3.3 Aircraft Wood Structures: Defects, Allowable Repairs & Gluing

Aircraft Wood Structures: Defects, Allowable Repairs & Gluing

FAA Airframe Mechanic Standard: Wood was the primary structural material in early aviation and remains widely used in aerobatic aircraft, gliders, homebuilt aircraft, and vintage restorations. Maintenance technicians must evaluate wood structural integrity, identify defects, prepare precision glued joints, and execute FAA-approved spar splices in accordance with 14 CFR Part 43 and AC 43.13-1B Chapter 1.

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1. Sitka Spruce: The Aircraft Structural Baseline

Under FAA airworthiness standards, Sitka Spruce (Picea sitchensis) is the gold standard species against which all other structural woods are compared and rated.

+-------------------------------------------------------------------------+
|                 SITKA SPRUCE STRUCTURAL BASELINE VALUES                 |
+-----------------------+------------------------+------------------------+
| Specific Gravity      | Moisture Content       | Slope of Grain         |
| - Min: 0.36           | - Allowable: 8% to 12% | - Max allowable: 1:15  |
| - Standard: 0.40      | - Optimum: 10% to 12%  | - Preferred: 1:20      |
|   (at 12% moisture)   | - Fungal rot > 15%     |   (primary spars)      |
+-----------------------+------------------------+------------------------+

Standard Physical Specifications (at 12% Moisture Content)

1. Specific Gravity: Minimum acceptable is 0.36; nominal standard is 0.40 (based on oven-dry weight and volume at $12%$ moisture content).
2. Moisture Content: Permissible range is $8%\text{ to }12%$. Moisture levels exceeding $15%$ sustain fungal growth and wood rot; moisture levels below $6%$ cause excessive fiber shrinkage and brittleness.
3. Annual Growth Rings: Minimum 6 rings per inch (measured radially along a cross section); 8 to 12 rings per inch is preferred for primary spar caps.
4. Slope of Grain: Maximum allowable grain slope for primary structural members (spars, spar flanges) is 1:15 (a $1\text{ inch}$ deviation over a length of $15\text{ inches}$). Preferred slope is 1:20.

Permissible Substitute Species

When Sitka spruce is unavailable, substitute species may be used provided structural dimensions are adjusted per AC 43.13-1B Table 1-1:

• Douglas Fir: $25%$ stronger and stiffer than spruce, but heavier ($1.25\times$ spruce weight). Harder to work and prone to splitting.
• Noble Fir: Slightly exceeds spruce in strength; direct 1:1 dimensional substitute.
• Western Hemlock: Equal or superior in strength to spruce; direct substitute if carefully inspected for shake.
• White Spruce: Slightly lower strength; requires a small increase in spar beam thickness.

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2. Classification of Wood Defects

Wood defects are categorized into unacceptable defects (which demand mandatory scrap or replacement) and allowable defects (permitted within strict dimensional limitations).

+-------------------------------------------------------------------------+
|                         WOOD DEFECT EVALUATION                          |
+------------------------------------+------------------------------------+
| UNACCEPTABLE (Mandatory Reject)    | ALLOWABLE (Within AC 43.13 Limits) |
+------------------------------------+------------------------------------+
| - Decay / Dry Rot / Dote           | - Pin knots (scattered, <= 1/4")   |
| - Compression Failures / Wood      | - Sound hard knots (<= 3/8" in     |
| - Shake / Checks / Splits          |   middle 1/3 of spar width)        |
| - Grain slope steeper than 1:15    | - Pitch pockets (isolated)         |
| - Spike knots & knot clusters      | - Mineral streaks (no decay)       |
+------------------------------------+------------------------------------+

A. Unacceptable Defects (Mandatory Rejection)

• Decay, Rot, or Dote: Any presence of fungal decay, crumbly dry rot (dote), or soft rot is cause for immediate, unconditional rejection of the piece. Fungal hyphae destroy lignin and cellulose cell walls, reducing structural strength to near zero.
• Compression Wood: High-density, reddish-brown wood formed on the lower side of leaning coniferous trees. It exhibits high longitudinal shrinkage, excessive warp, and brittle fracture characteristics.
• Compression Failures: Buckling of wood fibers caused by severe natural forces (heavy snow/wind bending live trees) or violent impact (felling across logs, flight overstress). Indicated by minute transverse hairline cracks perpendicular to the grain. Any compression failure in a structural member requires complete rejection.
• Shake: Longitudinal separation of wood fibers along annual growth rings. Completely destroys shear strength.
• Spike Knots & Knot Clusters: Knots running across the width or grouped closely together create severe stress concentrations.

B. Allowable Defects (AC 43.13-1B Standard Limits)

• Sound Hard Knots: Allowed up to $3/8\text{ inch}$ ($9.5\text{ mm}$) maximum diameter, provided they are confined to the middle third of the spar height, are located at least $1\text{ inch}$ from edges, and are separated by at least $20\text{ inches}$ from any other knot.
• Pin Knots: Small sound knots up to $1/4\text{ inch}$ ($6.4\text{ mm}$) diameter are allowed if well-scattered and causing minimal grain deviation.
• Pitch Pockets: Allowed in the middle third of the spar if at least $14\text{ inches}$ apart, length $\le 1.5\text{ inches}$, and depth $\le 1/8\text{ inch}$.
• Mineral Streaks: Dark mineral discolourations are acceptable provided careful microscopic/knife testing confirms no fungal decay or soft fibers.

