Wood, fabric and welding

Many older and homebuilt aircraft are wood, tube and fabric, and an A&P is expected to know how to inspect and repair them.

Wood

The reference wood is Sitka spruce; other woods can substitute if they match its strength. Wood for aircraft must be straight grained (the grain may slope no more than 1 in 15), with no compression wood, decay or large knots. Inspect for dry rot and failed glue joints by probing with a sharp tool and tapping, especially in low spots where water sits; check moisture, and that glue joints aren't separating. Modern repairs use resorcinol or epoxy glues, following the manufacturer's or AC 43.13's methods.

Fabric

Cotton and linen have given way to polyester fabric, applied under an FAA-approved covering system (an STC) with its own glues, fillers and finishes. Follow that system's manual exactly and don't mix products from different systems: the approval covers only the system as written. Polyester is glued on, then shrunk tight with an iron at set temperatures, stitched to the ribs where needed (rib stitching), and finished with coats that protect it from sunlight. Old fabric is checked for strength with a punch tester and replaced if it's lost too much.

Welding

• Oxy-acetylene: a neutral flame (equal oxygen and acetylene, a clean inner cone) for most steel. Too much acetylene gives a carburizing flame; too much oxygen an oxidizing one. Acetylene becomes unstable above 15 psi, so it's never used higher.
• TIG (gas tungsten arc welding): an arc from a tungsten electrode, shielded by argon or helium. Used for 4130 steel tube, stainless steel, aluminum and magnesium.
• Welded repairs to 4130 tube follow the manufacturer or AC 43.13; heat-treated parts generally can't be welded without being heat treated again.