5.3 Non-Metallic Structures: Composites, Wood, Fabric, and Plastics

Non-Metallic Structures: Hidden Damage and Material Control

Non-metallic structure is a broad Airframe ACS subject. It includes wood, fabric coverings, composite laminates, thermoplastics, acrylic windows, restraints, and upholstery. The common test theme is that each material fails and repairs differently. A method that is reasonable on aluminum may be harmful on composite, fabric, or acrylic.

Composite structures are often made from fibers, matrix resin, and sometimes a core such as honeycomb or foam. The fibers carry much of the load, the matrix supports and protects the fibers, and the core helps create stiffness with low weight. Damage can include delamination, disbonding, crushed core, punctures, resin cracks, burns, moisture intrusion, and broken fibers.

A tap test can help locate a delamination or disbond, but it is not a magic answer for every structure. The technician must know the material, access, reference standard, and inspection limit. Advanced non-destructive inspection may be required for critical or hidden damage. Impact damage that looks minor on the surface can spread inside a laminate.

Composite repair depends heavily on process control:

• Confirm the repair is allowed by the manufacturer's repair manual or other approved data.
• Check material identity, storage condition, and shelf life.
• Protect yourself from fibers, sanding dust, solvents, and resin systems.
• Prepare the surface without damaging sound plies.
• Maintain ply orientation, overlap dimensions, resin mix, vacuum bagging, pressure, and cure temperature.
• Inspect the repair for voids, contamination, bond quality, and finish requirements.

Wood and fabric require a different eye. Wood defects include decay, crushing, checks, splits, compression failures, poor glue joints, and moisture damage. Fabric defects include ultraviolet deterioration, loose tapes, tears, weak coatings, and damage near high-wear areas. Humidity matters because wood and fabric respond to moisture. The right adhesive, covering material, and approved method are central to an acceptable repair.

Acrylic windows and transparent plastics are vulnerable to crazing, scratches, chemical attack, and stress from improper drilling or fastener torque. Cleaning products must be approved for the material. A temporary side-window repair may be allowed by data, but it is not the same as a permanent structural repair. Thermoplastics add storage, handling, forming, and installation concerns.

The exam often asks for the safest choice when materials are uncertain. The strongest answer rejects improvisation. Do not mix resin by eye, use expired materials, substitute adhesives, or sand into sound fibers just to make the surface look smooth. Do not use a household cleaner on a windshield because it seems gentle. Do not accept fabric by appearance when approved inspection methods require strength or condition checks.

Good maintenance judgment for non-metallic structure is process discipline. Identify the material, inspect for hidden damage, protect personnel from exposure, use the approved repair system, and stop when the defect exceeds published limits.