M — Airframe (Cellule)Chapter 4 · 20 practice questions

Chapter 4: Aircraft Fabric Covering

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Chapter 7: Aircraft Fabric Covering

Overview

This chapter covers the principles, materials, and procedures involved in covering aircraft structures with fabric. Fabric covering remains a common method for light aircraft, providing a lightweight, aerodynamic surface. The chapter addresses fabric types, installation techniques, inspection criteria, and repair procedures, with emphasis on the regulatory standards found in AC 43.13-1B and manufacturer specifications. Understanding the interaction between fabric, dope, stitching, and structural components is essential for proper maintenance and airworthiness.

Diagram — Aircraft Fabric Covering Aircraft Fabric Covering — Cross-Section & Process Flow Fabric Covering Cross-Section Aircraft Structure (Rib / Frame) Antisabotage Tape Polyester Fabric (Dacron) Dope (Tautening) Topcoat / UV Protection Stitching (Modified Seine Knot) Fabric Covering Process Flow 1. Structure Prep Clean & Inspect 2. Attach Fabric Stitch & Tension 3. Apply Dope Tauten & Seal 4. Topcoat UV Protection 5. Inspection Per AC 43.13-1B Pass? Airworthy Repair Re-dope & Re-inspect Legend Structure (Rib/Frame) Polyester Fabric (Dacron) Dope / Topcoat Layers Stitching (Modified Seine Knot) Key References • AC 43.13-1B — Acceptable Methods • Manufacturer Specifications Regulatory Standard TC / FAA Approved Repair Schemes TEA Chapter 7 — Aircraft Fabric Covering | Cross-Section & Process Flow Diagram

Key Concepts

1. Fabric Materials

Modern aircraft covering primarily uses polyester fabric (e.g., Dacron) due to its superior strength, durability, and resistance to rot, moisture, and ultraviolet (UV) degradation. Grade A cotton fabric is now considered obsolete and is rarely used in new installations. Nylon has some applications but is not as common as polyester. Fiberglass is used for composite structures, not fabric covering.

Properties of Polyester Fabric:

  • High tensile strength
  • Excellent dimensional stability after shrinking
  • Resistance to fungal growth and rot
  • Good UV resistance when properly top-coated
  • Compatible with modern dope systems

2. Fabric Installation Process

Dope layer build-up Dope Layer Buildup Cross-sections — Aircraft fabric covering structure Dope Layer Buildup POLYESTER FABRIC (DACRON) Base fabric — heat-shrunk LAYER 1 — FILLING DOPE LAYER 2 — TENSION DOPE (SHRINKING) LAYER 3 — FILLING DOPE (SANDED) LAYER 4 — FINISHING DOPE (COLOR) LAYER 5 — UV PROTECTION (CLEAR) Application order (bottom to top) Brushed Finishing Wet dope surface BRUSH Steady brush movement BRUSHING TECHNIQUE Overlap each pass by 50% for uniform thickness Effect of Tension on Fabric BEFORE TENSION Loose fabric with wrinkles Dope AFTER TENSION Tight and smooth fabric NOTE Polyester (Dacron) uses heat for heat-shrinking. Dope maintains the tension. AME Training — Transport Canada — Aircraft Fabric Covering
Fabric covering process Fabric Covering Process STEP 1 Bare Structure Metal / wood / tube STEP 2 Attach Fabric Fabric laid over structure STEP 3 Shrink ♨ Heat / Dope Dacron heat-shrinking STEP 4 Sew Ribs Lock stitch (2 in) STEP 5 Tapes Reinforcement / finish tape STEP 6: DOPE / FINISH 1. Initial coats Impregnate and seal 2. Tension Fabric shrinkage 3. Light sanding Rough surface 4. Finish coats Color and UV protection Defects: • Blush (whitening) • Drumming • Sagging Tight, protected, aerodynamic fabric Inspection: drain grommets, seams, tension attach shrink sew reinforce Materials: Grade A cotton (obsolete) Dacron polyester (modern)
Fabric Covering — Full Process Fabric Covering — Complete Process Rib Fabric (Dacron/Cotton) Leading Edge Reinforcing & Finishing Tapes (Protects stitching) Lock Stitch (Rib Attachment) Dope Layers Nitrate Dope Butyrate Dope Inter-coat Sanding Paint / Finish Process: Structure Prep → Fabric Application → Stitching/Taping → Dope (Nitrate/Butyrate + Sanding) → Finish

The installation of fabric covering follows a systematic sequence to ensure proper tension, adhesion, and protection.

a. Pre-shrinking: Before applying dope, the fabric must be adequately pre-shrunk. Failure to do so can cause excessive tension after doping, leading to "drumming" – a condition where the fabric becomes overly taut and drum-like. Pre-shrinking removes initial wrinkles and prepares the fabric for subsequent coats.

