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.
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.
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.
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.
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:
88.Clean with approved solvent to remove contaminants
89.Abrade (sand) the surface to create a mechanical bond
90.Clean again to remove sanding residue
91.Feather the edges of damaged areas to create a scarf for repair plies
For metal surfaces:
93.Clean to remove all contaminants
94.Abrade (roughen) the surface
95.Clean again
96.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:
101.Drill a small hole at the center of the delamination
102.Inject epoxy resin to re-bond the layers
103.Allow to cure
104.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:
107.Remove damaged material
108.Feather the edges to create a scarf
109.Clean the area thoroughly
110.Apply repair plies (matching original material)
111.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:
123.Remove the damaged core
124.Scarf the surrounding core
125.Bond in a replacement core plug of the same material, cell size, and orientation
126.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
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.
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
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.
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
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
Common Relationships and Key Points
Material Selection Relationships
Application
Material
Key Property
Radome
Fiberglass
Radar transparency
Interior panels
Kevlar
Impact resistance, fire retardance
Primary structure
Carbon fiber
High strength-to-weight
Windows
Acrylic/Polycarbonate
Optical clarity, impact resistance
Propeller blades
Laminated wood
Strength, vibration damping
Damage Assessment Flow
251.Identify damage – Visual inspection, tap testing, NDI
252.Compare to allowable limits – SRM or manufacturer's data
253.Within limits – Document, return to service
254.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
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:
273.Follow approved data – Manufacturer's SRM or AC 43.13-1B
274.Match materials – Use the same or equivalent materials as the original
275.Control the environment – Temperature, humidity, and cleanliness
276.Proper surface preparation – The foundation of any bonded repair
277.Document everything – Logbook entries per 14 CFR 43.9
278.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.
