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.
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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.
Par 2-5 Page 2-4 TABLE 2-2. COTTON AND LINEN, TAPES AND THREADS Materials Specification Yarn Size Minimum Tensile Strength Yards Per Pound Use and Remarks Reinforcing tape, cotton.
MIL-T-5661E, Type 1 MIL-Y-1140H 150 lb. per 1/2 inch width.
Used as reinforcing tape on fabric and under rib lacing cord.
Strength of other widths approx. in proportion.
Lacing cord, prewaxed braided cotton.
Federal T-C-571F 40 lb. 310 minimum.
Lacing fabric to structures. Unless already waxed, must be lightly waxed before using.
Lacing cord, braided cotton.
MIL-C-5648A 80 lb. 170 minimum.
Lacing fabric to structures. Unless already waxed, must be lightly waxed before using.
Lacing cord thread, high tenacity cotton.
MIL-T-5660 Ticket No. 10. 62 lb. 480 minimum.
Lacing fabric to structures. Unless already waxed, must be lightly waxed before using.
Machine thread cotton Federal V-T-276H 20/4 ply 5 lb. 5,000 nominal. Use for all machine sewing Hand-Sewing thread cotton.
Federal V-T-276H Type III B 8/4 ply 14 lb. 1,650 nominal.
Use for all hand-sewing. Use fully waxed thread.
Finishing (Surface) tape cotton.
Same as fabric used. Same as fabric used.
Use over seams, leading edges, trailing edges, outer edges and ribs, pinked, raveled or straight edges.
2-6. PREPARATION OF THE STRUCTURE FOR COVERING. One of the most important items when covering aircraft is the proper preparation of the structure. Before covering, the airframe must be inspected and approved by a FAA-certified mechanic or repair station.
a. Battery Box Treatment. An asphaltic, rubber-based acid-proof coating should be applied to the structure in the area of a battery box, by brush, for additional protection from battery acid. Control cables routed in the area of the battery box should be coated with paralketone.
b. Worn Holes. Oversized screw holes or worn size 4 self-tapping screw holes through ribs and other structures used to attach fabric may be redrilled a minimum 1-1/2 hole diameter
Par 2-6 Page 2-5 distance from the original hole location with a # 44 (0.086) drill bit. Size 6 screws, drill bit size # 36 (0.1065), may be installed in stripped or worn holes drilled for size 4 screws, usually without redrilling. Worn holes for wire clips and wire barbs should be redrilled a minimum 1-1/2 hole distance from the original locations using a drill jig to ensure correct spacing, with the appropriate size drill bit. Drill bit size # 30 (0.128) may be used to redrill oversize holes for 1/8-inch diameter blind rivets a minimum 1-1/2 hole diameter distance from the original location.
c. Fairing Precautions. Aluminum leading edge replacement fairings installed in short sections may telescope during normal spar bending loads or from thermal expansion and contraction. This action may cause a wrinkle to form in the fabric, at the edge of the lap joint.
Leading edge fairing sections may be fastened together with rivets or screws to prevent telescoping after installation. Trailing edges should be adequately secured to prevent movement and wrinkles.
d. Dope Protection. Solvents found in nitrate and butyrate dope will penetrate, wrinkle, lift, or dissolve most one-part wood varnishes and one-part metal primers. All wood surfaces that come in contact with doped fabric should be treated with a protective coating such as aluminum foil, cellulose tape, or dope-proof paint to protect them against the action of the solvents in the dope. This can also be accomplished by recoating with a suitable, solvent resistant two-part epoxy varnish, which will be impervious to solvent penetration and damage after curing. Clad aluminum and stainless steel parts need not be dopeproofed.
(1) A solvent-sensitive primer on ferrous metal and aluminum alloy components which will be in contact with fabric may be protected from solvent damage by overcoating with a two-part epoxy primer. Epoxy primer meeting MIL-P-53022 (current revision) is acceptable.
(2) Small metal or wood surfaces, such as rib caps, to which fabric will not be dope bonded as a part of the particular fabric attachment procedure may be protected from dope damage by cellophane tape or aluminum foil.
e. Chafe Protection. Fabric and finishing tape is often cut through with sandpaper over sharp edges during the coating and finishing procedure and later polishing. All sharp metal edges or protruding screws, nails, rivets, and bolt heads should be covered with an antichafe tape to prevent cutting and wearing through the fabric after installation. Use appropriate non-bleeding cotton adhesive coated tape, finishing tape, or strips of fabric, cut from the fabric being used to cover the aircraft, doped in place.