b. Shrinking Process: After installation, the fabric is tightened using heat (for heat-shrinkable fabrics) or tautening dope. This process removes wrinkles and ensures the fabric conforms tightly to the airframe structure. The shrinking process does not significantly increase tensile strength; its purpose is to achieve proper tautness.

c. Dope Application: Dope serves multiple functions:

  • Seals the fabric pores
  • Provides tautness
  • Creates a smooth aerodynamic surface
  • Offers UV protection (when topcoats contain UV inhibitors)

Application Technique: Multiple thin coats with light sanding between each coat produce the smoothest finish. Sanding removes imperfections and provides a mechanical bond for subsequent coats. Thick coats can cause runs, orange peel, and other defects. Rollers are not typically used; spray or brush application is standard.

d. Doping Defects:

  • Blushing: A white haze on the dope surface caused by moisture condensation during drying, typically in high humidity. Moisture becomes trapped in the dope film, creating the whitish appearance.
Dope blushing — trapped moisture close-up Blush: Moisture Trapped in Dope Cross-section of dope film Ambient humidity (H₂O) Dope film drying (tacky surface — solvent evaporating) Blush zone — white veil (trapped water micro-droplets) Dried dope film Polyester fabric (Dacron) Structure / rib 0 50 µm Scale: × 500 Blush formation mechanism 1 Solvent evaporation Solvent evaporates from the film surface. The surface cools (evaporative cooling). Solvent ⟶ 2 Humidity condensation The cooled surface attracts ambient humidity. Water vapor condenses into micro-droplets. H₂O 3 Trapped in the film The dope film dries and solidifies. The droplets remain trapped in the film. Solidified film 4 Visible white veil Micro-droplets scatter light. Milky / white appearance — aesthetic defect. White veil Solution / Prevention • Apply dope in a low-humidity environment • Use a drying retarder in humid weather • Correct blush with a thin coat of thinner
  • Drumming: Excessive tautness caused by inadequate pre-shrinking before the first dope coat.
Drumming vs sagging fabric tension states Drumming vs Sagging: Fabric Tension States Before/after comparison of tension defects on fabric covering DRUMMING BEFORE (insufficient pre-shrinkage) AFTER (dope application) Initial normal tension ⚠ Insufficient pre-shrinkage before dope application Excessive tension Fabric hard like a drum Consequence: risk of structural damage — fabric can no longer absorb loads SAGGING BEFORE (properly tensioned fabric) AFTER (aging) Correct tension Loss of tension between ribs Consequence: loss of aerodynamic rigidity and poor aesthetic appearance SOLUTIONS Drumming: • Follow manufacturer's pre-shrinkage procedures • Check tension before dope application • Use a tensiometer to measure tension Sagging: • Regular monitoring of fabric tension • Re-tension with heat (polyester fabric) • Apply shrinking dope if appropriate • In severe cases: complete re-covering Reference: AC 43.13-1B — Chapter 4 Aircraft Fabric Covering AME Training — Transport Canada — M-AIRFRAME ch4: Aircraft Fabric Covering

3. Rib Stitching and Attachments

Rib stitching and load transfer Rib stitching and load transfer Wing cross-section — View of the rib and fabric covering RIB SPAR Anti-tear strip Locking stitch ≈ 2 in Reinforcement tape (over) Aero load Load transfer PRINCIPLE OF FABRIC ATTACHMENT TO THE RIB The locking stitch passes through the fabric, the anti-tear strip, and the rib, then forms a loop that secures the entire assembly. The anti-tear strip (below) and the reinforcement tape (above) distribute the load to prevent the thread from cutting the fabric under tension. Aerodynamic loads on the fabric are transferred to the rib through the stitches spaced approximately 2 inches apart. Finishing tape: Applied over junction seams (trailing edge, panels). FABRIC (Dacron) Lower surface fabric

Rib stitching secures the fabric to the wing ribs and prevents ballooning during flight.

a. Rib Stitch Spacing: According to AC 43.13-1B, rib stitching should be spaced approximately 2 inches apart. Closer spacing may be required at the leading edge where aerodynamic loads are higher. Wider spacing can allow the fabric to balloon in flight, reducing aerodynamic efficiency and potentially causing structural damage.

b. Stitch Types:

