Chapter 2: Non-Metallic Structures

Chapter: Non-Metallic Structures

Overview

This chapter covers the inspection, maintenance, and repair of non-metallic aircraft structures, including fiberglass, composite materials, honeycomb sandwich structures, plastics, wood, and adhesives. Modern aircraft increasingly utilize these materials for their high strength-to-weight ratios, corrosion resistance, and design flexibility. The chapter addresses the unique properties, failure modes, inspection techniques, and repair procedures specific to these materials, along with the regulatory framework governing their maintenance.

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Key Concepts

1. Types of Non-Metallic Materials in Aircraft Construction

Fiberglass

Fiberglass consists of fine glass fibers embedded in a resin matrix (typically polyester or epoxy). It offers excellent dielectric properties (important for radomes), good strength, and corrosion resistance. Fiberglass is used in radomes, fairings, wing tips, and control surfaces.

Composite Materials

Composites combine reinforcing fibers with a matrix material:

• Carbon fiber – High stiffness and strength, used in primary structures
• Kevlar (aramid) – Excellent impact resistance and damage tolerance, used in interior panels and ballistic protection
• Fiberglass – Good all-around properties, used in secondary structures

Honeycomb Sandwich Structures

These consist of thin face sheets bonded to a lightweight honeycomb core (aluminum, Nomex, or fiberglass). The core provides shear strength and stiffness while maintaining low weight. Used in flight control surfaces, floor panels, and radomes.

Plastics

• Acrylic (Plexiglas) – Used for windows and windshields
• Polycarbonate – Higher impact resistance, used for some windows
• Thermoplastics – Can be reformed with heat, used in various interior applications

Wood

Laminated wood is used in some propellers and vintage aircraft structures. It consists of multiple layers of wood veneer bonded with adhesive.

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2. Material Properties and Failure Modes

Delamination

Delamination is the separation of layers within a laminated composite structure. It can result from:

• Impact damage
• Moisture ingress
• Manufacturing defects
• Fatigue loading

Crazing

Crazing appears as tiny surface cracks in plastic materials. It indicates:

• Loss of structural integrity
• Degradation from UV exposure
• Chemical attack
• Stress concentration

Crazing weakens the material and can lead to crack propagation. Crazed plastic windows must be replaced – polishing may improve appearance but does not restore strength.

Barely Visible Impact Damage (BVID)

Composite structures can sustain internal damage (delamination, core crushing) while the surface appears nearly intact. This is particularly dangerous because:

• Internal damage may not be visible during routine inspection
• Structural strength can be significantly reduced
• Damage can propagate under load

Inspection techniques such as tap testing and ultrasonic testing are essential for detecting BVID.

Moisture Ingress

Composite and honeycomb materials are hygroscopic – they absorb moisture from the environment. Problems include:

• Prepreg materials absorb moisture before curing, which turns to steam during elevated-temperature cure, creating voids
• Honeycomb cores can corrode (aluminum core) or degrade when moisture enters
• Freeze-thaw cycles can cause internal damage as trapped water expands

Corrosion in Non-Metallic Assemblies

Aluminum components adjacent to non-metallic structures (e.g., window frames, honeycomb cores) can corrode when moisture is trapped. Corrosion must be removed and treated per AC 43.13-1B Chapter 6.

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3. Inspection Techniques

Visual Inspection

The first step in any inspection. Look for:

• Surface cracks, scratches, or punctures
• Discoloration (may indicate overheating or chemical attack)
• Delamination (visible as bulges or separation)
• Crazing on plastic surfaces
• Loose or missing fasteners
• Paint damage or blistering

Tap Testing (Coin Tapping)

A simple, effective method for detecting subsurface delamination and core damage:

• Tap the surface with a coin or specialized hammer
• A clear, sharp sound indicates a good bond
• A dull, hollow sound indicates delamination or disbonding

This technique is accepted by AC 43.13-1B for detecting delamination in composite structures.

Ultrasonic Testing

Uses high-frequency sound waves to detect internal flaws. Provides more detailed information than tap testing but requires specialized equipment and training.

Thermography

Detects subsurface defects by measuring temperature differences across the surface when heat is applied.

Moisture Detection

Moisture meters can detect water ingress in honeycomb structures. However, they do not detect core damage – tap testing or ultrasonic methods are needed for that.

