3.4 Aircraft Fabric Covering, Inspection & Doping/Finishing Processes

Aircraft Fabric Covering, Inspection & Doping/Finishing Processes

FAA Airframe Mechanic Standard: Aircraft fabric covering provides a lightweight, aerodynamically smooth envelope for airframes. AMTs must master fabric strength testing criteria under AC 43.13-1B Chapter 2, synthetic polyester heat-tautening procedures, rib-lacing attachment rules, dope chemistry compatibility, and finish defect troubleshooting.

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1. Aircraft Fabric Materials & Airworthiness Strength Standards

Aircraft covering systems are split historically and technologically into natural fabrics and synthetic polyester fabrics.

+-------------------------------------------------------------------------+
|                       AIRCRAFT FABRIC COMPARISON                        |
+------------------------------------+------------------------------------+
| NATURAL FABRICS (Organic)          | SYNTHETIC POLYESTER (Modern STC)   |
+------------------------------------+------------------------------------+
| - Grade A Cotton (AMS 3806/TSO-C15)| - Polyester / Dacron (Ceconite,    |
| - Organic cellulose fibers         |   Poly-Fiber, Superflite)          |
| - Subject to rot, mildew, fungus   | - Impervious to rot and mildew     |
| - Tautened chemically with dope    | - Tautened with calibrated heat    |
| - Requires fungicidal first coat   | - Infinite shelf life before dope  |
+------------------------------------+------------------------------------+

A. Natural vs. Synthetic Fabric Systems

• Grade A Cotton (AMS 3806 / TSO-C15): The historical aviation standard. New Grade A cotton has a minimum tensile breaking strength of $80\text{ lb/in}$ in both warp and fill. Cotton is organic, absorbs atmospheric moisture, and rots rapidly unless protected with fungicidal agents. Cotton tautens chemically as dope solvents evaporate.
• Synthetic Polyester Fabrics (Ceconite, Poly-Fiber, Superflite): Manufactured from continuous filament polyester (Dacron) under FAA Supplemental Type Certificates (STCs). Polyester is completely rot-proof, mildew-proof, and chemically stable. It is tautened mechanically using precision thermal clothing irons.

B. The FAA 70% Airworthiness Rule

Under AC 43.13-1B Chapter 2, fabric covering is deemed unairworthy when its tensile breaking strength deteriorates to less than $70%$ of the original required new strength for that aircraft category.

$\text{Minimum Airworthy Strength} = \text{Original Required Strength} \times 0.70$

+----------------------------------------------------------------------------------+
|                   FABRIC AIRWORTHINESS STRENGTH THRESHOLDS                       |
+-----------------------+---------------------+-------------------+----------------+
| Fabric Classification | Original New Rating | 70% Minimum Limit | Aircraft Class |
+-----------------------+---------------------+-------------------+----------------+
| Grade A Cotton        | 80 lb/in            | 56 lb/in          | Vne > 160 mph  |
| (Heavy Duty)          |                     |                   | or W/S > 9 psf |
+-----------------------+---------------------+-------------------+----------------+
| Intermediate Fabric   | 65 lb/in            | 46 lb/in          | Vne <= 160 mph |
|                       |                     |                   | and W/S <= 9   |
+-----------------------+---------------------+-------------------+----------------+
| Light / Glider Fabric | 50 lb/in            | 35 lb/in          | Gliders /      |
|                       |                     |                   | Light LSA      |
+-----------------------+---------------------+-------------------+----------------+

• High-Performance / Standard Category: For aircraft with never-exceed speed ($V_{NE}$) exceeding $160\text{ mph}$ or wing loading ($W/S$) exceeding $9\text{ lb/ft}^2$, Grade A cotton rating ($80\text{ lb/in}$) is required. Minimum allowable in-service strength is $56\text{ lb/in}$ ($80 \times 0.70 = 56$).
• Low-Performance Aircraft: For aircraft with $V_{NE} \le 160\text{ mph}$ and wing loading $\le 9\text{ lb/ft}^2$, Intermediate fabric rating ($65\text{ lb/in}$) is permitted. Minimum allowable in-service strength is $46\text{ lb/in}$ ($65 \times 0.70 = 45.5 \approx 46$).

