4.5 Composite Materials, Plastics & Control Cables

4.5 Composite Materials, Plastics & Control Cables

Advanced composites, high-performance polymers, transparent windshield thermoplastics, and high-strength flight control cables are critical components of general aviation, transport category, and military aircraft. An Aviation Maintenance Technician must understand composite mechanics, vacuum-bag consolidation, precision scarf repair design, optical plastic care, and cable rigging under FAA-H-8083-30B and AC 43.13-1B.

1. Advanced Reinforcing Fibers

A composite material consists of high-strength, high-modulus reinforcing fibers embedded within a matrix resin system. The fibers carry primary tensile and compressive flight loads, while the matrix binds the fibers together, transfers inter-laminar shear stresses, and shields fibers from environmental degradation.

Composite Structural Mechanics & Load Transfer:
  [ External Flight Tensile / Compressive Loads ]
                        │
                        ▼
  ┌──────────────────────────────────────────────────────────┐
  │ ═══════════════════════════════════════════════════════  │ <-- Structural Fibers
  │ ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░  │     (Carry 80-90% Tensile)
  │ ═══════════════════════════════════════════════════════  │
  │ ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░  │ <-- Matrix Resin System
  └──────────────────────────────────────────────────────────┘     (Transfers Shear & Compression)

Primary Reinforcing Fiber Types

2. Matrix Resin Systems (Thermosets vs. Thermoplastics)

1. Thermosetting Resins

Thermosetting polymers undergo an irreversible chemical cross-linking reaction (curing) when mixed with a curing agent/hardener and heated. Once cured, they cannot be melted or reshaped; heating beyond their degradation temperature decomposes the matrix.

• Epoxy Resins: The aerospace industry standard for structural laminates. Features superior adhesion, high mechanical strength, low cure shrinkage ($<1% - 2%$), and outstanding resistance to moisture, jet fuel, and Skydrol hydraulic fluid.
• Polyester Resins: Lower strength, high cure shrinkage ($6% - 8%$), low chemical resistance. Limited to non-structural fiberglass cowlings and light utility fairings.

2. Thermoplastic Resins

Thermoplastics consist of linear or branched polymer chains without chemical cross-links. They soften and melt when heated and re-solidify upon cooling, allowing them to be repeatedly reshaped and hot-fusion welded.

• Aviation Thermoplastics: PEEK (Polyetheretherketone), PPS (Polyphenylene Sulfide), and PEI (Polyetherimide). They deliver exceptional impact toughness, infinite room-temperature shelf life, and rapid manufacturing cycles.

3. Core Materials for Sandwich Structures

Sandwich construction consists of two thin, high-strength composite or metallic face sheets bonded to a thick, lightweight core. The face sheets resist in-plane bending tension and compression, while the lightweight core carries out-of-plane shear loads and stabilizes the thin skins against compressive buckling.

Honeycomb Sandwich Panel Mechanics:
  ════════════════════════════════════════════════  <-- Upper Face Sheet (Compression)
  ░░░░░░░[ Adhesive Layer / Primer ]░░░░░░░░░░░░░░
  │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │  <-- Honeycomb Core (Carries Shear)
  │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │      Ribbon (L) Direction = Max Shear
  ░░░░░░░[ Adhesive Layer / Primer ]░░░░░░░░░░░░░░
  ════════════════════════════════════════════════  <-- Lower Face Sheet (Tension)

Core Classifications

1. Nomex Honeycomb: Aramid fiber paper dipped in phenolic resin. Extremely lightweight, fire-resistant, and corrosion-proof; standard for cabin floorboards, control surfaces, and engine nacelles.
2. Aluminum Honeycomb (5052 / 5056 Alloy): High shear strength-to-weight ratio; susceptible to internal galvanic corrosion if water enters the cells.
3. Core Ribbon Direction (L-Direction): The ribbon direction represents the continuous foil/paper web of the honeycomb and possesses the highest shear strength. During repair, the replacement core plug's ribbon direction must align with the original part's ribbon direction.
4. Structural Foams: Rigid closed-cell foams, including PVC (Divinycell), Polymethacrylimide (Rohacell), and Syntactic foams (epoxy filled with hollow microscopic glass microballoons).

4. Prepregs, Vacuum Bagging & Curing Processes

Pre-Impregnated Fabrics (Prepregs)

Prepregs are structural fabrics pre-impregnated by the manufacturer with a formulated, partially cured (B-stage) epoxy resin matrix. To prevent the resin from advancing to full C-stage cure, prepregs must be stored in sealed moisture-barrier bags in freezers at $0^\circ\text{F}$ ($-18^\circ\text{C}$). Technicians must track and log the material's cumulative room-temperature exposure (out-time).