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4. Repair Procedures

General Principles

• Follow the manufacturer's instructions – The SRM (Structural Repair Manual) is the primary authority
• AC 43.13-1B provides acceptable methods when manufacturer data is unavailable
• Use approved materials – Match the original material as closely as possible
• Proper surface preparation is critical for bond strength

• Control the environment – Temperature and humidity affect material properties and curing

Surface Preparation

Proper surface preparation is the most critical step before bonding:

For composite surfaces:

1. Clean with approved solvent to remove contaminants
2. Abrade (sand) the surface to create a mechanical bond
3. Clean again to remove sanding residue
4. Feather the edges of damaged areas to create a scarf for repair plies

For metal surfaces:

1. Clean to remove all contaminants
2. Abrade (roughen) the surface
3. Clean again
4. Apply primer if specified

Critical: Release agents prevent bonding and must never be used on repair surfaces.

Repair of Fiberglass Structures

Small Delaminations:

For delaminations approximately 1 inch in diameter, away from edges:

1. Drill a small hole at the center of the delamination
2. Inject epoxy resin to re-bond the layers
3. Allow to cure
4. Apply a fiberglass patch to restore strength and protect the area

Punctures and Holes:

Even if damage is within allowable limits, punctures must be repaired to restore structural integrity and moisture resistance:

1. Remove damaged material
2. Feather the edges to create a scarf
3. Clean the area thoroughly
4. Apply repair plies (matching original material)
5. Cure under vacuum bag pressure

Surface Scratches:

For shallow scratches that do not penetrate structural plies:

• Seal with resin to prevent moisture ingress
• This is sufficient for cosmetic damage

Cracks:

• Stop-drill the ends of cracks to arrest propagation
• Apply a patch to restore strength
• Restore any protective coatings (e.g., conductive coating for lightning protection)

Repair of Honeycomb Sandwich Structures

Crushed Core:

Crushed honeycomb core cannot be reliably reformed or filled. The standard repair:

1. Remove the damaged core
2. Scarf the surrounding core
3. Bond in a replacement core plug of the same material, cell size, and orientation
4. Repair the face sheets as required

Corroded Core:

Corroded core must be removed and replaced – draining or injecting corrosion inhibitor does not address the structural damage.

Cutting Replacement Core:

• Use sharp tools to avoid crushing the cells
• Cut slightly oversized, then trim for a snug (not forced) fit
• Never use a mallet (will crush the core) or heat (will damage it)

Repair of Composite Materials

Material Matching:

Repairs must use materials that match the original as closely as possible. For example:

• Kevlar panels require Kevlar repair material (fiberglass would not restore impact resistance)
• Interior panels require fire-retardant materials

Prepreg Repairs:

Prepreg materials are pre-impregnated with resin and require careful handling:

• Store at proper temperature (typically frozen)
• Allow to thaw before use
• Protect from moisture absorption (hygroscopic nature)
• Control temperature and humidity in the repair environment

Wet Lay-Up:

• Mix resin and hardener in the manufacturer's specified ratio
• Mix thoroughly in a clean, dry container
• Never mix on the repair surface (can lead to incorrect ratio and contamination)
• If resin begins to cure (gel) before lay-up is complete, start over with fresh materials – the cure cannot be reversed

Bonded Repairs

Adhesive Selection:

• Epoxy – Structural bonding, rigid
• Contact cement – Decorative laminates to wood or metal (interior panels)
• RTV silicone – Flexible, rubber gaskets and seals
• Structural adhesives – For load-bearing bonds

Shelf Life:

Adhesives have a limited shelf life. Expired adhesive must be discarded – testing, extended curing, or other workarounds are not acceptable for critical applications.

Vacuum Bagging:

The vacuum bag applies atmospheric pressure to the repair:

• Consolidates the plies
• Removes trapped air
• Ensures proper adhesion
• Reduces voids

A leaking vacuum bag must be replaced – patching may not be reliable, and increasing pressure could damage the part.

Cure Temperature Monitoring:

• Use multiple thermocouples to monitor temperature across the repair area
• Ensures even heating, no hot or cold spots
• Excessive heat can over-cure, degrade, or burn the resin
• Insufficient heat results in incomplete cure

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5. Radome-Specific Considerations

Radomes protect weather radar antennas while allowing radar signals to pass through. Key considerations:

Electrical Properties

• Radomes must remain transparent to radar frequencies
• Repairs must use materials and techniques that maintain electrical properties
• Improper materials (e.g., metal) or excessive resin can cause radar attenuation or beam distortion

Lightning Protection

Many radomes have:

• Lightning diverter strips – Provide a path for lightning current
• Conductive coatings – Dissipate static charge

After any repair, the conductive coating and diverter strips must be restored to maintain the lightning protection path.