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2. Fabric Strength Testing Methods

When inspecting fabric during an annual or 100-hour inspection, an AMT evaluates fabric condition using mechanical testers and laboratory pull tests.

A. Mechanical Punch Testers (Field Screening)

• Maule Punch Tester: A spring-loaded blunt steel pin pressed perpendicularly against the fabric until the scale indicates the test force (e.g., $56\text{ lb/in}$ equivalent). If the fabric withstands the load without puncturing, the fabric passes. If the pin penetrates the fabric, the fabric is unairworthy.
• Seyboth Tester: Uses a penetrating needle and a color-coded indicator (green = airworthy, yellow = questionable, red = unairworthy). Seyboth punches a small hole that must be patched.

B. Laboratory 1-Inch Strip Tensile Pull Test (The Definitive Standard)

Mechanical punch testers are non-destructive screening tools; the only FAA-recognized definitive standard for borderline fabric is the 1-inch strip pull test:

1. A test strip measuring $1\text{ inch wide by }6\text{ inches long}$ is cut from an upper surface exposed to maximum sunlight (e.g., upper wing surface or top of fuselage).
2. All dope and paint coatings must be completely stripped from the center $2\text{ inches}$ of the fabric sample using MEK or acetone.
3. The strip is clamped into a calibrated tensile test machine with jaws spaced $3\text{ inches}$ apart and pulled at a rate of $12\text{ in/min}$ until failure. If breaking force is $\ge 56\text{ lb}$, the fabric is airworthy.

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3. Covering Application & Heat-Shrinking Synthetic Fabric

A. Application Methods

• Envelope Method: Factory pre-sewn fabric slipcovers matching the airframe geometry are slid over the wing or fuselage like an envelope, then glued and stitched at root and trailing edges.
• Blanket Method: Flat sheets of fabric are draped over the structure, joined at edges and structural members using approved fabric cement (e.g., Poly-Tak, New DMU) with a minimum seam overlap of $1\text{ to }2\text{ inches}$.

B. Calibrated Heat-Shrinking Temperature Schedule

Synthetic polyester fabrics shrink predictably when exposed to specific temperatures. Shrinking must be done using a calibrated electric household or covering iron with an accurate surface thermometer.

  Temperature (°F)
     ^
 375°+ - - - - - - - - - - - - - - - - - - - - - - - CRITICAL MAXIMUM (DO NOT EXCEED!)
     |
 350°+-------------------------+                     3rd Pass: Final Tautening (350°F)
     |                        /| 
 300°+--------------+        / |                     2nd Pass: Intermediate Shrink (300°F)
     |             /|       /  |
 250°+-----+      / |      /   |                     1st Pass: Smooth Wrinkles (250°F)
     |    /|     /  |     /    |
   0°+---+--+----+---+----+----+------------------> Steps

1. First Pass ($250^\circ\text{F} / 121^\circ\text{C}$): Smooths out wrinkles and establishes initial skin contour.
2. Second Pass ($300^\circ\text{F} / 149^\circ\text{C}$): Intermediate shrinkage and initial tautening.
3. Final Pass ($350^\circ\text{F} / 177^\circ\text{C}$): Imparts full structural tension and final flight tautness.
4. CRITICAL UPPER SAFETY LIMIT: NEVER EXCEED $375^\circ\text{F}$ ($190^\circ\text{C}$). At $375^\circ\text{F}$ and above, polyester fiber filaments melt and permanently lose tensile strength, rendering the covering unairworthy.

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4. Fabric Attachment & Rib Lacing Standards

Aerodynamic suction on the upper wing surface during flight tends to pull fabric off the wing ribs. Mechanical fasteners or stitching are mandatory to secure fabric to rib caps.

