3.3 Wood, Fabric, Plastics & Sealants

Wood, Fabric, Plastics & Sealants

Quick Summary: In addition to advanced composites, certifying aircraft maintenance engineers must maintain mastery over specialized non-metallic materials governed by EASA Part-66 Module 6.3: structural aircraft timber and plywood, synthetic fabric coverings, transparent cockpit glazing, and fuel-resistant aerospace sealants. This section examines the strict inspection standards, chemical vulnerabilities, and repair protocols governing these critical materials.

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1. Aircraft Structural Wood and Plywood

While largely superseded by metals and composites in commercial transports, structural wood remains vital in gliders, aerobatic aircraft, vintage restorations, and utility airplanes. All aircraft wood standards stem from FAA AC 43.13-1B and international airworthiness specifications.

The Reference Standard: Sitka Spruce

Sitka Spruce (Picea sitchensis) is the international benchmark against which all aircraft structural timbers are evaluated:

• Specific Gravity: Must be at least 0.36 (AN-W-2 / MIL-S-6073, kiln-dried aircraft spruce). FAA AC 43.13-1B Table 1-1 quotes no specific-gravity figure at all — it rates Sitka spruce as the 100% strength reference against which every other species is compared. There is no higher reference value: 0.36 is both the minimum and the benchmark.
• Moisture Content: Permissible range is strictly 8% to 12%. Moisture below 8% causes wood to become brittle and prone to sudden catastrophic cracking. Moisture exceeding 12% invites rapid fungal decay and causes severe shrinkage/warping across the grain as the timber dries.
• Slope of Grain: The grain deviation must not exceed a slope of 1:15 (1 unit lateral deviation per 15 units of longitudinal length). A grain slope steeper than 1:15 drastically degrades tensile and bending allowable stresses.
• Annual Growth Rings: Must display a minimum of 6 rings per inch (some primary spar specifications mandate 8 rings/inch) to guarantee structural density.

                  Slope of Grain Limit: Maximum 1 in 15
        ┌─────────────────────────────────────────────────────────────┐
        │    /─────────────────────────────────────────────────────── │
        │   /  ◄─── 1 Unit Lateral Deviation                          │
        └──/──────────────────────────────────────────────────────────┘
           ◄──────────────── 15 Units Longitudinal Length ────────────►

Permissible Wood Substitutions

Other timber species may substitute for Sitka spruce provided specific gravity and cross-sectional sizing adjustments are made:

• Douglas Fir: Specific gravity 0.45 (+12% heavier than spruce); delivers higher strength but is prone to splintering during working.
• Noble Fir: Specific gravity 0.40; identical strength properties to spruce, acceptable direct substitute.
• Western Hemlock: Specific gravity 0.44; slightly higher strength, acceptable if inspected carefully for hidden ring shakes.

Aircraft Plywood

Aircraft structural plywood consists of an odd number of thin hardwood veneers laminated together with phenolic or resorcinol waterproof adhesives under heat and pressure (specifications MIL-P-6070 or BS 2V35):

• Birch (Betula alleghaniensis) & African Mahogany: The primary aircraft plywood veneers. Birch provides extreme shear resistance and bearing strength; mahogany is lighter and provides excellent skin stability.
• Grain Alignment: Veneers are laminated with alternate grain directions at 90° to balance tensile and shear properties, or at 45° for specialized wing spar shear webs.

Approved Wood Adhesives

• Resorcinol-Formaldehyde: The premier structural wood adhesive. Forms a dark purple/reddish-brown bondline. Fully waterproof, impervious to fungi, fuels, and solvents, and cures at room temperature (above 21°C / 70°F).
• Epoxy Resins: Two-part structural epoxies (e.g., T-88, Aerodux) provide excellent gap-filling capabilities and require lower clamping pressure than resorcinol.
• Casein Glue: Derived from milk curd proteins. STRICTLY PROHIBITED in modern aircraft maintenance. Casein is not waterproof and deteriorates rapidly when exposed to humidity, supporting fungal rot that destroys the joint.