(1) Small holes cut through the fabric to accommodate flying wires, control cables, and fittings, must be reinforced with finishing tape or fabric patches cut from the same fabric used for the covering.
(2) Areas needing additional chafe protection such as control cables routed firm against the fabric surface should be protected with patches cut from cotton duck, leather, or plastic.
These patches may be sewn, doped, or cemented in place, as appropriate.
(3) Any drag and anti-drag wires in the wings should be protected from chafing at cross points.
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f. Inter-Rib Bracing. Use a woven fabric tape of the same quality and width as that used for the rib lace reinforcing, where so incorporated in the wing design by the original aircraft manufacturer. When the original routing for the inter rib bracing is not known, the tape will be routed diagonally, alternating between the top and bottom of each rib cap on each successive rib, if a single pair, half way between the front and rear spars. The number of tape pairs will duplicate the original aircraft manufacturer’s installation. Tapes will be routed continuously from the wing butt to the wingtip bow, with one turn of tape around each intermediate rib cap strip.
Care should be given to position the tape so as not to interfere with control cables, bellcranks or push-pull rods.
g. Preparation of Plywood Surfaces for Covering. Prior to covering plywood surfaces, prepare the surface by sanding, cleaning, and applying sealer and dope. When plywood surfaces are to be covered with light weight glass fiber deck cloth instead of fabric, no sealer or dope should be applied to the plywood as it would inhibit penetration of epoxy resin.
(1) Sand plywood surfaces as needed to remove old loose dope or varnish residue to provide a clean bonding surface. Remove any oil, grease, or other contamination with a suitable solvent such as naphtha. Small, rough areas and irregularities in the plywood surface and around any plywood repairs may be filled and smoothed with an appropriate commercial grade wood filler. Filling large warp depressions on plywood surfaces with a wood filler for cosmetic purposes is not acceptable.
(2) After cleaning and sanding all plywood surfaces, seal the wood grain with a suitable solvent resistant two-part epoxy varnish. After the varnish has thoroughly dried, apply two brush or spray coats of clear dope, allowing sufficient drying time between coats.
2-7. FABRIC SEAMS. Seams parallel to the line of flight are preferable; however, spanwise seams are acceptable.
a. Sewn Seams.
(1) Machine-sewn-seams should be double stitched using any of the styles illustrated in Figure 2-1, Image A, B, C, or D. A machine-sewn seam used to close an envelope at a wingtip, wing trailing edge, empennage and control surface trailing edge, and a fuselage longeron may be made with a single stitch when the seam will be positioned over a structure. (See Figure 2-1, Image E.) The envelope size should accommodate fittings or other small protrusions with minimum excess for installation. Thick or protruding leading edge sewn seams should be avoided on thin airfoils with a sharp leading edge radius because they may act as a stall strip.
FIGURE 2-1. FABRIC SEAMS 1/16" → -3/8". - 1/16" 116→ +197 +116 C STITCHING STITCHING→ A. PLAIN OVERLAP SEAM, STRAIGHT EDGE B. FRENCH FELL SEAM 1/16" → - 1/16" -1/4"
STITCHING C. FOLDED FELL SEAM
STITCHING STEP 1 1/16" -3/8* STEP 2 * STITCHING E. SINGLE STITCH SEAM TO CLOSE AN
ENVELOPE WHEN THE STITCH WILL
BE LOCATED OVER A STRUCTURE D. MODIFIED FOLDED FELL SEAM SEWN IN
TWO STEPS
(2) Hand sew, with plain overthrow or baseball stitches at a minimum of four stitches per inch, or permanent tacking, to the point where uncut fabric or a machine-sewn seam is reached. Lock hand sewing at a maximum of 10 stitch intervals with a double half hitch, and tie off the end stitch with a double half hitch. At the point where the hand-sewing or permanent tacking is necessary, cut the fabric so that it can be doubled under a minimum of 3/8 inch before sewing or permanent tacking is performed. (See Figure 2-2.)