+-------------------------------------------------------------------------+
|                    RIB STITCHING (LACING) STANDARDS                     |
+------------------------------------+------------------------------------+
| Cord Type                          | Waxed braided polyester / linen    |
| Reinforcing Tape                   | Placed under cord to prevent tears |
| Knot Type                          | Modified Seine Knot (self-locking) |
| Spacing Rule                       | Spacing closer in prop slipstream  |
|                                    | and at higher airspeeds (Vne)      |
+------------------------------------+------------------------------------+

A. Rib Stitching / Lacing

• Reinforcing Tape: A continuous strip of woven reinforcing tape is applied over the fabric directly above each rib cap before stitching to prevent the cord from cutting through the fabric.
• Lacing Cord: High-strength waxed polyester or braided linen cord.
• Knot Specifications: The Modified Seine Knot is standard for continuous rib lacing. Unlike standard knots, the modified Seine knot is self-locking; if a cord loop breaks elsewhere on the rib, the knot prevents adjacent stitches from loosening.
• Stitch Spacing Criteria: Spacing between rib stitches is governed by aircraft $V_{NE}$ and whether the rib is located inside the propeller slipstream or outside the slipstream (per AC 43.13-1B rib stitch spacing curves). Stitches are spaced closer together in the propeller slipstream and at higher airspeeds.

B. Alternative Attachment Methods & Finishing Tape

• Alternative Fasteners: Sheet metal screws with plastic washers, Martin fabric clips, wire clips, or blind pop rivets (used on metal rib caps).
• Surface (Finishing) Tape: Pinked-edge (sawtooth cut) or heat-cut surface tape is doped over all stitch lines, leading edges, trailing edges, and seams to seal fasteners and streamline airflow. Pinked edges provide greater bonding surface area and prevent fabric unraveling.
• Drain Grommets & Inspection Rings: Plastic drain grommets are glued at the lowest trailing point of every wing bay and control surface to drain water condensation. Plastic inspection rings are installed over internal pulleys, bellcranks, and fuel tank connections.

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5. Doping & Finishing Chemistry

Dope coats protect fabric from weather, tauten natural fibers, and provide a durable aerodynamic finish.

+-------------------------------------------------------------------------+
|                   FABRIC FINISHING COAT SEQUENCE                        |
+-------------------------------------------------------------------------+
| [1] Topcoats: Polyurethane Enamel or Cellulose Acetate Butyrate (CAB)   |
| [2] UV Barrier Coats: Aluminum Pigmented Dope (2 to 3 cross coats)      |
| [3] Clear Primer Coats: Clear Dope / Poly-Brush (with Fungicide)        |
| [4] Tautened Synthetic or Natural Fabric Base                           |
+-------------------------------------------------------------------------+

A. Finishing Sequence

1. First Coat (Clear Dope / Primer): Clear nitrate dope (on cotton) or vinyl primer (Poly-Brush on polyester). For organic fabrics, a fungicide additive is mandatory in the first coat to prevent rot.
2. Intermediate UV Barrier Coats (Aluminum Pigmented Dope): Clear dope mixed with fine aluminum powder paste. The aluminum flakes float to the surface, creating an opaque metallic shield that completely blocks ultraviolet (UV) radiation. Unprotected polyester or cotton degrades and loses tensile strength rapidly when exposed to solar UV radiation.
3. Topcoats (Color Coats): Applied over the sanded aluminum barrier coats using cellulose acetate butyrate (CAB) dope or flexible polyurethane enamel.

B. Dope Chemical Compatibility Rule

• Nitrate Dope: Cellulose dissolved in nitric acid solvents. Offers exceptional adhesion and tautening, but is highly flammable.
• Butyrate Dope (CAB): Cellulose dissolved in butyric acid. Significantly lower flammability, highly flexible, and weather-resistant, but exhibits lower initial adhesion.
• THE INCOMPATIBILITY RULE: $\text{Butyrate Dope CAN be applied OVER Nitrate Dope. } (\checkmark)$ $\mathbf{Nitrate\ Dope\ must\ NEVER\ be\ applied\ OVER\ Butyrate\ Dope. } (\boldsymbol{\times})$ Reason: Solvents in nitrate dope attack and soften the underlying butyrate base, causing severe blistering, lifting, and complete finish failure.

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6. Finishing Defects & Troubleshooting