Defect Limits and Mandatory Rejection Criteria

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2. Aircraft Fabric Covering Systems

Fabric covering provides an ultra-lightweight aerodynamic envelope for wings, fuselages, and control surfaces on utility aircraft, gliders, and light aerobatic trainers.

                         Fabric Covering System Evolution
                                        │
        ┌───────────────────────────────┴───────────────────────────────┐
        ▼                                                               ▼
  Historical Grade A Cotton                                  Modern Synthetic Polyester
• Organic mercerized cotton (MIL-C-5643)                   • Polyethylene terephthalate (Ceconite, Poly-Fiber)
• Deteriorates from moisture, mildew, UV                   • Rot-proof, mildew-proof, inorganic
• Tautened chemically with dope shrinkage                  • Tautened thermally with calibrated iron
• Replaced when strength drops < 70% (56 lbs/in)           • UV-shielded with aluminium silver dope

Synthetic Polyester vs. Legacy Cotton

• Grade A Cotton (Historical Standard): Manufactured from organic long-staple cotton treated with caustic soda (mercerized). Minimum new tensile strength was 80 lbs/inch. Cotton was vulnerable to moisture, fungal mildew, and atmospheric acids, requiring periodic punch-testing with a Seyboth or Maule tester. When tensile strength dropped below 70% of original (56 lbs/inch), the fabric was deemed unairworthy and required complete stripping.
• Synthetic Polyester (Ceconite, Poly-Fiber, Dacron): The modern airworthiness standard. High-tenacity continuous filament polyester fabric. It is completely immune to moisture rot, mildew, and fungal attack and maintains its structural strength indefinitely unless degraded by actinic ultraviolet radiation.

Thermal Heat-Shrinking Calibration

Unlike cotton (which shrank unpredictably via chemical dope solvent evaporation), polyester fabric is shrunk mechanically to precise, repeatable drum-tightness using a calibrated household or industrial iron:

• Pass 1 (Wrinkle Removal & Smoothing): Calibrated iron set at 105°C to 120°C (225°F to 250°F).
• Pass 2 (Intermediate Tautening): Calibrated iron set at 149°C (300°F).
• Pass 3 (Final Full Tension): Calibrated iron set at exactly 177°C (350°F) to achieve final flight tension.
• Critical Danger: Never apply an iron exceeding 190°C (375°F). Temperatures above 190°C permanently anneal, melt, and destroy the polyester molecular chains, causing the fabric to go slack and fail structurally in flight.

Mechanical Attachment to Wing Ribs

Aerodynamic suction on the upper wing surface tries to rip the fabric envelope away from the wing structure. The fabric must be mechanically anchored to each wing rib:

• Rib Stitching (Rib Lacing): Flat, braided, waxed polyester cord is laced around the rib profile through reinforced holes, tied with a modified seine knot.
• Reinforcing Tape: Heavy cotton or polyester tape laid directly over the fabric envelope along the rib cap before stitching to prevent the lacing cord from pulling through the taut fabric.
• Stitch Spacing: Determined by maximum aircraft dive speed ($V_d$). Spacing is closer in the slipstream area (typically 1.25 to 2 inches) and wider outside the slipstream (2 to 3 inches).

Finishing Dopes & Ultraviolet (UV) Protection

• Cellulose Nitrate Dope: Formulated from dissolved nitrocellulose. Provides superior fabric encapsulation and adhesion, serving as the universal base primer coat. Highly flammable.
• Cellulose Acetate Butyrate (CAB) Dope: Formulated with cellulose acetate and butyric acid. Significantly less flammable than nitrate, highly flexible, and weather-resistant; used for build and top coats.
• Actinic Ultraviolet (UV) Shielding: While polyester does not rot from moisture, solar UV radiation attacks polyester molecules, causing embrittlement and catastrophic tearing within 6 to 12 months. Protection is achieved by applying aluminium-pigmented butyrate dope ("silver dope") in 2 to 3 heavy cross-coats over the clear primer. The microscopic aluminium flakes overlap like fish scales, creating an opaque, 100% reflective barrier against UV light before color coats are sprayed.

