Par 2-22 Page 2-32
b. After the first dope coat has dried at least 1 hour at 70 °F, a second heavy coat of clear dope is applied by brush to fill the fabric weave and prevent pinholes. The installation of finish tape around the perimeter of the plywood surfaces, leading edges, and other wear points, is optional but recommended for wear and chafe protection. The application of aluminum-pigmented dope coating, sanding, and finish coats will be the same as that specified for fabric panel areas. Reinforcement grommets are not required on drain holes through plywood surfaces.
2-23. COATING APPLICATION DEFECTS.
a. Blushing. The appearance of light shaded dull areas on the surface as dope dries is the result of moisture in the atmosphere condensing on a surface due to the cooling effect of the fast-evaporating components of dope thinner escaping from the coating. Blushing can occur at any temperature when the humidity is above 65 percent. There are several ways to remedy this problem. The drying time may be slowed by adding up to 1 quart of blush retarder to 1 gallon of dope or by increasing the temperature of the dope room and eliminating any cooling draft from blowing across the surface. Blushed surfaces may be reworked by spraying several, closely timed coats of a 50/50 blend of blush retarder and dope thinner to soften and return the dope surface to the original liquid state. Blush retarder, mixed with dope, may delay the full drying time by several days, but will eventually escape from the dope film if the room temperature is maintained an average of 70 °F.
b. Pinholes. Voids between the fabric threads that are not filled with the first coats of dope are called pinholes. They may be caused by fabric contamination, such as oil or fingerprints, but are usually the result of improper dope application. Pinholes are usually found in a second layer of fabric such as finishing tapes and reinforcing patches or over underlying, non-porous structures; such as leading edges, turtle decks, and plywood surfaces. Any non-porous structure under fabric will act as a backstop and will resist complete dope penetration into the fabric.
Microscopic cavities between the backstop and fabric collect escaping solvent vapors during the drying process and balloon up through the surface leaving pinholes, or become pinholes when the top of the balloon is sanded. Moisture, in the fabric or on the backstop surface, also interferes with complete dope penetration, resulting in pinholes. The remedy for pinholes, at any stage before the final finish coat, is to add blush retarder to low viscosity dope and carefully brush over the affected surfaces to penetrate into and fill the pinholes. Discontinue brushing after five or six strokes to avoid leaving brush marks.
c. Orange Peel. A rough spray gun-applied finish, similar to the texture of an orange peel, may be caused by one or more of the following conditions:
(1) Viscosity of material being sprayed is too high.
(2) Air temperature is too high.
(3) Spraying in direct sunlight, onto a hot surface or in a drafty/windy condition, which causes a fast solvent evaporation.
(4) Spray gun, tip, cap, and/or needle are not properly matched for the type material being sprayed.
Par 2-23 Page 2-33
(5) Volume of air available from the compressor not sufficient for spray gun.
(6) Wrong thinner used and drying too fast.
(7) Spray gun not properly adjusted. The spray gun should be adjusted to a uniform spray pattern with the material atomized to deposit fine, wet particles that merge and form a smooth film.
d. Blisters. One or more of the following conditions my cause blisters:
(1) Freshly coated surface placed in hot sunlight or high temperature area to accelerate drying time, causing the vapor from rapidly evaporating solvents to be trapped.
(2) Excessive high air pressure used to spray heavy coats which “blasts” air bubbles into the coating.
(3) Water or oil in air supply.
e. Runs, Sags, and Curtains. These defects may be caused by one or more of the following conditions:
(1) Viscosity of material being sprayed is too low.
(2) Coats applied too heavily.
(3) Insufficient drying time between coats.
(4) Spray gun held too close to work surface.
(5) Improperly adjusted spray gun.
f. Spray Gun Laps and Streaks. These defects may be caused by one or more of the following conditions:
(1) Spray gun not properly adjusted to spray a wet, smooth surface.
(2) Overspray on a partially dried surface.
