Damage you cannot see, in materials that expire

Forty-eight elements — joint largest in Airframe — and only four need a bench. That gap is the first thing to understand about this unit.

1. Seventeen elements you will never perform

K1–K6 are wood. K7–K17 are fabric covering. Seventeen of twenty-nine knowledge elements, and not one of the twelve skills touches either.

The single place fabric appears in a skill is S6 — locate and explain the procedures for tying a modified seine knot — and it is a Locate: documentation satisfies it.

This is deliberate, and it is not unique to this unit. The same pattern holds for welding in AF-A Concepts §1, for glass cockpits in AF-H, and for topcoat spraying in GEN-G Concepts §8. The ACS asks a mechanic to know technologies the certificate does not require them to do.

That is coherent as a standard. You may be handed a fabric-covered airplane for an inspection and you must not be ignorant of how it is built, what its defects look like, or what its repair demands. You are not being certified to recover one.

So read this material for recognition and judgment: what wood defects are unacceptable, what moisture does to both wood and fabric, where covering deteriorates first, and — most usefully — when the answer is "this goes to someone who does this work."

2. Composite damage has no perceptual signature

K18, K19, S3, S8 in GEN-K. This is why the unit exists in its modern form.

Metal tells you it is damaged. It dents, cracks visibly, tears, corrodes with a color and a texture. You can see it, and often you can see the extent.

Composite frequently does not. The failure modes that matter most — disbond between skin and core, and delamination between plies — happen inside the laminate. The surface can be undamaged, unmarked and cosmetically perfect over a structure that has lost most of its strength.

An impact that leaves no visible mark can leave substantial internal damage. This is why:

• tap testing exists (GEN-K Procedures Procedure 4) — it listens for the change in response that a disbond produces;
• any impact should be investigated, not judged by what the surface looks like;
• extent is harder to bound than in metal, and S11's repairability judgment is correspondingly harder.

The habit: on composite, treat the absence of visible damage as unproven rather than as evidence.

3. Materials that expire, and systems you may not mix

K21 storage and shelf life. R4 storage. R7 shelf life. R6 use of materials that are not part of an approved repair system.

Resin systems have a shelf life and it is not advisory. Expired resin cures to the wrong properties — lower strength, incomplete cure, poor adhesion — and the finished repair looks exactly like a good one. You cannot inspect your way to knowing.

Some materials also require cold storage, and their life is counted differently in and out of the freezer. A prepreg left out overnight may have consumed a large part of its remaining life.

R6 is the wider point: a composite repair is a system. The resin, the hardener, the fabric, the adhesive and the cure schedule were qualified together. Substituting one component — a different hardener, a similar-looking cloth, a resin from another manufacturer — produces something nobody has tested, and the repair scheme in the SRM no longer describes what you built.

This puts composite materials in the same group as fire bottles and ELT batteries (AF-M §2): time-based airworthiness that no inspection reveals.

4. Mixing is measurement, not cooking

R5, measuring and mixing materials for composite construction.

Two-part systems are specified by ratio, and the ratio is by weight or by volume — never assume which (GEN-H Concepts §3). Getting it wrong does not produce something that obviously fails; it produces something that cures soft, or brittle, or not at all in the middle.

Three things that catch people:

• Pot life starts when the parts meet. Mixed resin is a clock, and a large batch generates its own heat, which shortens the clock further.
• Mix thoroughly, including the sides and bottom of the pot. Unmixed hardener leaves uncured resin.
• Temperature and humidity affect cure, which is GEN-G Concepts §5's point in a different material.

5. R3 — the exposure is real

Exposure to materials used in composite repair.

Uncured resins and hardeners are sensitizers: repeated skin exposure can produce a permanent allergic response, after which you cannot work with the material at all. This is a career-ending outcome that arrives quietly and does not reverse.

Dust from sanding cured composite is a respiratory hazard, and carbon fiber dust is also electrically conductive — it gets into equipment and causes faults.

Gloves, respiratory protection appropriate to the material, ventilation, and containment of dust. K28 and the safety data sheet govern (the equipment and materials specification §B4).

6. S11 — repairability, harder than in metal

Same judgment as AF-A Concepts §5, with an extra difficulty: you are assessing the extent of damage you cannot directly see.

The sequence:

1. Map the extent by tap test, and by whatever other method the SRM specifies.
2. Establish what is damaged — skin only, skin and core, or through-structure. The repair differs completely.
3. Check the SRM's damage limits for that structure and location.
4. Check whether a repair is permitted there — repairs near edges, fittings, or other repairs are often restricted.
5. Confirm the repair system is available and in date (§3).

Check step 5 before starting, not after preparing the damage. A repair opened up and then stopped for want of in-date material is worse than one not started.

7. Windows are a material with its own rules

K26, K27, S4, S5. Transparent plastics are easily damaged by the things used to clean everything else.

• Crazing — a network of fine cracks — is caused by solvents and by stress, and it is permanent.
• Clean with the approved product and a clean soft cloth, flooding rather than rubbing, so grit does not scratch.
• Never use a solvent that is not approved for the material — GEN-G Concepts §4.
• Temporary repairs to side windows exist and are limited (S5, a Locate).

8. What consumes this unit

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