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

Composite Construction: Fiber + Matrix

A composite combines a fiber reinforcement that carries the load with a matrix (resin) that bonds the fibers, transfers load between them, and holds the shape. The fibers do the work, so ply orientation matters — strength runs along the fibers, and a repair must duplicate the original number of plies and their orientation. Common reinforcements:

The usual matrix is epoxy resin (sometimes polyester or phenolic; high-temp areas use thermoplastics like PEEK). Resins are temperature- and humidity-sensitive: a prepreg (pre-impregnated cloth) has a freezer shelf life and out-time that must be tracked, and wet lay-up resin must be mixed at the exact ratio or it will not cure to strength.

Honeycomb Sandwich and Hidden Damage

Many control surfaces, floors, and panels are honeycomb sandwich: two thin face skins bonded to a lightweight core (aluminum, Nomex/aramid, or foam). This gives huge stiffness for the weight, but it creates the biggest inspection trap in composites — damage can be invisible from the surface. An impact may leave a barely visible dent while the core is crushed and the skin is delaminated or disbonded underneath. Water intrusion into the core is especially serious: it adds weight, breaks the skin-to-core bond, and freezes/expands at altitude.

Inspection methods escalate with suspicion:

• Visual — look for dents, blisters, resin starvation, paint cracks, fluid stains.
• Coin (tap) test — tap with a light coin or tap hammer; a sharp ring = sound bond, a dull thud = disbond/delamination/water. Good for thin-skin areas only.
• Ultrasonic / thermography / radiography — map subsurface damage and trapped water when the tap test or impact history warrants it.

After any impact, overheat, or lightning strike, the technician must treat the area as suspect until NDI clears it — appearance is not enough.

Composite Repair Discipline and Plastics/Windows

A structural composite repair restores the load path by replacing lost plies. Key controls:

1. Remove damage to sound material (scarf or stepped taper, typically a wide taper ratio so load feeds gradually into the repair).
2. Surface prep — clean, dry, abrade; carbon needs care to avoid galvanic issues at metal interfaces.
3. Ply layup — match fiber type, ply count, and orientation; stagger ply edges; respect overlap.
4. Cure — control temperature, time, and pressure (vacuum bag/heat blanket or autoclave) per the data; under-cure means low strength, over-temp damages resin.
5. Post-repair — inspect (tap/NDI), and rebalance control surfaces if weight changed.

Transparent plastics (windshields, windows) are usually acrylic (Plexiglas) or stretched acrylic/polycarbonate. Inspect for crazing (fine surface cracks), scratches, and crack growth; stop-drill crack tips only per data; clean with approved cleaners and a clean cloth (never dry-wipe or use solvents that craze acrylic). Wear and chemical/dust hazards matter: composite dust and resins are respiratory and skin irritants, so use PPE and dust control. Reject any unapproved repair system — a strong-looking patch built with the wrong resin or cure is not airworthy.

Wood and Fabric (Legacy)

Legacy structures still appear on the test. Wood structure (spars, ribs) is inspected for decay/dry rot, compression failures across the grain, glue-joint separation, splits, and moisture. Approved aircraft woods (e.g., Sitka spruce and substitutes) must meet grain-slope and defect limits; glue joints must be tight and the right adhesive. Probe suspect soft spots; a screwdriver that sinks into a spar signals rot.

Fabric covering (cotton, linen, or modern polyester such as Ceconite/Dacron) is judged mainly by strength:

• Tested with a punch tester (or pull test) at suspect, sun-exposed areas.
• Fabric is unairworthy when it falls below 70% of the strength required for a new covering of that aircraft (for example, on an aircraft requiring ~56 lb/in new fabric, 70% ≈ 46 lb/in is the floor — verify against the actual approved value).
• Inspect for deteriorated finish (dope/UV protection), tears, loose rib lacing, and ringworm/ringwork separation.

The common thread across all non-metallic materials: the material dictates the method, hidden damage is the real danger, and only approved materials and procedures return the part to service.

Resin Handling, Cure Control, and Common Traps

Resin chemistry quietly drives several exam questions. Two-part epoxy must be mixed at the manufacturer's exact ratio by weight or volume; too much or too little hardener yields a soft, weak, or brittle cure that looks fine but has low strength. Prepreg materials are stored frozen and have a tracked shelf life and out-time (the cumulative time at room temperature); exceeding either means the resin may not fully cure, so the material is scrapped.

During cure the temperature, ramp rate, dwell time, and pressure all matter — a vacuum bag with a heat blanket and thermocouples is common in the field, and under-curing (too cool or too short) gives low strength while over-temperature degrades the matrix. The technician verifies the cure (often a separate witness coupon) before declaring the repair sound.

A few traps recur. Carbon fiber is electrically conductive, so a carbon part in contact with aluminum sets up galvanic corrosion of the aluminum unless an isolation layer (fiberglass ply or sealant) is used; this also affects lightning protection and bonding/grounding. Aramid (Kevlar) absorbs water and frays when cut, so edges must be sealed and special shears used. After any composite repair that adds or removes material on a control surface, the surface's mass balance must be re-checked because added resin or plies shift the CG.

Finally, an unapproved repair scheme is never airworthy no matter how strong it appears: a wet lay-up substituted where a prepreg co-cure is required, or the wrong resin system, fails the data test. Treating composite work as a controlled-process problem — right material, right ratio, right orientation, right cure, verified and documented — is the judgment the ACS rewards.