(
3) Spray pattern not sufficiently over-l apped on each pass.
(
4) Viscosity of material being sprayed is too high.
(5) Metallic finishes sprayed too heavily allowing metallic pigments to move or flow after deposit, causing a marbled appearance.
2-24. –2-29. [RESERVED.]
Par 2-30 Page 2-34 SECTION 3. INSPECTION AND TESTING
2-30. GENERAL. All components of the covering should be inspected for general condition.
Loose finishing tape and reinforcing patches; chafing under fairings; brittle, cracking, peeling, or deteriorated coatings; fabric tears and rock damage; broken or missing rib lacing; and rodent nests are unacceptable. The entire fabric covering should be uniformly taut with no loose or wrinkled areas, or excess tension which can warp and damage the airframe.
a. Excess Tension. There are no methods or specifications for measuring acceptable fabric tension other than observation. Excess tension may warp critical components, such as longerons, wing rib, and trailing edges out of position, weakening the airframe structure.
(1) Excess tension with cotton, linen, and glass fiber fabric covering is usually caused by excessive dope film on a new covering, or continuous shrinking of an originally satisfactory dope film as the plasticizers migrate from the dope with age. Heat from sun exposure accelerates plasticizer migration.
(2) Excess tension with polyester fabric, coated with dope, is usually caused by the combined tension of the heat-tautened polyester fabric and continuous shrinking of the dope film as the plasticizers migrate from the dope with age.
b. Loose Fabric. Fabric that flutters or ripples in the propeller slipstream, balloons, or is depressed excessively in flight from the static position, is unacceptable.
(1) Loose or wrinkled cotton, linen, and glass fabric covering may be caused by inadequate dope film; poor quality dope; fabric installed with excess slack; or by a bent, broken, or warped structure.
(2) Loose or wrinkled polyester fabric covering, finished with coatings other than dope, may be caused by inadequate or excessive heat application; excess slack when the fabric was installed; or bent or warped structure. Polyester fabric which does not meet aircraft quality specifications will very likely become loose after a short period of time.
(3) Glass fabric covering should be tested with a large suction cup for rib lacing cord failure and reinforcing tape failure caused by chafing on all wing ribs and other structural attachments throughout the airframe. Particular attention should be given to the area within the propeller slipstream. If failure is indicated by the covering lifting from the static position, the rib lacing cord and reinforcing tape must be reinstalled with double the number of original laces.
NOTE: Temporary wrinkles will develop in any fabric coated and finished with dope, when moisture from rain, heavy fog, or dew is absorbed into a poor-quality dope film, causing the film to expand. Temporary wrinkles may also develop with any type of thick coatings, on any type of fabric, when an aircraft is moved from a cold storage area to a warm hangar or parked in the warming sunshine, causing rapid thermal expansion of the coating.
c. Coating Cracks. Fabric exposed through cracks in the coating may be initially tested for deterioration by pressing firmly with a thumb to check the fabric’s strength. Natural fibers
Par 2-30 Page 2-35 deteriorate by exposure to UV radiation, mildew, fungus from moisture, high acid-content rain, dew, fog, pollution, and age. Polyester filaments will deteriorate by exposure to UV radiation.
(1) Glass fabric will not deteriorate from UV exposure, but will be deteriorated by acid rain, dew fallout, and chaffing if loose in the prop blast area.
(2) Cotton, linen, and glass fabric coverings are dependent solely on the strength and tautening characteristics of the dope film to carry the airloads. Dope coatings on heat tautened polyester fabric will also absorb all the airloads because the elongation of polyester filaments are considerably higher than the dope film. Polyester fabric that is coated with materials other than dope, is dependent solely on the heat tautening and low-elongation characteristics of the polyester filaments to develop tension and transmit the airloads to the airframe without excess distortion from a static position.