Fabric stitch types comparison Fabric Sewing Stitch Types Chapter 4 — Aircraft Fabric Covering | Transport Canada — AME Training Lock Stitch (Lock Stitch / Rib Stitch) Wing rib Interlocking loops securing the fabric to the structure. Distributes load over the rib. Baseball Stitch (Baseball / Herringbone Stitch) Flexible zigzag joining two pieces of fabric (patches). Retains some elasticity. Running Stitch (Running Stitch) Simple and quick stitch. Used for temporary work or marking. Not very strong. Blanket Stitch (Blanket Stitch) Stitch that wraps the edge of the fabric. Protects against fraying. Comparison Table — Usage and Characteristics Lock Stitch Baseball Stitch Running Stitch Blanket Stitch Appearance Regular interlocking loops, continuous thread under the rib Appearance Diagonal zigzag crossed (herringbone) regular Appearance Dotted straight line, spaced vertical passes Appearance Loops wrapping the edge of the fabric, forming a selvedge Primary use Attaching the fabric to the wing ribs or fuselage Primary use Joining patches for repairs of torn fabric Primary use Temporary work, marking, holding provisionally Primary use Finishing fabric edges to prevent fraying Characteristic Strong attachment, distributes the load over the rib. Requires tapes Characteristic Strong and flexible seam, maintains the elasticity of the fabric Characteristic Not suitable for structural repairs. Low strength Characteristic Protects the edge, prevents fraying and tearing Anti-tear tape + reinforcing tape Finishing tape over No specific tape No specific tape
  • Lock Stitch (Rib Stitch): Used for attaching fabric to wing ribs. It consists of a series of interlocking loops that secure the fabric firmly to the rib structure.
  • Baseball Stitch (Herringbone Stitch): Used for attaching fabric patches to existing fabric. It provides a strong, flexible seam suitable for repairs.
  • Running Stitch: Used for temporary work, not for structural attachment.
  • Blanket Stitch: Used for edge finishing, not for structural attachment.

c. Anti-Tear Strips (Rib Lacing Tape): Placed under the rib stitching to distribute the load and prevent the thread from cutting through the fabric. They do not provide structural strength to the wing or act as a moisture barrier.

Anti-tear strip and reinforcing tape sandwich Tear strips and reinforcement tape at the rib ENLARGED RIB CUTAWAY WING RIB (SECTION) TEAR STRIP REINFORCEMENT TAPE TENSION TENSION FUNCTION OF THE STRIPS TEAR STRIP • Placed UNDER the stitch point • On the rib, between the fabric and the structure • Distributes the stitch load • Prevents the thread from cutting the fabric • Acts like a washer REINFORCEMENT TAPE • Placed ABOVE the stitching • Covers the stitch points • Distributes the load over the area • Often glued with dope • Ensures a smooth finish PRINCIPLE Both strips work together to distribute the load and protect the fabric from tearing under tension. Tear strip Reinforcement tape FABRIC (DACRON) STRUCTURE (WOOD) Stitch point Fig. 4-12 — Rib stitching detail

d. Reinforcing Tape (Rib Tape): Applied over the rib stitching to distribute the load and prevent the thread from cutting into the fabric under tension. It does not significantly strengthen the stitching, provide smoothness, or act as a moisture barrier.

4. Drain Grommets

Drain grommets and moisture drainage path Drain Grommets and Moisture Path Fabric Wing Cross-Section — Managing Internal Condensation Spar Rear spar Rib Rib Rot (critical zone) Rot (critical zone) Drain grommet Drain grommet Condensation flow Key Inspection Points — Drain Grommets Location: Wing low points only Function: Drain condensation and moisture Prevent corrosion and rot Inspection: Ensure they are not blocked Fabric around: no rot ⚠ Critical Point — Fabric Rot Unevacuated condensation accumulates at low points and causes rot of the cotton (Grade A) and corrosion of internal structures. Fungicide does not restore strength — rotted fabric must be replaced. Inspect regularly

Drain grommets are installed at the low points of a fabric-covered wing to allow condensation and moisture to drain out. Their primary purpose is to prevent corrosion of the internal structure and rot of the fabric. They are not for ventilation, inspection, or static ports. Regular inspection of drain grommets is essential, as rot often begins in these areas due to moisture accumulation.

5. Anti-Tear and Finishing Tapes

a. Anti-Tear Strips: As described above, these are placed under rib stitching to prevent fabric tearing.

b. Finishing Tape: Applied over fabric seams (e.g., at the trailing edge) to protect the stitching and prevent the seam from unraveling due to airflow. It also provides some aerodynamic smoothness, but the primary purpose is to secure the seam.

Repair Procedures

1. Patch Repairs

Fabric repair decision Fabric Repair Decision Damage size / location → repair method per AC 43.13-1B 1. DAMAGE ASSESSMENT Inspect damaged area Measure size and depth Rot? Tear? Impact? Damage > 1 in? (diameter) YES NO SMALL DAMAGE • Sewn or glued patch • Overlap: 2 in min. • Baseball stitch for sewing LARGE DAMAGE • Panel replacement • Sewn + glued patch • Overlap: 2 in min. SPECIAL DAMAGE LOCATION Trailing edge Sew + finishing tape Rotted area Remove and replace APPROVED DATA REQUIRED • Major repair: approved by Transport Canada (STC, form 24B) • Minor repair: AC 43.13-1B procedure acceptable 2-INCH RULE Minimum 2 in overlap in all directions T 2 in min.