Vacuum Bagging Processing Stack (Layer Sequence from Tool Outward):
  [ 1. Tool / Mold Base with Release Agent ]
  [ 2. Composite Laminate Plies (Correct Orientation: 0°, ±45°, 90°) ]
  [ 3. Peel Ply / Release Fabric (Textured surface for bonding) ]
  [ 4. Perforated Release Film (Allows gas/resin escape) ]
  [ 5. Bleeder Cloth (Absorbs excess resin; controls fiber/resin ratio) ]
  [ 6. Solid Barrier Release Film (Isolates bleeder from breather) ]
  [ 7. Breather Felt (Provides continuous open air channel) ]
  [ 8. Thermocouple Temperature Sensors (Monitors thermal cycle) ]
  [ 9. Vacuum Bag Film & Sealant Butyl Tape (Seals entire assembly) ]
  [ 10. Vacuum Port (Draws 22 to 28 inHg vacuum pressure) ]

Vacuum Bagging Consolidation Mechanics

• Vacuum Level: A vacuum of at least $22\text{ to }28\text{ inHg}$ ($74\text{ to }95\text{ kPa}$) is drawn, exerting atmospheric pressure ($11 - 14\text{ psi}$) uniformly across the laminate to consolidate plies, eliminate trapped air/volatiles, and remove excess resin into the bleeder.
• Controlled Cure Cycle (Hot Bonder):
  1. Ramp Up: Temperature increased at $3^\circ\text{F} - 5^\circ\text{F}$ per minute to avoid thermal shock.
  2. Soak / Dwell: Temperature held at curing level ($250^\circ\text{F}$ or $350^\circ\text{F} \pm 10^\circ\text{F}$) for $60$ to $120$ minutes.
  3. Ramp Down (Cooling): Controlled cooling at $<5^\circ\text{F}$ per minute until below $140^\circ\text{F}$ before releasing vacuum, preventing thermal stress warpage.

5. Composite Damage Evaluation & Repair Principles

Inspection Methods

• Tap Testing (Acoustic Coin Tapping): The technician lightly taps the laminate surface with a specialized acoustic hammer or coin. Solid, well-bonded laminates produce a clear, crisp ringing metallic tone; internal delaminations, voids, or core disbonds produce a dull, flat, dead thud.
• Ultrasonic NDI: Pulse-echo A-scan and through-transmission C-scan map delamination boundaries and depth.
• Radiography (X-Ray): Detects water ingress inside honeycomb core cells.

Flush Stepped Scarf Repair (AC 43.13-1B)

When damaged composite plies must be restored flush with the original aerodynamic contour:

1. Scarf Taper Ratio: The damaged zone is carefully scarfed (taper sanded) using a pneumatic sander with a standard taper ratio of $1 : 30\text{ to }1 : 50$ (representing $1/2^{\prime\prime}\text{ to }1^{\prime\prime}$ of scarf width per individual ply of thickness).
2. Ply Orientation Matching: Each replacement ply must duplicate the exact fiber material, weave pattern, and orientation angle ($0^\circ, +45^\circ, -45^\circ, 90^\circ$) of the removed ply layer.
3. Extra Surface Ply: A final extra layer of light fiberglass or fine fabric (cosmetic/protection ply), extending at least $1\text{ inch}$ beyond the scarf boundary, is applied over the outer surface for environmental and sanding protection.

6. Aircraft Transparent Plastics (Acrylic vs. Cellulose Acetate)

Aircraft windshields, canopy enclosures, and cabin windows are manufactured from transparent thermoplastic resins.

Transparent Plastics Physical Identification & Reaction:
  [ Acrylic (Plexiglas / Lucite / MIL-PRF-5425) ]
    ├─ Chemical Test: Softens slowly in acetone; remains clear
    ├─ Burn Test: Burns with steady blue/yellow flame, little smoke, sweet fruity odor
    └─ Optical Property: Highly craze-resistant, superior UV clarity

  [ Cellulose Acetate (Older Aircraft) ]
    ├─ Chemical Test: Dissolves rapidly in acetone; turns milky white
    ├─ Burn Test: Burns with sputtering yellow flame, heavy black smoke, pungent vinegar odor
    └─ Optical Property: Yellows, embrittles, and crazes rapidly under UV sunlight

Material Identification Tests

• Acetone Chemical Test: A drop of acetone is applied to a scrap corner. Cellulose acetate softens and dissolves almost immediately, turning milky white; acrylic softens very slowly and remains optically clear.
• Burn Test: When ignited, cellulose acetate burns with a sputtering yellow flame, dense dark smoke, and a pungent vinegar-like acetic acid odor. Acrylic burns with a steady, quiet blue-yellow flame, little smoke, and a sweet, fruity ester odor.