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6. Plastic Windows and Windshields

Inspection

• Check for crazing (tiny surface cracks)
• Check for cracks (can propagate rapidly)
• Check for warping or distortion
• Check for discoloration or chemical attack

Repair vs. Replacement

• Crazing – Window must be replaced (polishing does not restore strength)
• Cracks – Window must be replaced (stop drilling is not an approved repair for plastic windows)
• Warping – Reject the part; do not attempt to flatten or heat (can damage material)

Installation

• Install without undue stress
• Ensure proper fit to prevent stress cracks
• Use proper sealants and fasteners

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7. Wood Structures

Inspection

• Check for glue line separation
• Check for cracks, splits, or delamination
• Check for moisture damage or rot
• Check for insect infestation

Glue Line Separation

A separation along a glue line in a laminated wood propeller is a structural defect that cannot be safely repaired. The propeller is unairworthy and must be replaced.

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8. Environmental Control During Repairs

Temperature

• Prepreg materials require controlled temperature storage
• Cure temperature must be closely monitored
• Heat lamps and heat blankets require thermocouple monitoring

Humidity

• High humidity causes moisture absorption in prepreg materials
• Absorbed moisture turns to steam during elevated-temperature cure
• Steam creates voids and weakens the laminate

Cleanliness

• Repair areas must be clean and dry
• Contamination can cause bond failure
• Use approved solvents for cleaning

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Regulations and Procedures

14 CFR 43.13(a) – Performance Standards

Maintenance must be performed using methods acceptable to the Administrator. This includes:

• Manufacturer's instructions
• AC 43.13-1B (Acceptable Methods, Techniques, and Practices)
• FAA-approved data

14 CFR 43.9 – Maintenance Records

After any maintenance or repair:

• Describe the work performed
• Record the date
• Include signature and certificate number of the person performing the work

14 CFR 43.17 – Airframe Major Repairs

Repairs to primary composite structures must follow:

• FAA-approved methods
• Manufacturer's SRM
• May require FAA Form 337

Allowable Damage Limits (ADL)

• Published in the SRM
• Damage within limits is airworthy
• Document the finding in the logbook
• Repair can be scheduled for a later time

Manufacturer's Instructions

• When a manufacturer specifies a particular repair system, substitutions require approval
• Using a different system without authorization violates approved data requirements
• If the manufacturer's manual lacks a repair procedure, contact the manufacturer for approved data

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Common Relationships and Key Points

Material Selection Relationships

Damage Assessment Flow

1. Identify damage – Visual inspection, tap testing, NDI
2. Compare to allowable limits – SRM or manufacturer's data
3. Within limits – Document, return to service
4. Exceeds limits – Repair per approved data or replace

Repair Decision Factors

• Structural vs. non-structural component
• Primary vs. secondary structure
• Damage size and location
• Manufacturer's instructions vs. AC 43.13-1B
• Material availability and shelf life
• Environmental conditions during repair

Critical Safety Points

• Never use expired adhesives – bond strength cannot be guaranteed
• Never repair crazed or cracked plastic windows – replace them
• Never repair glue line separation in wood propellers – replace the propeller
• Never substitute materials without approval – mechanical properties must match
• Never continue a lay-up after resin has gelled – start over with fresh materials
• Never skip surface preparation – bond strength depends on it
• Never ignore environmental conditions – moisture and temperature affect cure quality

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Summary

Non-metallic structures require specialized knowledge for proper inspection, maintenance, and repair. Understanding material properties, failure modes, and repair techniques is essential for the AME. The key principles are:

1. Follow approved data – Manufacturer's SRM or AC 43.13-1B
2. Match materials – Use the same or equivalent materials as the original
3. Control the environment – Temperature, humidity, and cleanliness
4. Proper surface preparation – The foundation of any bonded repair
5. Document everything – Logbook entries per 14 CFR 43.9
6. Know when to replace – Some damage cannot be repaired

These principles ensure that repairs restore the structural integrity and performance of non-metallic aircraft components, maintaining airworthiness and safety.