When fabric damage occurs, the repair method depends on the size and location of the damage.

a. Patch Size Requirements: According to AC 43.13-1B, a patch must extend at least 2 inches beyond the damaged area in all directions. For example, a 4-inch diameter damaged area requires a patch at least 8 inches in diameter (4 + 2 + 2).

Fabric patch size rule (2-inch overlap) Patch Sizing Rule (2 inches) LEGEND Damaged area (hole) Repair patch Baseball stitch Top View — Damaged Area with Patch Existing fabric 8 in patch (Ø min. 8 in) Hole 4 in 4 in 8 in (4 + 2 + 2) 2 in 2 in 2 in 2 in Baseball stitch PATCH CALCULATION Hole: 4 in diameter Required overlap: 2 in on each side Min. patch = 4 + 2 + 2 Min. patch = 8 in KEY RULES Patch ≥ 2 in beyond the damaged area Overlap ≥ 2 in Stitch: baseball stitch Adhesive: dope (not epoxy) Reference: AC 43.13-1B Chapter 4 — Fabric Covering Repairs

b. Patch Attachment:

  • Adhesive: Fabric cement (dope) is the standard adhesive for attaching fabric patches to polyester fabric coverings. Epoxy is too rigid and can cause cracking. Polyester resin is for composites, not fabric. Cyanoacrylate is not suitable for large areas.
  • Stitching: The baseball stitch (herringbone stitch) is used for attaching fabric patches. It provides a strong, flexible seam.

c. Small Tear Repair: A small tear (e.g., 1/2 inch) near a rib can be repaired with a fabric patch that overlaps the damaged area by at least 2 inches. Sewing alone may not restore strength. Dope alone will not provide structural repair. Replacing the entire covering is excessive for small damage.

d. Trailing Edge Tear: For a tear along the trailing edge, sewing the tear and covering with finishing tape is appropriate. A patch would be difficult to apply smoothly on the trailing edge. Replacing the entire covering is excessive for localized damage.

e. Rot Repair: Rotted fabric must be removed and replaced with a patch. Fungicide will not restore strength. Replacing the entire covering is excessive for a small area. Dope alone will not repair the rot.

2. Inspection Criteria

Regular inspection of fabric-covered aircraft is essential for airworthiness.

a. Signs of Aging:

  • Sagging fabric between ribs: A common sign of aging where the fabric has relaxed or lost its tautness. Rib stitching tension would affect the fabric at the ribs, not between them. Dope application issues would cause other defects. Damaged ribs would cause localized distortion.
  • Rot near drain grommets: Indicates moisture accumulation and requires immediate repair.

b. Tension Checks: Fabric tension should be uniform across the structure. Excessive tension (drumming) or insufficient tension (sagging) indicates problems that require correction.

Important Regulations and Standards

The primary reference for fabric covering procedures is AC 43.13-1B, which provides acceptable methods, techniques, and practices for aircraft fabric covering. Key requirements include:

  • Rib stitch spacing: Approximately 2 inches, with closer spacing at leading edges
  • Patch overlap: Minimum 2 inches beyond damaged area
  • Stitch types: Lock stitch for rib attachment, baseball stitch for patches
  • Dope application: Multiple thin coats with sanding between coats
  • Drain grommets: Required at low points of wings

Relationships Between Concepts

  1. Fabric Tension and Stitching: Proper rib stitch spacing prevents fabric ballooning, while anti-tear strips and reinforcing tape prevent the stitching from cutting through the fabric. Both work together to maintain aerodynamic shape.
  2. Dope Application and Fabric Tension: The shrinking process (heat or tautening dope) removes wrinkles and tightens fabric. Inadequate pre-shrinking leads to drumming after doping. Multiple thin coats with sanding produce the smoothest finish.
  3. Moisture and Fabric Degradation: Drain grommets prevent moisture accumulation, which causes rot. Blushing occurs when moisture is trapped in dope during high humidity. Regular inspection of drain grommets is critical for preventing fabric deterioration.
  4. Repair Size and Method: Small tears can be patched, while rot requires removal and replacement. The 2-inch overlap rule ensures adequate bonding area. Stitch type (baseball stitch) and adhesive (dope) must be compatible with the fabric material.
  5. Fabric Material and System Compatibility: Polyester fabric requires compatible dope systems. Using incorrect adhesives (epoxy, cyanoacrylate) can cause cracking or poor adhesion. Modern polyester fabric has replaced Grade A cotton due to superior durability and resistance.

Diagram

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