(3) Cracks in coatings will allow any type of exposed fabric to deteriorate. Cracks should be closed by sealing or removing the coatings in the immediate area and replace with new coatings, or recover the component.
2-31. FABRIC IDENTIFICATION. Cotton Fabric meeting TSO-C15 or TSO-C14 can be identified by an off-white color and thread count of 80 to 94 for TSO-C14b and 80 to 84 for TSO-C15d in both directions.
a. Aircraft linen conforming to British specification 7F1 may be identified by a slightly darker shade than cotton fabric and irregular thread spacing. The average thread count will be about the same as Grade A fabric (TSO-C15d). The non-uniformity of the linen thread size is also noticeable, with one thread half the size of the adjacent thread. When viewed under a magnifying glass, the ends of the cotton and linen fiber nap may be seen on the backside. The nap is also seen when the coating is removed from the front or outside surface. A light-purple color showing on the back side of cotton or linen fabric indicates a fungicide was present in the dope to resist deterioration by fungus and mildew.
b. Polyester fabric conforming to TSO-C14b or TSO-C15d is whiter in color than cotton or linen. The fabric styles adapted for use as aircraft covering have a variety of thread counts, up to 94, depending on the manufacturing source, weight, and breaking strength. Polyester is a monofilament and will not have any nap or filament ends showing.
c. Glass fabric is manufactured white in color, and one source is precoated with a blue-tinted dope as a primer and to reduce weave distortion during handling. Thread count will be approximately 36 threads per inch. Glass fabrics are monofilament and will not have any nap or filament ends showing unless they are inadvertently broken.
d. When a small fabric sample can be removed from the aircraft and all the coatings removed, a burn test will readily distinguish between natural fabric, polyester, and glass fabric.
Cotton and linen will burn to a dry ash, polyester filaments will melt to a liquid and continue burning to a charred ash, and glass filaments, which do not support combustion, will become incandescent over a flame.
Par 2-32 Page 2-36
2-32. COATING IDENTIFICATION. Nitrate or butyrate dope must be used to develop tension on cotton, linen, and glass fabrics. When a small sample can be removed, burn tests will distinguish nitrate dope-coated fabric from butyrate dope-coated fabric by its immediate ignition and accelerated combustion. Butyrate dope will burn at less than one-half the rate of nitrate dope.
Coating types other than nitrate or butyrate dope may have been used as a finish over dope on cotton, linen, and glass fiber fabric coverings.
a. If the fabric type is determined to be polyester, coating identification should start by reviewing the aircraft records and inspecting the inside of the wings and the fuselage for the required fabric source identification stamps for covering materials authorized under the STC.
The manual, furnished by the holder of the STC-approved fabric, should be reviewed to determine whether the coatings are those specified by the STC.
b. Coating types, other than those authorized by the original STC, may have been used with prior FAA approval, and this would be noted in the aircraft records. The presence of dope on polyester can be detected by a sample burn test.
2-33. STRENGTH CRITERIA FOR AIRCRAFT FABRIC. Minimum performance standards for new intermediate-grade fabric are specified in TSO-C14b, which references AMS 3804E. Minimum performance standards for new Grade A fabric are specified in TSO-C15d, which references AMS 3806G.
a. The condition of the fabric covering must be determined on every 100-hour and annual inspection, because the strength of the fabric is a definite factor in the airworthiness of an airplane. Fabric is considered to be airworthy until it deteriorates to a breaking strength less than 70 percent of the strength of new fabric required for the aircraft. For example, if grade-A cotton is used on an airplane that requires only intermediate fabric, it can deteriorate to 46 lb. per inch width (70 percent of the strength of intermediate fabric) before it must be replaced.
b. Fabric installed on aircraft with a wing loading less than 9 lb. psf, and a Vne less than 160 mph, will be considered unairworthy when the breaking strength has deteriorated below 46 lb. per inch width, regardless of the fabric grade. Fabric installed on aircraft with a wing loading of 9 lb. psf and over, or a Vne of 160 mph and over, will be considered unairworthy when the breaking strength has deteriorated below 56 lb. per inch width.
c. Fabric installed on a glider or sailplane with a wing loading of 8 lb. psf and less, and a Vne of 135 mph or less, will be considered unairworthy when the fabric breaking strength has deteriorated below 35 lb. per inch width, regardless of the fabric grade.
