clear dope is brushed ing across the surface. Blushed surfaces may firmly through the fabric to soften the under- be reworked by spraying several, closely-timed lying dope pre-coat, insuring a good bond. coats of a 50/50 blend of blush retarder and Brushing techniques should be accomplished dope thinner to soften and return the dope sur- by moving the brush from one side across to face to the original liquid state. Blush retarder, the opposite side to remove all air bubbles and mixed with dope, may delay the full drying thoroughly saturate the fabric. This is indi- time by several days, but will eventually es- cated by the plywood grain being easily visible cape from the dope film if the room tempera- through the translucent fabric. Except for very ture is maintained an average of 70 °F. small imperfections or small dents in the ply- wood surface, voids are not permissible be- b. Pinholes. Voids between the fabric tween the fabric and plywood surfaces. Voids threads that are not filled with the first coats of may allow the fabric to balloon from the ply- dope are called pinholes. They may be caused wood surface, creating adverse handling char- by fabric contamination, such as oil or finger acteristics.

Par 2-22 Page 2-29 AC 43.13-1B 9/8/98 prints, but are usually the result of improper (6) Wrong thinner used and drying too dope application. Pinholes are usually found fast. in a second layer of fabric such as finishing tapes and reinforcing patches or over underly- (7) Spray gun not properly adjusted. ing, non-porous structures; such as leading The spray gun should be adjusted to a uniform edges, turtle decks, and plywood surfaces. spray pattern with the material atomized to de- Any non-porous structure under fabric will act posit fine, wet particles that merge and form a as a backstop and will resist complete dope smooth film. penetration into the fabric. Microscopic cavi- ties between the backstop and fabric collect d. Blisters. One or more of the following escaping solvent vapors during the drying pro- conditions my cause blisters: cess and balloon up through the surface leav- ing pinholes, or become pinholes when the top (1) Freshly coated surface placed in hot of the balloon is sanded. Moisture, in the fab- sunlight or high temperature area to accelerate ric or on the backstop surface, also interferes drying time, causing the vapor from rapidly with complete dope penetration, resulting in evaporating solvents to be trapped. pinholes. The remedy for pinholes, at any stage before the final finish coat, is to add (2) Excessive high air pressure used to blush retarder to low viscosity dope and care- spray heavy coats which “blasts” air bubbles fully brush over the affected surfaces to pene- into the coating. trate into and fill the pinholes. Discontinue brushing after five or six strokes to avoid (3) Water or oil in air supply. leaving brush marks. e. Runs, Sags, and Curtains. These de- c. Orange Peel. A rough spray gun- fects may be caused by one or more of the applied finish, similar to the texture of an or- following conditions: ange peel, may be caused by one or more of the following conditions: (1) Viscosity of material being sprayed is too low. (1) Viscosity of material being sprayed is too high. (2) Coats applied too heavily.

(2) Air temperature is too high. (3) Insufficient drying time between coats. (3) Spraying in direct sunlight, onto a hot surface or in a drafty/windy condition, (4) Spray gun held too close to work which causes a fast solvent evaporation. surface.

(4) Spray gun, tip, cap, and/or needle (5) Improperly adjusted spray gun. are not properly matched for the type material being sprayed. f. Spray Gun Laps and Streaks. These defects may be caused by one or more of the (5) Volume of air available from the following conditions: compressor not sufficient for spray gun.

Page 2-30 Par 2-23 9/8/98 AC 43.13-1B

(1) Spray gun not properly adjusted to (4) Viscosity of material being sprayed spray a wet, smooth surface. is too high.

(2) Overspray on a partially-dried sur- (5) Metallic finishes sprayed too heav- face. ily allowing metallic pigments to move or flow after deposit, causing a marbled appearance. (3) Spray pattern not sufficiently over- lapped on each pass. 2-24.2-29. [RESERVED.]