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3. Transparent Aircraft Plastics: Acrylic vs. Polycarbonate

Cockpit windscreens, fighter canopies, passenger cabin windows, and sensor fairings rely on transparent thermoplastic polymers.

                               Transparent Glazing
                                        │
        ┌───────────────────────────────┴───────────────────────────────┐
        ▼                                                               ▼
  Acrylic (PMMA / Plexiglas)                                Polycarbonate (Lexan)
• Exceptional optical clarity (>92%)                       • Extreme ballistic impact resistance
• Biaxially stretched for craze resistance                 • Scratches easily (very soft surface)
• Moderate impact resistance                               • Cockpit bird-strike windshields
• Vulnerable to aromatic solvents                          • Sensitive to UV yellowing & solvents

Acrylic (Polymethyl Methacrylate, PMMA)

• Trade Names: Plexiglas, Perspex, Lucite.
• Cast Acrylic vs. Stretched Acrylic:
  • Cast Acrylic: Formed by pouring liquid methyl methacrylate monomer between glass plates. Good optical clarity, but relatively brittle.
  • Biaxially Stretched Acrylic (MIL-P-25690): Formed by heating cast acrylic and mechanically stretching it simultaneously in two orthogonal directions by 100% to 150%. Stretching aligns the tangled polymer chains parallel to the surface. This dramatically increases fracture toughness, impact resistance, and resistance to crazing. Stretched acrylic is the universal standard for pressurized passenger airliner cabin windows and pressurized cockpit windshield outer plies.
• Optical Transmission: Exceeds 92% visible light transmission with minimal optical distortion.

Polycarbonate (PC)

• Trade Names: Lexan, Makrolon.
• Mechanical Characteristics: Possesses unrivaled impact and ballistic toughness (approximately 30 times tougher than cast acrylic). It is the standard core material for bird-strike resistant transport aircraft cockpit windshields and fighter jet canopies.
• Limitations: Polycarbonate has a very soft surface that scratches easily without proprietary polysiloxane hard-coatings. It is also susceptible to UV-induced yellowing and solvent stress-cracking.

Acrylic vs. Polycarbonate Glazing Comparison

The Crazing Phenomenon

Crazing is the formation of a dense, localized network of microscopic fissures in the plastic surface, oriented perpendicular to the direction of applied tensile stress:

• Mechanism: Caused by the synergistic combination of sustained tensile stresses (from cabin pressurization, aerodynamic loads, or over-torqued mounting bolts) and chemical solvent attack.
• Chemical Culprits: Acetone, methyl ethyl ketone (MEK), benzene, toluene, lacquer thinners, alcohol/isopropyl, and household window cleaners containing ammonia.
• Structural Penalty: Crazing severely obscures pilot vision (creating blinding glare under sunlight) and acts as stress-concentration notches that reduce structural strength by up to 50%, eventually propagating into catastrophic window blowout.

         Tensile Stress  ◄═════════════════════════════════════►  Tensile Stress
                              │   │   │   │   │   │   │
                              ▼   ▼   ▼   ▼   ▼   ▼   ▼
                 Microscopic Fissures (Perpendicular to Tensile Stress)
                 Triggered by: Tensile Stress + Solvent (Acetone/Alcohol)

Cleaning and Maintenance Procedures

• Step 1 (Grit Flush): Flood the plastic surface with abundant, clean, lukewarm water to flush away abrasive dust, sand, and grit particles. Never wipe dry plastic with a rag!
• Step 2 (Bare Hand Washing): Wash gently using mild neutral soap and water, wiping with the bare hands. The human palm is sensitive enough to feel microscopic grit particles (which would otherwise be pressed into the soft plastic by a wash mitt or sponge) before they can gouge the surface.
• Step 3 (Drying): Pat dry with a clean, damp chamois or soft cotton flannel. Rubbing with dry paper towels generates an electrostatic charge that immediately attracts airborne grit and causes fine swirl scratches.
• Step 4 (Polishing): Minor scratches may be polished out using specialized micro-abrasive plastic polish applied with clean cotton flannel in circular motions, buffed clean with dry flannel.