2-34. FABRIC TESTING. Mechanical devices used to test fabric by pressing against or piercing the finished fabric are not FAA approved and are used at the discretion of the mechanic to base an opinion on the general fabric condition. Punch test accuracy will depend on the individual device calibration, total coating thickness, brittleness, and types of coatings and fabric.
Mechanical devices are not applicable to glass fiber fabric that will easily shear and indicate a very low reading regardless of the true breaking strength. If the fabric tests in the lower breaking strength range with the mechanical punch tester or if the overall fabric cover conditions are poor, then more accurate field tests may be made. Cut a 1-1/4-inch wide by 4-inch long sample from a
Par 2-34 Page 2-37 top exposed surface, remove all coatings and ravel the edges to a 1-inch width. Clamp each end between suitable clamps with one clamp anchored to a support structure while a load is applied (see Table 2-1) by adding sand in a suitable container suspended a few inches above the floor. If the breaking strength is still in question, a sample should be sent to a qualified testing laboratory and breaking strength tests made in accordance with ASTM D5035.
NOTE: ASTM D1682 has been discontinued but is still referred to in some AMS. The grab test method previously listed in ASTM D1682, sections 1 through 16, has been superseded by ASTM D5034. The strip testing method (most commonly used in aircraft) previously listed in ASTM D1682, sections 17 through 21, has been superseded by ASTM D5035.
2-35. REJUVENATION OF THE DOPE FILM. If fabric loses its strength, there is nothing to do but remove it and recover the aircraft. But if the fabric is good and the dope is cracked, it may be treated with rejuvenator, a mixture of very potent solvents and plasticizers, to restore its resilience. The surface of the fabric is cleaned and the rejuvenator sprayed on in a wet coat, and the solvents soften the old finish so the plasticizers can become part of the film. When the rejuvenator dries, the surface should be sprayed with two coats of aluminum-pigmented dope, then sanded and a third coat of aluminum-pigmented dope applied, followed with the colored-dope finish. When repairing, rejuvenating, and refinishing covering materials approved under an STC, instructions in the manual furnished by the material supplier should be followed.
2-36. –2-41. [RESERVED.]
Par 2-42 Page 2-38 SECTION 4. REPAIRS TO FABRIC COVERING
2-42. GENERAL. All materials used to make repairs to fabric covering must be of a quality at least equal to the original materials. Workmanship and repair methods must be made in a manner that will return the fabric covering to its original airworthy condition.
a. Any combination of seams hand-sewn and overlapped and doped may be used to make repairs and install new fabric sections. (See paragraph 2-7.)
b. All pigmented dope coats, including aluminum-pigmented coats, should be removed to the clear dope preliminary coats before installing a new fabric section or finishing tape on the old fabric. The appropriate dope thinner (see paragraph 2-20) may be applied with a brush to soften the old dope. The softened coats can then be removed down to the clear dope coats by scraping with a dull-bladed knife while supporting the fabric from the back side. Removing the old dope by sanding is optional.
c. Avoid allowing dope to run down the back side of the fabric or drip through the wing onto the back side of the opposite surface, which will cause cosmetic damage and will show in a high gloss finish.
d. Repairing a new fabric section over two adjacent wing ribs is considered a major repair. A logbook entry and an FAA Form 337 must be processed accordingly.
e. All fabric patch edges not covered with a finishing tape should have a pinked edge or a 1/4-inch raveled edge.