Plastic Care, Craze Prevention & Maintenance Rules

• Crazing: The formation of microscopic hairline surface fissures caused by tensile stresses combined with exposure to chemical vapors (gasoline, paint thinners, alcohol, acetone, benzene, or Skydrol). Once crazed, the plastic suffers significant loss of impact strength and optical transparency.
• Cleaning Rules (FAA-H-8083-30B):
  • Wash with copious amounts of clean water and mild soap using bare hands or clean microfiber cloths to rinse away abrasive grit. Never rub dry plastic with a shop rag.
  • Masking Paper Removal: If protective masking paper adhesive has hardened on plastic sheets, soften the adhesive by applying kerosene, aliphatic naphtha, or warm soapy water. Never scrape with razor blades or metal tools.
• Temporary Crack Repair (Stop-Drilling): If a crack develops in a windshield or window, its propagation must be arrested immediately by stop-drilling a $#30\text{ to }#40$ hole ($0.098^{\prime\prime} - 0.128^{\prime\prime}$) through the exact terminus (tip) of the crack to relieve stress concentrations.

7. Aircraft Control Cables & Rigging Hardware

Aircraft flight control cables transmit mechanical inputs from cockpit flight controls to ailerons, elevators, rudders, and trim tabs under MIL-DTL-83420.

Aircraft Control Cable Cross-Sectional Construction:
       7 x 7 Non-Flexible Cable                   7 x 19 Extra-Flexible Cable
          (49 Total Wires)                            (133 Total Wires)
          ┌───┬───┬───┐                              ┌───────┬───────┐
          │ * │ * │ * │                              │ ***** │ ***** │
      ┌───┼───┼───┼───┼───┐                      ┌───┼───────┼───────┼───┐
      │ * │ * │ * │ * │ * │                      │***│ ***** │ ***** │***│
      └───┼───┼───┼───┼───┘                      └───┼───────┼───────┼───┘
          │ * │ * │ * │                              │ ***** │ ***** │
          └───┴───┴───┘                              └───────┴───────┘
  Straight runs, trim tabs, engine controls    Primary flight controls around small pulleys

Cable Construction Classes

• $7 \times 7$ Non-Flexible Cable: Composed of $7$ strands of $7$ wires each ($49$ wires total). Moderate flexibility; used for straight runs, engine controls, trim tabs, and mechanical position indicators.
• $7 \times 19$ Extra-Flexible Cable: Composed of $7$ strands of $19$ wires each ($133$ wires total). Maximum flexibility; mandatory for primary flight control cable systems that route around small-diameter pulleys and directional fairleads.
• Materials: Manufactured from Galvanized Carbon Steel (high fatigue resistance, zinc-coated) or Corrosion-Resistant Steel (CRES / Stainless Steel).

Swaged Terminal Fittings & Go/No-Go Gauge Inspection

• Terminal Fittings: MS20667 (fork end), MS20668 (eye end), MS20664 (ball end), and AN669 (turnbuckle swaged terminal).
• Swaging Process: The terminal sleeve is hydraulically or rotary-swaged over the cable shank. The metal flows into the cable valleys to create a joint exceeding the rated cable breaking strength.
• Go / No-Go Gauge Inspection (AC 43.13-1B):
  • After swaging, the outside diameter of the sleeve must be measured along its entire length using a precision Go / No-Go Swage Gauge.
  • The sleeve must slip into the 'Go' slot of the gauge, confirming that the sleeve has been compressed to the correct final diameter. If the sleeve is too large (fits into 'No-Go' slot), it was under-swaged; if it is over-swaged or distorted, it is condemned.
• Proof-Load Testing: Swaged cable assemblies must be proof-load tested to $60%$ of the cable's minimum rated breaking strength for at least $5\text{ seconds}$ without slippage before installation.

Cable Wear Limits, Broken Wires & Rigging Tensiometers

• Broken Wire Inspection: Wipe the cable with a rag to snag broken wire ends, or bend the cable into a reverse loop across pulley contact zones.
• Rejection Criteria: AC 43.13-1B mandates cable replacement if more than 3 broken wires per strand in any 7-wire strand or more than 6 broken wires in any 1-cable-lay (pitch length) are discovered.
• Cable Tensiometer Calibration: Flight control cable tension is measured using a calibrated mechanical or digital tensiometer. Because airframe aluminum expands and contracts with temperature changes faster than steel cables, the measured cable tension MUST be adjusted for ambient temperature using the aircraft manufacturer's temperature-tension compensation rigging chart.