Thermal Expansion & Installation Clearances

Acrylic exhibits a coefficient of thermal expansion roughly 3 to 8 times higher than aluminium alloys. When installing transparent panels in airframes:

1. Bolt holes drilled through acrylic must be oversized by 3.0 to 4.0 mm (1/8 inch) to allow free thermal expansion and contraction over operational temperature extremes (-55°C at altitude to +50°C on hot tarmacs).
2. Fasteners must pass through synthetic spacer bushings and elastomeric grommets.
3. Mounting clamping strips must be torqued only to snug engagement; rigid clamping generates localized tensile stresses that initiate crazing around fastener perimeters.

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4. Aerospace Sealants & Integral Fuel Tank Sealing

Aerospace elastomer sealants provide hermetic containment of aviation fuels, maintain fuselage pressurization, and prevent environmental corrosion at structural faying surfaces.

                             Aerospace Sealant Chemistries
                                           │
         ┌─────────────────────────────────┴─────────────────────────────────┐
         ▼                                                                   ▼
  Polysulfide Sealants                                            Silicone Sealants
• Two-part synthetic rubber (PR-1422, PR-1440)                    • High temperature (-65°C to +260°C)
• Manganese dioxide curing agent                                  • Firewalls, de-icing ducts, engine pylons
• Exceptional resistance to Jet-A, Avgas, Skydrol                 • STRICTLY PROHIBITED in Fuel Tanks
• Standard for Integral Fuel Tanks ("Wet Wings")                  • (Silicone dissolves in hydrocarbon fuel!)

Sealant Chemistries

• Polysulfide Sealants (AMS-S-8802, PR-1422, PR-1440): Two-part elastomeric polymers with sulfur-sulfur linkages in their molecular backbone. Polysulfides provide unrivaled resistance to aviation fuels (Jet-A, Jet-B, Avgas), hydraulic fluids (Skydrol), water, and weathering. Temperature service range is -54°C to +121°C (-65°F to +250°F), with short excursions to +180°C.
• Polythioether Sealants (AMS 3277, PR-2001): Advanced sulfur-containing elastomers that are free of liquid polysulfide polymers. They deliver up to 30% lower density (significant weight savings), faster cure rates at low temperatures, and lower volatile organic compounds (VOCs).
• Silicone Sealants: Retain elastomeric flexibility across extreme temperatures (-65°C to +260°C). Used for engine firewalls, pylon fairings, and pneumatic de-icing boots.

[!CAUTION] Critical Fuel Tank Sealing Rule: Silicone sealants swell, soften, and dissolve completely when exposed to hydrocarbon aviation fuels. Silicone must NEVER be used in integral fuel tanks, fuel access doors, or fuel system plumbing. Only certified polysulfide or polythioether sealants are permitted.

Integral Fuel Tank ("Wet Wing") Sealing Geometry

Modern transports utilize the interior cavities of the wing structure as fuel tanks ("wet wings") without bladder cells. Three distinct seal geometries are required:

1. Faying Surface Seal: A thin layer of low-viscosity sealant applied between overlapping structural surfaces (e.g., skin to spar cap) before inserting fasteners. When fasteners are torqued, excess sealant squeezes out, ensuring zero void space.
2. Fillet Seal: A continuous triangular bead applied along all internal structural seams, rib-to-skin junctions, and stiffener boundaries using a pneumatic sealant extrusion gun and shaped spatula (typically 6 to 10 mm fillet width).
3. Fastener Head / Nut Cap Seal: A brush coat or pre-molded cured sealant cap applied over the ends of fasteners, nuts, and collars to eliminate fuel weep paths through fastener holes and isolate fasteners from potential lightning strike sparking.