Par 2-23 Page 2-31 (and 2-32)

9/27/01 AC 43.13-1B CHG 1

SECTION 3. INSPECTION AND TESTING

2-30. GENERAL. All components of the (2) Loose or wrinkled polyester fabric covering should be inspected for general con- covering, finished with coatings other than dition. Loose finishing tape and reinforcing dope, may be caused by inadequate or exces- patches; chafing under fairings; brittle, crack- sive heat application; excess slack when the ing, peeling, or deteriorated coatings; fabric fabric was installed; or bent or warped struc- tears and rock damage; broken or missing rib ture. Polyester fabric which does not meet air- lacing; and rodent nests are unacceptable. The craft quality specifications will very likely be- entire fabric covering should be uniformly taut come loose after a short period of time. with no loose or wrinkled areas, or excess ten- sion which can warp and damage the airframe. (3) Glass fabric covering should be tested with a large suction cup for rib lacing a. Excess Tension. There are no methods cord failure and reinforcing tape failure caused or specifications for measuring acceptable fab- by chafing on all wing ribs and other structural ric tension other than observation. Excess ten- attachments throughout the airframe. Particu- sion may warp critical components, such as lar attention should be given to the area within longerons, wing rib, and trailing edges out of the propeller slipstream. If failure is indicated position, weakening the airframe structure. by the covering lifting from the static position, the rib lacing cord and reinforcing tape must (1) Excess tension with cotton, linen, be reinstalled with double the number of origi- and glass fiber fabric covering is usually nal laces. caused by excessive dope film on a new cov- ering, or continuous shrinking of an originally NOTE: Temporary wrinkles will de- satisfactory dope film as the plasticizers mi- velop in any fabric coated and fin- grate from the dope with age. Heat from sun ished with dope, when moisture from exposure accelerates plasticizer migration. rain, heavy fog, or dew is absorbed into a poor-quality dope film, causing (2) Excess tension with polyester fabric, the film to expand. Temporary wrin- coated with dope, is usually caused by the kles may also develop with any type of combined tension of the heat tautened polyes- thick coatings, on any type of fabric, ter fabric and continuous shrinking of the dope when an aircraft is moved from a cold film as the plasticizers migrate from the dope storage area to a warm hangar or with age. parked in the warming sunshine, causing rapid thermal expansion of b. Loose Fabric. Fabric that flutters or the coating. ripples in the propeller slipstream, balloons, or is depressed excessively in flight from the c. Coating Cracks. Fabric exposed static position, is unacceptable. through cracks in the coating may be initially tested for deterioration by pressing firmly with (1) Loose or wrinkled cotton, linen, and a thumb to check the fabric’s strength. Natural glass fabric covering may be caused by inade- fibers deteriorate by exposure to ultraviolet ra- quate dope film; poor quality dope; fabric in- diation, mildew, fungus from moisture, high stalled with excess slack; or by a bent, broken, acid-content rain, dew, fog, pollution, and age. or warped structure. Polyester filaments will deteriorate by expo- sure to UV radiation.

Par 2-30 Page 2-33 AC 43.13-1B 9/8/98

(1) Glass fabric will not deteriorate back side of cotton or linen fabric indicates a from UV exposure, but will be deteriorated by fungicide was present in the dope to resist de- acid rain, dew fallout, and chaffing if loose in terioration by fungus and mildew. the prop blast area. b. Polyester fabric conforming to (2) Cotton, linen, and glass fabric cov- TSO-C14b or TSO-C15d is whiter in color erings are dependent solely on the strength and than cotton or linen. The fabric styles adapted tautening characteristics of the dope film to for use as aircraft covering have a variety of carry the airloads. Dope coatings on heat- thread counts, up to ninety-four (94), depend- tautened polyester fabric will also absorb all ing on the source, weight, and the airloads because the elongation of polyes- breaking strength. Polyester is a monofilament ter filaments are considerably higher than the and will not have any nap or filament ends dope film. Polyester fabric that is coated with showing. materials other than dope, is dependent solely on the heat tautening and low-elongation char- c. Glass fabric is manufactured white in acteristics of the polyester filaments to develop color, and one source is precoated with a blue- tension and transmit the airloads to the air- tinted dope as a primer and to reduce weave frame without excess distortion from a static distortion during handling. Thread count will position. be approximately 36 threads per inch. Glass fabrics are monofilament and will not have any (3) Cracks in coatings will allow any nap or filament ends showing unless they are type of exposed fabric to deteriorate. Cracks inadvertently broken. should be closed by sealing or removing the coatings in the immediate area and replace d. When a small fabric sample can be with new coatings, or recover the component. removed from the aircraft and all the coatings removed, a burn test will readily distinguish 2-31. FABRIC IDENTIFICATION. between natural fabric, polyester, and glass Cotton Fabric meeting TSO-C15 or TSO-C14 fabric. Cotton and linen will burn to a dry ash, can be identified by an off-white color and polyester filaments will melt to a liquid and thread count of 80 to 94 for TSO-C14b and continue burning to a charred ash, and glass 80 to 84 for TSO-C15d in both directions. filaments, which do not support combustion, will become incandescent over a flame. a. Aircraft linen conforming to British specification 7F1 may be identified by a 2-32. COATING IDENTIFICATION. slightly darker shade than cotton fabric and ir- Nitrate or butyrate dope must be used to de- regular thread spacing. The average thread velop tension on cotton, linen, and glass fab- count will be about the same as Grade A fabric rics. When a small sample can be removed, (TSO-C15d). The non-uniformity of the linen burn tests will distinguish nitrate dope-coated thread size is also noticeable, with one thread fabric from butyrate dope-coated fabric by its half the size of the adjacent thread. When immediate ignition and accelerated combus- viewed under a magnifying glass, the ends of tion. Butyrate dope will burn at less than the cotton and linen fiber nap may be seen on one-half the rate of nitrate dope. Coating types the backside. The nap is also seen when the other than nitrate or butyrate dope may have coating is removed from the front or outside been used as a finish over dope on cotton, surface. A light-purple color showing on the linen, and glass fiber fabric coverings.