f. Where the edge of a new fabric section will be located within 1 inch of a structural member to which the fabric is attached by rib lacing or other methods, the new fabric section should be extended 3 inches past the structural member. Before installing new finishing tape, duplicate the original rib lacing or other attachments without removing the original rib lacing or attachment. Removing the original finishing tape is optional.
g. When installing large sections of fabric on a wing or other components, all machine-sewn and/or overlapped and doped seams should be made in accordance with the guidelines specified in paragraphs 2-7 and 2-8.
h. When repairing a covering material other than cotton or linen, which was approved with the manufacturer’s type certificate (TC), or approved under the authority of an STC, follow the repair instructions furnished by the aircraft manufacturer or supplier of the STC-approved covering materials.
2-43. REPAIR OF TEARS AND ACCESS OPENINGS. When all the original fabric is intact, an opening may be repaired by sewing the two sides together with a curved needle as illustrated in Figures 2-13 and 2-14. The fabric edges should be pulled together uniformly with no wrinkles.
Before sewing, remove the old dope coats down to the clear dope coats a distance of 2 inches on each side of the opening. The hand-sewn thread quality should be at least equal to that specified in Table 2-2 and treated with wax (paraffin-free or beeswax) to prevent fraying, or use the proper
thread on the STC application. After sewing, apply a coat of clear dope over the cleaned area and install a 3-inch wide finishing tape, centered over the stitches.
FIGURE 2-13. REPAIR OF TEARS IN FABRIC 1/4" MIN 1/4" MAX. SPACING .5°|
START SEWING AT
THE POINT
BASEBALL STITCH
LOCK STITCHING WITH A HALF HITCH EVERY 10 STITCHES MAXIMUM.
LOCK STITCHING AT END
WITH DOUBLE HALF
HITCH KNOT
MATERIAL SHOULD BE AT
LEAST AS GOOD AS ORIGINAL Par 2-43 Page 2-39
Par 2-43 Page 2-40 FIGURE 2-14. HAND-STITCH DETAIL
a.
Finishing tapes should be well saturated with dope and smoothed out with no voids or wrinkles during installation. After drying for 1 hour at 70 ºF, additional coats of clear dope followed with pigmented dope are applied as detailed in paragraph 2-21.
b. If the opening is more than 8 inches long but less than 16 inches long in any direction, the finishing tape width should be increased to 4 inches.
c. The finishing tape width should be increased to 6 inches if the opening is more than 16 inches long in any direction, is located on a wing top surface, or the aircraft Vne speed is greater than 150 mph.
2-44. SEWN-PATCH REPAIR. Openings that cannot be repaired by closing with stitches may be repaired by sewing in a new fabric section. The edges of the fabric around the opening should
Par 2-44 Page 2-41 be trimmed straight on four sides to facilitate the installation of straight sections of finishing tape over the stitches.
a. After cutting out the damaged section and removing the coatings as detailed in paragraph 2-42, the new fabric section should be sized to allow folding both edges of the fabric back 1/2-inch to increase the stitch tear resistance. Temporarily attach the four corners in position with thread. Start with a double thread with a square knot at the end (see Figure 2-14) and continue stitching in the manner described in Figures 2-13 and 2-14. When the stitching is complete, wet the new fabric section as described in paragraph 2-21 to remove any creases. After drying, apply one coat of clear dope on the new fabric, as described in paragraph 2-21. When the first coat of dope has dried 1 hour at 70 °F, apply a 3-inch wide finishing tape, centered over the stitches. The finishing tape should be well saturated with dope and smoothed out with no voids or wrinkles. Additional coats of clear dope and pigmented dope are then applied to obtain the desired tautness and finish, as described in paragraph 2-21.
b. If the opening is more than 8 inches but less than 16 inches long in any direction, the finishing tape should be 4 inches wide.