                      Integral Fuel Tank Fillet & Faying Seals

            Skin Panel ═══════════════════════════════════════════════════
                           │ [Faying Surface Seal: Squeezed Between] │
            Spar Flange ───┴─────────────────────────────────────────┴───
                                ╱◄── [Fillet Seal: Triangular Bead]
                               ╱
                              *──────────────── (Internal Fuel Cavity)

Surface Cleaning: The Two-Cloth Method

Adhesion failure is the leading cause of fuel tank leaks. Contaminants must be removed using the mandatory Two-Cloth Solvent Cleaning Method:

• Solvent Selection: Methyl ethyl ketone (MEK), acetone, or high-purity isopropyl alcohol (IPA).
• Protocol:
  • Cloth 1 (Wetting): A clean, lint-free cotton cloth wetted with solvent is wiped across the joint to dissolve grease, oils, and hydraulic residue.
  • Cloth 2 (Drying): A second, clean, dry, lint-free cloth immediately wipes away the dissolved contaminants and solvent before it can evaporate. If solvent is allowed to dry naturally on the metal, the evaporated solvent simply redeposits a thin film of oil across the surface, destroying sealant adhesion.

Sealant Mixing and Cure Parameters

• Two-Part Mixing: Base compound (Part A, white/grey) and accelerator/curing agent (Part B, black manganese dioxide). The two parts must be thoroughly blended until a completely uniform, streak-free grey color is obtained. Marbling or streaks indicate unmixed components that will never cure.
• Application Time (Working Life) Classes:
  • Class A (Brush Grade): Low viscosity (pourable/brushable) for fastener coating; working life 1/2 to 2 hours.
  • Class B (Fillet Grade): Thixotropic extrusion grade (does not sag or slump on vertical surfaces); applied with extrusion gun; working life designated by suffix (e.g., B-1/2 = 30 min, B-2 = 2 hours, B-4 = 4 hours).
  • Class C (Faying Surface Grade): Long open-time sealant that remains workable for 24 to 48 hours to permit large panel assembly before skin riveting.
• Cure Dynamics: Polysulfide curing is an oxidation reaction catalyzed by manganese dioxide that requires ambient moisture. Curing is optimal at 25°C (77°F) and 50% relative humidity. In extremely dry winter air, polysulfide curing slows drastically. Complete tack-free cure and full Shore A hardness (35 to 50) typically requires 24 to 72 hours.

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5. Maintenance Traps & Practical Scenarios

Scenario 1: The Crazed Windshield Disaster

A line maintenance technician uses commercial spray glass cleaner containing ammonia and shop paper towels to clean the acrylic windscreen of an executive jet.

• The Trap: Within three weeks, the windscreen develops a dense network of milky, glittering crazing fissures. Under runway landing lights, optical glare completely blinds the flight crew. The windscreen must be removed and replaced at extreme expense ($45,000).
• The Rule: Clean transparent acrylic solely with clean water, mild neutral soap, bare hands, and cotton flannel. Never use ammonia, alcohols, or dry paper towels.

Scenario 2: The Improper Fuel Tank Sealant

During an unscheduled line turn, a technician repairs a leaking fuel tank access panel using high-temperature red silicone sealant found in the engine shop.

• The Trap: Jet-A fuel dissolves the silicone into a sticky sludge within 48 hours. Sloughed silicone chunks migrate into the fuel booster pump suction screen, triggering low fuel pressure warnings in flight.
• The Rule: Silicone sealants are completely incompatible with aviation fuels. Only approved polysulfide or polythioether elastomers may be applied to aircraft fuel cells.