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a. If the fabric type is determined to be width, regardless of the fabric grade. Fabric polyester, coating identification should start installed on aircraft with a wing loading of by reviewing the aircraft records and inspect- 9 lb. per square foot and over, or a Vne of ing the inside of the wings and the fuselage for 160 mph and over, will be considered unair- the required fabric source identification stamps worthy when the breaking strength has deterio- for covering materials authorized under the rated below 56 lb. per inch width. STC. The manual, furnished by the holder of the STC-approved fabric, should be reviewed c. Fabric installed on a glider or sail- to determine whether the coatings are those plane with a wing loading of 8 lb. per square specified by the STC. foot and less, and a Vne of 135 mph or less, will be considered unairworthy when the fabric b. Coating types, other than those breaking strength has deteriorated below 35 lb. authorized by the original STC, may have been per inch width, regardless of the fabric grade. used with prior FAA approval, and this would be noted in the aircraft records. The presence 2-34. FABRIC TESTING. Mechanical de- of dope on polyester can be detected by a sam- vices used to test fabric by pressing against or ple burn test. piercing the finished fabric are not FAA ap- proved and are used at the discretion of the 2-33. STRENGTH CRITERIA FOR mechanic to base an opinion on the general AIRCRAFT FABRIC. Minimum perform- fabric condition. Punch test accuracy will de- ance standards for new intermediate-grade fab- pend on the individual device calibration, total ric are specified in TSO-C14b, which refer- coating thickness, brittleness, and types of ences AMS 3804C. Minimum performance coatings and fabric. Mechanical devices are standards for new Grade A fabric are specified not applicable to glass fiber fabric that will in TSO-C15d, which references AMS 3806D. easily shear and indicate a very low reading regardless of the true breaking strength. If the a. The condition of the fabric covering fabric tests in the lower breaking strength must be determined on every 100-hour and an- range with the mechanical punch tester or if nual inspection, because the strength of the the overall fabric cover conditions are poor, fabric is a definite factor in the airworthiness then more accurate field tests may be made. of an airplane. Fabric is considered to be air- Cut a 1-1/4-inch wide by 4-inch long sample worthy until it deteriorates to a breaking from a top exposed surface, remove all coat- strength less than 70 percent of the strength of ings and ravel the edges to a 1-inch width. new fabric required for the aircraft. For exam- Clamp each end between suitable clamps with ple, if grade-A cotton is used on an airplane one clamp anchored to a support structure that requires only intermediate fabric, it can while a load is applied (see table 2-1) by add- deteriorate to 46 pounds per inch width ing sand in a suitable container suspended a (70 percent of the strength of intermediate fab- few inches above the floor. If the breaking ric) before it must be replaced. strength is still in question, a sample should be sent to a qualified testing laboratory and b. Fabric installed on aircraft with a breaking strength tests made in accordance wing loading less than 9 lb. per square foot with American Society of Testing Materials (psf), and a Vne less than 160 mph, will be (ASTM) publication D5035. considered unairworthy when the breaking strength has deteriorated below 46 lb. per inch