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 opening not over 8 inches in length in any direction, 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 around the opening may be secured by extending 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 applying a coat of clear dope around the opening, then positioning the patch over the opening. Brush out any void or wrinkles while saturating only the fabric overlap area. After the first coat of clear dope around the edge has dried 1 hour at 70 ºF, wet the fabric patch to remove any creases as described in paragraph 2-21. After drying, apply additional coats of clear dope and pigmented dope over the entire patch as described in paragraph 2-21.
c. If the opening is less than 8 inches in length in any direction and the aircraft Vne speed is greater than 150 mph, a 2-inch wide finishing tape should be installed on all sides, centered on the edge of the 2-inch overlap patch.
d. If the opening is more than 8 inches but less than 16 inches in length, in any direction on an aircraft with a Vne speed less than 150 mph, it may be repaired with a doped patch, which is overlapped 1/4 of the opening maximum dimension. The maximum overlap should not exceed 4 inches.
Par 2-45 Page 2-42
e. If the opening is more than 8 inches but less than 16 inches in length in any direction, the repair is located on a wing top surface, and the aircraft Vne speed is greater than 150 mph, the patch overlap should be 4 inches and a 2-inch wide finishing tape installed on all sides, centered on the edge of the patch.
f. If the opening is more than 16 inches in length in any direction and the Vne speed is less than 150 mph, the patch overlap should be 4 inches and the finishing tape should be 4 inches in width, centered on the edge of the patch.
g. If the opening is more than 16 inches in length in any direction and the Vne speed is greater than 150 mph, the patch overlap should be 4 inches and the finishing tape should be 6 inches in width, centered on the edge of the patch.
2-46. –2-51. [RESERVED.]
Par 3-1 Page 3-1 CHAPTER 3. FIBERGLASS AND PLASTICS SECTION 1. REPAIR OF LIGHT LOAD LAMINATE STRUCTURES
3-1. GENERAL. There is a wide variation in the composition and structural application of laminates, and it is essential that these factors be given major consideration when any restoration activities are undertaken. To a similar extent, there also exist many types of laminate structure repairs that may or may not be suitable for a given condition. For this reason, it is important that the aircraft or component manufacturer’s repair data be reviewed when determining what specific type of repair is permissible and appropriate for the damage at hand.
NOTE: Review material safety data sheets for material to be used. When handling materials, prepreg fabrics, or parts with prepared surfaces, observe shelf life. Latex gloves and approved masks must be worn.
a. The materials used in the repair of laminate structures must preserve the strength, weight, aerodynamic characteristics, or electrical properties of the original part or assembly.
Preservation is best accomplished by replacing damaged material with material of identical chemical composition or a substitute approved by the manufacturer.
b. To eliminate dangerous stress concentrations, avoid abrupt changes in cross-sectional areas. When possible, for scarf joints and facings, make small patches round or oval-shaped, and round the corners of large repairs. Smooth and properly contour aerodynamic surfaces.
c. Test specimens should be prepared during the actual repair. These can then be subjected to a destructive test to establish the quality of the adhesive bond in the repaired part.
To make this determination valid, the specimens must be assembled with the same adhesive batch mixture and subjected to curing pressure, temperature, and time identical to those in the actual repair.
3-2. FIBERGLASS LAMINATE REPAIRS. The following repairs are applicable to fiberglass laminate used for nonstructural fairing, covers, cowlings, honeycomb panel facings, etc. Prior to undertaking the repair, remove any paint by using normal dry sanding methods. Bead blasting may be used but caution must be exercised not to abrade the surfaces excessively.
NOTE: Chemical paint strippers must not be used.
NOTE: These repairs are not to be used on radomes or advanced composite components, such as graphite (carbon fiber) or Kevlar.
CAUTION: Sanding fiberglass laminates gives off a fine dust that may cause skin and/or respiratory irritation unless suitable skin and respiration protection is used. Sanding also creates static charges that attract dirt or other contaminants.
a. Check for voids and delamination by tap testing. (See Chapter 5.) When the surface of a fiberglass laminated structure is scratched, pitted, or eroded; first wash with detergent and