Inspecting and repairing non-metallic structure

Six procedures.

There is no tutorial component for this unit, and that is deliberate. Wet lay-up produces vapor and dust that need ventilation and containment, and AF-B Concepts §5's sensitization risk is not something to meet unsupervised at a kitchen table. Composite lay-up is learned in the shop, at the ventilated bench, with an instructor, in the Airframe in-person course. Riveting (AF-A Hands-on) is also course work in the shop, though it is safe to practice at home; flaring (GEN-D Hands-on) is worked at home in the General bench kit.

Procedure 1 — Establish the material and the system (before anything)

1. Identify the structure: solid laminate, sandwich with honeycomb or foam core, thermoplastic, transparency, or wood or fabric — in which case see AF-B Concepts §1.
2. Identify the repair system the SRM specifies — resin, hardener, fabric, adhesive, cure schedule, as a matched set.
3. Check the material is in date and correctly stored (AF-B Concepts §3).
4. Read the safety data sheet and set up ventilation, PPE and containment.

Check — name the repair system and its expiry. If either is unknown, you do not have an approved repair, you have materials.

Failure — substituting a component of the system because it looks equivalent. R6.

Procedure 2 — Inspect composite structure (S3, S1)

1. Visual first: surface cracks, dents, punctures, delamination visible at edges, resin starvation, discoloration from heat, and any sign of impact.
2. Tap test the area and beyond it (GEN-K Procedures Procedure 4), establishing the sound of good structure first.
3. Map the boundary of any change in response and mark it.
4. Check for moisture ingress — a sandwich structure that has taken water is heavier, and it will damage itself in a freeze.
5. Inspect fasteners in composite (S1) for elongation, pull-through and correct type.
6. Confirm by the SRM's specified method where required.

Check — an impact with no visible damage still needs tap testing. The absence of a mark is not evidence of the absence of damage (AF-B Concepts §2).

Failure — inspecting only the visible dent. Internal damage typically extends well beyond it.

Procedure 3 — Clean and inspect acrylic transparencies (S4)

1. Flood the surface with clean water first to float grit off.
2. Clean with the approved product and a clean, soft, dedicated cloth — never a shop rag, which carries grit and solvent.
3. Wipe in straight lines, not circles, with no pressure.
4. Inspect for crazing, scratches, delamination of laminated panels, and edge condition.
5. Assess against the SRM's limits — position matters, since damage in the pilot's field of view is judged more strictly.

Check — look through the window from the pilot's seat, not from outside. That is where its serviceability is decided.

Failure — cleaning with a solvent that is not approved. Crazing is permanent and the panel is scrap (AF-B Concepts §7).

Procedure 4 — Prepare a composite surface for painting (S7)

Cross-referenced to GEN-G Procedures Procedure 6 — the same task, taught there.

The composite-specific points: abrade by hand and lightly, only to key the surface; do not expose fiber; use only strippers and solvents approved for the laminate.

Check — shiny fiber means you have gone through the resin-rich surface layer. That is damage, not preparation.

Procedure 5 — Perform a lay-up repair (S12)

The station element, performed at the ventilated bench.

1. Remove damage to the boundary established in Procedure 2, to the SRM's method.
2. Taper or step the edges as the scheme specifies — the overlap is what transfers load, and a butt joint transfers nothing.
3. Clean and dry. Moisture in the laminate will steam during cure and ruin the repair.
4. Cut plies to the specified size, orientation and sequence. Fiber orientation is structural, and a ply laid at the wrong angle is not the ply the scheme called for.
5. Mix resin by the specified ratio and method (AF-B Concepts §4). Note the pot life.
6. Wet out and lay up in the specified sequence.
7. Bag it: peel ply, bleeder, breather, bag and sealant tape as the scheme specifies.
8. Draw vacuum and check for leaks — hold the vacuum with the pump isolated and watch the gauge.
9. Cure to the specified schedule, monitoring temperature.
10. Debag, inspect, and tap test the repair.

Check — the vacuum must hold with the pump off. A bag that leaks does not consolidate the laminate, and a repair cured without consolidation is porous and weak. Check the bag before committing the resin, because after mixing you are on the clock.

Failure — plies laid in the wrong orientation. Invisible once cured, and the repair does not carry the load it was designed for.

Procedure 6 — Determine repairability (S11)

1. Map the extent (Procedure 2).
2. Establish what is damaged — skin, core, or through-structure.
3. Compare against the SRM's damage limits for that location.
4. Check location restrictions — edges, fittings, control surfaces, proximity to other repairs.
5. Confirm materials are available and in date (AF-B Concepts §3).
6. Decide, and record the reasoning.

Check — do step 5 before opening the damage. A repair started and then halted for want of in-date material leaves the structure worse than it was found.

The checks, collected

License

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