Par 2-32 Page 2-35 AC 43.13-1B 9/8/98

NOTE: ASTM publication D1682 has is cracked, it may be treated with rejuvenator, a been discontinued but is still referred mixture of very potent solvents and plasticiz- to in some Aerospace Material Speci- ers, to restore its resilience. The surface of the fication (AMS). The grab test method fabric is cleaned and the rejuvenator sprayed previously listed in ASTM D1682, sec- on in a wet coat, and the solvents soften the tions 1 through 16, has been super- old finish so the plasticizers can become part seded by ASTM publication D5034. of the film. When the rejuvenator dries, the The strip testing method (most com- surface should be sprayed with two coats of monly used in aircraft) previously aluminum-pigmented dope, then sanded and a listed in ASTM D1682, sections 17 third coat of aluminum-pigmented dope ap- through 21, has been superseded by plied, followed with the colored-dope finish. ASTM publication D5035. When repairing, rejuvenating, and refinishing covering materials approved under an STC, in- 2-35. REJUVENATION OF THE DOPE structions in the manual furnished by the mate- FILM. If fabric loses its strength, there is rial supplier should be followed. nothing to do but remove it and recover the aircraft. But if the fabric is good and the dope 2-36.2-41. [RESERVED.]

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SECTION 4. REPAIRS TO FABRIC COVERING

2-42. GENERAL. All materials used to rib lacing or other methods, the new fabric make repairs to fabric covering must be of a section should be extended 3 inches past the quality at least equal to the original materials. structural member. Workmanship and repair methods must be made in a manner that will return the fabric NOTE: Before installing new finish- covering to its original airworthy condition. ing tape, duplicate the original rib lacing or other attachments without a. Any combination of seams hand-sewn removing the original rib lacing or at- and overlapped and doped may be used to tachment. Removing the original fin- make repairs and install new fabric sections. ishing tape is optional. (See paragraph 2-7.) g. When installing large sections of fab- b. All pigmented dope coats, including ric on a wing or other components, all ma- aluminum-pigmented coats, should be re- chine-sewn and/or overlapped and doped moved to the clear dope preliminary coats be- seams should be made in accordance with the fore installing a new fabric section or finishing guidelines specified in paragraphs 2-7 and 2-8. tape on the old fabric. The appropriate dope thinner (see paragraph 2-20) may be applied h. When repairing a covering material with a brush to soften the old dope. The sof- other than cotton or linen, which was ap- tened coats can then be removed down to the proved with the manufacturer’s clear dope coats by scraping with a dull-bladed (TC), or approved under the authority of an knife while supporting the fabric from the back STC, follow the repair instructions furnished side. Removing the old dope by sanding is by the aircraft manufacturer or supplier of the optional. STC-approved covering materials.

c. Avoid allowing dope to run down the 2-43. REPAIR OF TEARS AND ACCESS back side of the fabric or drip through the wing OPENINGS. When all the original fabric is onto the back side of the opposite surface, intact, an opening may be repaired by sewing which will cause cosmetic damage and will the two sides together with a curved needle as show in a high gloss finish. illustrated in figures 2-13 and 2-14. The fabric edges should be pulled together uniformly with d. Repairing a new fabric section over no wrinkles. Before sewing, remove the old two adjacent wing ribs is considered a major dope coats down to the clear dope coats a dis- repair. A log book entry and an FAA tance of 2 inches on each side of the opening. Form 337 must be processed accordingly. The hand-sewn thread quality should be at least equal to that specified in table 2-2 and e. All fabric patch edges not covered treated with wax (paraffin-free or beeswax) to with a finishing tape should have a pinked prevent fraying, or use the proper thread on the edge or a 1/4-inch raveled edge. STC application. After sewing, apply a coat of clear dope over the cleaned area and install f. Where the edge of a new fabric sec- a 3-inch wide finishing tape, centered over the tion will be located within 1 inch of a struc- stitches. tural member to which the fabric is attached by

Par 2-42 Page 2-37 AC 43.13-1B 9/8/98

FIGURE 2-13. Repair of tears in fabric.

