3.4 Wooden Structures: Construction, Preservation & Repair

What 6.3.2 Actually Requires

Sub-module 6.3.2 Wooden structures is a level 1 requirement for A and B1/B3 and is not applicable to B2. Level 1 means you must be able to give a general description and recognise the terminology — but the syllabus lists six distinct bullets, and questions are drawn from all of them:

1. Construction methods of wooden airframe structures
2. Characteristics, properties and types of wood and glue used in aeroplanes
3. Preservation and maintenance of wooden structures
4. Types of defects in wood material and wooden structures
5. Detection of defects in wooden structures
6. Repair of wooden structures

Section 3.3 covered bullet 2 (species selection, grain slope, adhesives). This section covers the other five. The authoritative industry reference is FAA AC 43.13-1B, Chapter 1, which European authorities accept where the aircraft's own maintenance manual is silent.

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Construction Methods

Wooden airframes are almost entirely wings and control surfaces built around spars, with a plywood or fabric covering.

Spars

Ribs, Skins and Joints

• Ribs are either truss type (capstrips with diagonal bracing and gussets) or plywood-web type.
• Gussets are thin plywood plates glued across joints to transfer load between members; they are the wooden equivalent of a splice plate.
• Plywood skin on a monocoque or semi-monocoque fuselage carries shear and torsion as a stressed skin.
• Compression struts and drag/anti-drag wires triangulate the wing internally between spars.

The critical structural point is that the glue joint, not the wood, is the designed load path in most joints. A wooden aircraft is a glued structure with wood fillers.

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Adhesives and Bonding

• Casein — the earliest aircraft glue. Moisture-sensitive, supports fungal attack, and deteriorates in service; found only on legacy aircraft and normally replaced when a structure is rebuilt.
• Resorcinol-formaldehyde — the classic dark-red waterproof aircraft glue; excellent durability but requires close-fitting joints and clamping pressure.
• Phenol-formaldehyde — hot-pressed, used in the manufacture of aircraft plywood rather than in field repair.
• Urea-formaldehyde (plastic resin) — has been used, but deteriorates in hot, moist environments and is not preferred for structural repair.
• Epoxy — modern, gap-tolerant, used under approved data.

Bonding requires controlled moisture content, temperature, open and closed assembly time and clamping pressure. Aircraft wood is bought kiln-dried to roughly 10–17 % moisture content under the classic AN-W-2 specification, and glue manufacturers typically require the wood to be at 8–12 % at the moment of bonding. A starved joint — too much clamping pressure or too-thin glue — is as dangerous as no glue at all, because it looks perfect.

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Preservation and Maintenance

• Keep the aircraft hangared, dry and well ventilated. Prolonged damp is the single biggest killer of wooden structure.
• The finish coat (usually varnish) is the last line of defence against water entry. Finish failure results from prolonged water exposure, wood splitting, ultraviolet exposure or surface abrasion — and finish failure precedes decay.
• Keep drain holes open at the lowest point of every enclosed bay, and keep inspection panels and access covers serviceable.
• Protect end grain, which absorbs moisture many times faster than side grain.
• Watch for shrinkage effects: as a member dries it shrinks tangentially most, radially less, and negligibly along the grain. This loosens fittings and wire bracing and splits members. Bushings are deliberately made slightly short so that when the wood shrinks they do not protrude and hold the fitting off the wood.
• Do not over-tighten fittings — crushing the underlying wood is a recognised damage mechanism.

Types of Defect

Defects in the material (rejection criteria)

Defects in service

Decay is fungal. Fungi need the wood's moisture content to reach 20 % or greater to grow. Decayed wood shows softness, swelling while wet, excessive shrinkage when dry, cracking and discoloration. Repair or replace the wood if any amount or form of decay is found — there is no "acceptable" decay.

Splitting follows grain lines and is driven by drying shrinkage; glue-joint failure shows as a dark line or a visible gap; impact and overload damage comes from heavy landings or aerodynamic overload; fitting crush comes from over-torqued bolts.

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Detecting Defects

Wooden structure is inspected mostly through access panels, so preparation matters more than instrumentation:

• Keep the aircraft in a dry, well-ventilated hangar with all inspection covers and access panels removed for as long as possible before the final inspection, and take a moisture-meter reading at the preliminary inspection. If the moisture content is high, dry the aircraft thoroughly first — a wet structure hides the condition of its bonded joints.
• Visual inspection with a strong light and an inspection mirror through the openings; look for dark stains around fittings (a classic sign of water and glue failure), varnish cracking and rust streaks from fittings.
• Tap test with a light plastic mallet or coin: a sharp ring indicates sound bonded wood; a dull or dead sound indicates decay or a failed glue line.
• Probing with a blunt awl at suspect areas; sound wood resists, decayed wood is soft and stringy.
• Smell — a musty odour in a closed bay is a decay indicator.
• Moisture meter readings above about 20 % mean conditions are right for fungal attack.
• Radiography for internal decay in a spar that cannot be opened.

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Repairing Wooden Structure

The scarf joint

The scarf joint is the most satisfactory method of making an end joint between two solid wood members. It works by converting an end-grain butt (which glue cannot hold) into a long, shallow, side-grain overlap.

• Reinforcement plates must be used on all scarf repairs to spars, and the scarf slopes shown in the AC's figures are minimum slopes — never steeper.
• Reinforcement plates are feathered off at a 5:1 slope at their ends.
• For plywood, the scarf slope of 1 in 12 is the steepest slope permitted for all kinds of plywood.
• Cut the scarf in the general direction of the grain slope, and match the grain direction of any replacement plywood web or reinforcement plate to the original.

Splice location rules

• A splice may not be made under wing attachment fittings, landing-gear fittings, engine-mount fittings, or lift and interplane strut fittings, and these fittings may not overlap any part of the splice.
• A spar splice must not be made adjacent to a previous splice or to a reinforcing plate. Spacing between two splices, or between a splice and a reinforcing plate, must be at least three times the length of the longer splice.
• Splicing under minor fittings (drag wire, anti-drag wire, compression strut) is acceptable subject to the plate not interfering with attachment or alignment.

Other repairs

• Longitudinal spar cracks and local damage (except in box spars) may be repaired by bonding spruce or plywood plates of sufficient thickness to develop the longitudinal shear on both sides of the spar, extending well beyond the ends of the cracks.
• Box spar webs are always spliced and reinforced with the same type of plywood as the original; solid wood must never replace a plywood web, and the face grain of replacement webs and reinforcement plates must run in the same direction as the original.
• Plywood skin damage is repaired by surface (overlay) patch, scarf patch, or plug patch. The scarf patch is the best repair for damaged plywood skin; the surface patch is quicker but aerodynamically inferior.
• Spar replacement is a major repair, and a solid spar may be replaced by a laminated spar (or the reverse) provided the material quality matches and external reinforcements are reproduced as on the original.

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Exam Traps

• 1:15 is the maximum grain slope for the material. 1:12 is the steepest plywood scarf slope. 5:1 is the reinforcement-plate feather. 3× is the minimum splice spacing multiplier. Four different numbers — do not blend them.
• Fungal decay needs 20 % moisture content; the bonding range is 8–12 %. A question about decay wants 20 %.
• Any decay condemns the piece — there is no allowable depth.
• Compression wood is a growth defect; compression failures are buckled fibres from overload or rough handling. Both are rejections but they are different defects.
• Plywood is stronger in shear than solid wood of the same thickness — this is why webs are plywood and why solid wood may never substitute for a plywood web.