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FIGURE 2-2. TYPICAL METHODS OF ATTACHING FABRIC
USE A T-PIN THROUGH THE FABRIC
STRAPS TO TEMPORARILY SECURE
THE FABRIC COVERING AND REMOVE AS HAND SEWN SEAM PROGRESSES MINIMUM 4 STITCHES PER INCH.
TOP COVER
- FORMER (A) ATTACHING FABRIC AT AILERON CUTOUT
TOP COVER, TOP COVER *FINISH WITH TAPE SPACING 1-1/4.
OR CLOSER
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.
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(3) After hand sewing is complete, any temporary tacks used to secure the fabric over wood structures may be removed.
(4) Cover a sewn spanwise seam on a wing’s leading edge with a minimum 4-inch wide pinked-edged surface tape with the tape centered on the seam.
(5) Cover a spanwise-sewn seam at the wing trailing edge with pinked-edge surface tape that is at least 3 inches wide. For aircraft with never exceed speeds in excess of 200 miles per hour (mph), cut V notches at least 1 inch in depth and 1/4 inch in width in both edges of the surface tape when used to cover spanwise seams on trailing edges of control surfaces. Space notches at intervals not exceeding 6 inches. On tape less than 3 inches wide, the notches should be 1/3 the tape width. In the event the surface tape begins to separate because of poor adhesion or other causes, the tape will tear at a notched section, thus preventing progressive loosening of the entire length of the tape which could seriously affect the controllability of the aircraft. A loose tape acts as a trim tab only on a movable surface. It becomes a spoiler on a fixed surface and has no effect at the trailing edge other than drag.
(6) Make spanwise-sewn seams on the wing’s upper or lower surfaces in a manner that will minimize any protrusions. Cover the seams with finishing tape at least 3 inches wide, centering the tape on the seam.
(7) Sewn seams parallel to the line of flight (chordwise) may be located over ribs.
However, careful attention must be given to avoid damage to the seam threads by rib lace needles, screws, rivets, or wire clips that are used to attach the fabric to the rib. Cover chordwise seams with a finishing tape at least 3 inches wide with the tape centered on the seam.
b. Doped Seams.
(1) For an overlapped and doped spanwise seam on a wing’s leading edge, overlap the fabric at least 4 inches and cover with finishing tape at least 4 inches wide, with the tape centered at the outside edge of the overlap seam.
(2) For an overlapped and doped spanwise seam at the trailing edge, lap the fabric at least 3 inches and cover with pinked-edge surface tape at least 4 inches wide, with the tape centered on the outside edge of the overlap seam.
(3) For an overlapped and doped seam on wingtips, wing butts, perimeters of wing control surfaces, perimeters of empennage surfaces, and all fuselage areas, overlap the fabric 2 inches and cover with a finishing tape that is at least 3 inches wide, centered on the outside edge of the overlap seam.
(4) For an overlapped and doped seam on a wing’s leading edge, on aircraft with a never-exceed velocity (Vne) speed up to and including 150 mph, overlap the fabric 2 inches and cover with a finishing tape that is at least 3 inches wide, with the tape centered on the outside edge of the overlap seam.
(5) For an overlapped and doped seam on the perimeter of a wing (except a leading edge), perimeters of wing control surfaces, perimeters of empennage surfaces, and all areas of a
Par 2-7 Page 2-10 fuselage, on aircraft with a Vne speed up to and including 150 mph, overlap the fabric 1 inch and cover with a finishing tape that is at least 3 inches wide, centered on the outside edge of the overlap seam.
2-8. COVERING METHODS. The method of fabric attachment should be identical, as far as strength and reliability are concerned, to the method used by the manufacturer of the airplane being recovered or repaired. Carefully remove the old fabric from the airframe, noting the location of inspection covers, drain grommets, and method of attachment. Cotton or linen fabric may be applied so that either the warp or fill-threads are parallel to the line of flight. Either the envelope method or blanket method of covering is acceptable.
a. The Envelope Method. A wing envelope may be developed by two methods. Machine sew together, side by side multiple fabric sections, cut to reach chordwise around the wing, starting and ending at the trailing edge with a minimum of 1 inch excess length. The sewn envelope is then positioned around the wing and secured with closely spaced T-Head pins at the wingtip and trailing edge. Excess material may then be trimmed. Carefully remove the envelope and complete by machine sewing at the wingtip and along the trailing edge, except where the geometry of the wing (aileron and flap cut out) would prevent the sewn envelope from being reinstalled. After reinstalling the envelope, the un-sewn sections and butt end are then closed by hand-sewn or overlapped and doped seams in accordance with the aircraft Vne speed. (See subparagraph 2-7b.)