a. Finishing tapes should be well satu- opening should be trimmed straight on four rated with dope and smoothed out with no sides to facilitate the installation of straight voids or wrinkles during installation. After sections of finishing tape over the stitches. drying for 1 hour at 70 °F, additional coats of clear dope followed with pigmented dope are a. After cutting out the damaged section applied as detailed in paragraph 2-21. and removing the coatings as detailed in para- graph 2-42, the new fabric section should be b. If the opening is more than 8 inches sized to allow folding both edges of the fabric long but less than 16 inches long in any direc- back 1/2-inch to increase the stitch tear resis- tion, the finishing tape width should be in- tance. Temporarily attach the four corners in creased to 4 inches. position with thread. Start with a double thread with a square knot at the end (see fig- c. The finishing tape width should be in- ure 2-14) and continue stitching in the manner creased to 6 inches if the opening is more than described in figures 2-13 and 2-14. When the 16 inches long in any direction, is located on a stitching is complete, wet the new fabric sec- wing top surface, or the aircraft Vne speed is tion as described in paragraph 2-21 to remove greater than 150 mph. any creases. After drying, apply one coat of clear dope on the new fabric, as described in 2-44. SEWN-PATCH REPAIR. Openings paragraph 2-21. When the first coat of dope that cannot be repaired by closing with stitches has dried 1 hour at 70 °F, apply a 3-inch wide may be repaired by sewing in a new fabric finishing tape, centered over the stitches. The section. The edges of the fabric around the finishing tape should be well saturated with

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c. The finishing tape width should be 6 inches wide if the opening is over 16 inches long in any direction, is located on a wing top surface, or the aircraft Vne speed is greater than 150 mph.

2-45. DOPED-ON PATCH REPAIR. An Step 1. Tie the thread ends together with a opening not over 8 inches in length in any di- square knot. rection, on an aircraft with a Vne speed less than 150 mph, may be repaired with a 2-inch overlapped and doped patch. The opening should be trimmed to eliminate any irregular edges and old pigmented dope coats removed as described in paragraph 2-42.

a. When installing a fabric patch over a small opening, the loose edge of the fabric Step 2. Start the stitch by routing through the around the opening may be secured by ex- loop, positioning the square knot on the inside. tending a series of small threads, from the edge across the opening, to the opposite side. After the patch is completed, the threads may be clipped and removed through an access port or left in place.

b. The fabric patch is installed by ap- plying a coat of clear dope around the opening, then positioning the patch over the opening. Brush out any void or wrinkles while saturat- ing only the fabric overlap area. After the first coat of clear dope around the edge has dried 1 hour at 70 °F, wet the fabric patch to remove any creases as described in paragraph 2-21. Step 3. Complete the hand-stitch and secure the After drying, apply additional coats of clear stitching with a half hitch at a maximum each dope and pigmented dope over the entire patch 10 stitches. as described in paragraph 2-21. FIGURE 2-14. Hand-stitch detail. c. If the opening is less than 8 inches in dope and smoothed out with no voids or wrin- length in any direction and the aircraft Vne kles. Additional coats of clear dope and pig- speed is greater than 150 mph, a 2-inch wide mented dope are then applied to obtain the de- finishing tape should be installed on all sides, sired tautness and finish, as described in para- centered on the edge of the 2-inch overlap graph 2-21. patch.

b. If the opening is more than 8 inches d. If the opening is more than 8 inches but less than 16 inches long in any direction, but less than 16 inches in length, in any direc- the finishing tape should be 4 inches wide. tion on an aircraft with a Vne speed less than

Par 2-44 Page 2-39 AC 43.13-1B 9/8/98

150 mph, it may be repaired with a doped f. If the opening is more than 16 inches patch, which is overlapped 1/4 of the opening in length in any direction and the Vne speed is maximum dimension. The maximum overlap less than 150 mph, the patch overlap should be should not exceed 4 inches. 4 inches and the finishing tape should be 4 inches in width, centered on the edge of the e. If the opening is more than 8 inches patch. but less than 16 inches in length in any direc- tion, the repair is located on a wing top sur- g. If the opening is more than 16 inches face, and the aircraft Vne speed is greater than in length in any direction and the Vne speed is 150 mph, the patch overlap should be 4 inches greater than 150 mph, the patch overlap should and a 2-inch wide finishing tape installed on be 4 inches and the finishing tape should be all sides, centered on the edge of the patch. 6 inches in width, centered on the edge of the patch.

2-46.2-51. [RESERVED.]

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