📖 The lesson
Wooden structures
Wood offers a good strength-to-weight ratio, is easy to work and repair, and absorbs vibration. It is still found in gliders, vintage and some light aircraft (spars, ribs, plywood skins). Its weaknesses are moisture absorption, decay and variable quality.
- Sitka spruce is the reference aircraft wood. Approved substitutes include Douglas fir, Noble fir and Western hemlock, with adjusted dimensions where their properties differ.
- Plywood (often mahogany or birch) — odd numbers of thin veneers glued with grain at right angles — gives skins, gussets and webs strength in two directions.
- Laminated wood — several layers glued with the grain parallel — is used for spars and curved members.
- Wood must be properly seasoned; repairs are made at a moisture content of roughly 8–12%.
Adhesives
Resorcinol-formaldehyde (waterproof, cures at room temperature) and epoxy are used for repairs; phenol-formaldehyde for factory-made plywood. Older casein and urea-formaldehyde glues deteriorate with age and moisture — joints made with them must be inspected especially carefully. Follow the glue's mixing, open-assembly and clamping times and temperatures.
Inspection of wood structures
- Look for moisture and decay: dark stains, softness, musty smell. Probe suspect wood with a sharp tool — decayed wood is soft and the probe sinks in.
- Check glue joints for separation (a thin feeler blade must not enter); look along trailing edges and at rib-to-spar joints.
- Check for crushing under fittings and bolt heads, elongated bolt holes, cracks and compression failures (fine wrinkles across the grain).
- Tap testing, opening inspection panels and drain holes, and checking the protective finish (varnish) complete the picture.
Repairs
- Spars are spliced with long, shallow scarf joints (typically 1 in 10 to 1 in 12), sometimes with reinforcing plates; splices are not permitted in certain areas (under fittings, where the manufacturer prohibits).
- Plywood skins are repaired with splayed patches, surface patches or plug patches depending on size.
- All repair wood must meet the same species, grade and moisture requirements, and repairs are refinished to keep moisture out.
Fabric covering
- Cotton (Grade A) and linen — the traditional organic fabrics, tautened with dope. They rot and lose strength with age and UV.
- Polyester (Dacron-type, Ceconite, Poly-Fiber) — the modern standard: strong, does not rot, and is shrunk taut with a calibrated iron rather than by dope.
- Glass fabric — very durable, used with specific systems.
- Dopes: nitrate dope gives good adhesion and penetration but is highly flammable; butyrate dope is more fire-resistant and tautens more. Polyester systems use non-tautening coatings.
- Ultraviolet light rapidly degrades fabric, so aluminium-pigmented coats are applied under the colour coats.
- Attachment: fabric is cemented at edges and secured to ribs by rib stitching (rib lacing), screws, rivets or clips at the manufacturer's spacing; reinforcing tape goes under the stitches, surface tape (pinked edges) over stitches, seams and edges. Drain grommets and inspection rings are fitted.
- Each covering system is an approved package (often an STC): use only its listed products and follow its manual.
Testing and repair of fabric
- Fabric is airworthy only while it keeps at least 70% of the strength required of new fabric. Punch testers (for example Maule or Seyboth) give a field indication on the doped surface; a laboratory strip tensile test is definitive.
- Look for UV-rotted or brittle areas, cracked dope, tears, chafing at structure, loose tapes, broken rib stitching and mildew.
- Small tears are sewn (baseball stitch) and covered with a doped-on patch; larger damage is repaired with a sewn-in patch or by replacing the panel between structural members, following the system's manual or AC 43.13-1B.