(1) An alternative method, when fabric of sufficient width is available, is to sew together, side-by-side, two sections of fabric, placing the seams spanwise on the leading edge, then fit and sew the wingtip and trailing edge in the same manner as the multiple piece chordwise envelope.
(2) An envelope may be developed for the fuselage in the same manner, with a final closing along a longeron by hand-sewn or overlapped and doped seams in accordance with the aircraft Vne speed.
b. The Blanket Method. A blanket is developed by sewing together, side-by-side, multiple sections of fabric with the seams chordwise or two wide sections of fabric, side-by-side, placing the seam spanwise on the leading edge, the same as an envelope. Close the three remaining sides with a hand-sewn seam or overlapped and doped seams in accordance with the aircraft Vne speed. Small components may be covered by wrapping one piece of fabric over a straight leading or trailing edge, then closing three sides with handstitched or overlapped and doped seams in accordance with the aircraft Vne speed.
NOTE: All overlapped and doped seams will be made only over underlying supporting structures extending the full width of the seam.
c. Machine-Sewn Alternate. An alternate to machine-sewn seams on a wing envelope or blanket is to use two sections of wide fabric spanwise. Attach the fabric with overlapped and doped seams at the leading and trailing edge, wingtip and wing butt, in accordance with the aircraft Vne speeds. (See subparagraph 2-7b.) Smaller components may be covered in the same manner. The fuselage may be covered with multiple fabric sections with overlapped and doped
Par 2-8 Page 2-11 seams on the longerons or other wide fabric-forming structures in accordance with the aircraft Vne speed. (See subparagraph 2-7b.)
d. Holes in Fabric. Never cut any holes in the fabric for inspection panels, spar fittings, or drain grommets; or attach the fabric to the airframe with rib lacing screws, rivets, clips, or rib stitch cord until the fabric has been semi-tauted and stabilized with several coats of dope.
2-9. REINFORCING TAPE.
a. Reinforcing tape should be securely bonded to the fabric surface with dope before cord lacing or installation of hardware. Where multiple attachments are in close proximity, such as on a wing rib, continuous reinforcing tape should be installed, extending at least 1 inch past the last attachment at each end. Random or wide spacing, such as on fuselage stringers or empennage surfaces, may be reinforced with 2-inch lengths of reinforcing tape centered on the attachment location.
b. Reinforcing tapes should be of the appropriate width for hardware attachment such as screws, rivets, wire clips, etc., which pierce the center of the tape. Reinforcing tape under cord lacing should be the same width as the rib to which the fabric is laced and may be comprised of multiple widths positioned side-by-side to achieve the required width.
c. When the aircraft Vne speed is over 250 mph, anti-tear strips, cut from the same quality fabric used to cover the aircraft, are recommended for use under reinforcing tape on the entire top surface of the wing and on the portion of the wing’s bottom surface in the propeller slipstream. The propeller slipstream is considered to be the propeller diameter plus one outboard rib. The anti-tear strip should be installed completely around the wing, beginning and ending at the trailing edge in the propeller slipstream, and installed from the trailing edge over the leading edge and back under to the front spar on the balance of the ribs. Anti-tear strips should extend 1/2 inch past the wing rib cap edges and be thoroughly bonded 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.
a. Fabric should be attached to aircraft components to prevent ballooning due to aerodynamic forces, in the identical manner and locations as used by the original aircraft manufacturer. Any deviation from the original method(s) of attachment, such as screws, rivets, wire clips, lacing cord, etc., are considered a major alteration and in conflict with the aircraft type design data. Obtain FAA approval on any deviation.
NOTE: When the type of rib lace knot used by the original aircraft manufacturer is not known, the modified seine knot shown in Figure 2-4 through 2-9c will be used.