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3. Structural Glues, Working Times & Clamping Pressures

In wood aircraft construction, the adhesive joint must be stronger than the wood itself. A properly executed glue joint tested to destruction will exhibit $100%$ wood fiber shear failure rather than adhesive bond line separation.

A. Approved vs. Obsolete Adhesives

• Resorcinol-Formaldehyde & Phenol-Resorcinol: The traditional FAA-approved synthetic resin adhesive for structural aircraft wood. Two-part system (liquid resin + powder hardener) with a distinctive dark reddish-purple/brown bond line. Fully waterproof, fungus-proof, and impervious to temperature extremes. Requires a minimum shop curing temperature of $70^\circ\text{F}$ ($21^\circ\text{C}$).
• Aerospace Structural Epoxies (e.g., T-88, West System): 100% solids, neutral pH epoxies specifically formulated for wood. Completely waterproof, excellent gap-filling capabilities, and cures reliably at room temperature ($60^\circ\text{F}\text{ to }70^\circ\text{F}$) under low clamping pressure.
• Casein Glues (OBSOLETE): Derived from milk curd proteins. Casein glue is water-soluble, supports fungal growth, and deteriorates rapidly when exposed to humidity. Casein is obsolete and no longer approved for outdoor or primary structural aircraft repairs.

B. Assembly Time Definitions

• Open Assembly Time: Time elapsed between applying glue to the wood surfaces and bringing the mating surfaces into contact.
• Closed Assembly Time: Time elapsed between mating the glued surfaces together and applying full clamping pressure.
• Rule: Exceeding allowable assembly times causes the glue to dry or "skin over," resulting in a weak, defective joint.

C. Clamping Pressure Standards

Proper clamping pressure forces air out, creates a microscopic glue film ($0.002\text{ to }0.005\text{ in}$ thick), and drives adhesive into the cellular pore structure of the wood: $\text{Softwoods (Spruce, Fir, Hemlock): } \mathbf{125\text{ to }150\text{ psi}}$ $\text{Hardwoods (Birch, Ash, Maple, Oak): } \mathbf{150\text{ to }200\text{ psi}}$

• Insufficient Pressure ($< 125\text{ psi}$): Results in a thick, porous, weak bond line filled with voids.
• Excessive Pressure ($> 200\text{ psi}$): Starves the joint by squeezing out necessary adhesive before cross-linking occurs.

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4. Structural Wood Spar & Rib Repairs

A. Scarf Splicing of Solid Wood Spars

Solid wood spars carrying flight bending loads must be repaired using a precision scarf joint.

                              SOLID SPAR SCARF SPLICE

 <--------------------------------- Scarf Length -------------------------------->
 
 ---------------------------------------\-----------------------------------------
                                         \   Glued Scarf Joint (10:1 to 15:1 Slope)
                                          \
 ------------------------------------------\--------------------------------------
 [====== Reinforcing Birch Plywood Plate ======] (Beveled 5:1 on Ends)

1. Scarf Slope: AC 43.13-1B mandates a minimum scarf slope of 10:1 (preferable 15:1 for highly loaded solid wood spar caps).
   • Example: For a spar with a thickness of $1.5\text{ inches}$, a 10:1 scarf joint requires a cut length of: $\text{Scarf Length} = 1.5\text{ in} \times 10 = 15.0\text{ inches}$
2. Prohibited Splice Locations:
   • A spar splice must NEVER be made directly under wing attachment fittings, strut fittings, landing gear brackets, or aileron/flap hinge brackets.
   • No splice may be located such that the splice overlap ends closer than the depth of the spar to any structural fitting.
3. Reinforcing Plates: The scarf joint must be reinforced on both sides with birch aircraft plywood (MIL-P-6070). Reinforcing plates must extend past the scarf ends and have their edges beveled at a 5:1 slope to prevent abrupt stress risers.

B. Built-Up Box Spars & I-Beam Spars

• Web Splices: Plywood web splices require a 10:1 or 12:1 scarf joint backed by a solid spruce backing block.
• Staggering Rule: In a built-up box spar, splices in the upper and lower spar cap strips and the front and rear plywood webs must be staggered longitudinally. No two splices may occur in the same structural bay.

C. Wing Rib Repairs

• Cap Strip Scarfing: A cracked or broken rib cap strip is spliced using a 10:1 scarf joint backed with spruce reinforcement blocks and birch plywood gussets glued and nailed to both sides.
• Gusset Plates: Rib truss joints utilize aircraft-grade birch or mahogany plywood gusset plates. Nails used during assembly are small brass or cement-coated steel wire brads driven with a small hammer or nail gun to maintain pressure while glue cures; nails are not relied upon for structural shear transfer.