Aircraft Wood Structures

Wood in Aircraft Construction

• Solid, laminated, and plywood are the three common forms of wood used.
• The same plywood type originally used should be used when splicing or reinforcing plywood webs.
• Knots, checks, splits, pitch pockets, cross grain, curly grain, decay, and dry rot are examples of wood defects.
• Wood sections with hard knots are acceptable within limits.
• Casein glue in older wooden aircraft requires careful inspection for deterioration.
• Resorcinol glue, Phenol-formaldehyde glue, and epoxy resin glue are used in modern aircraft.
• Compression wood is not acceptable for structural repairs.

Working with Wood

• Wood pieces should be kept in the same room for 24 hours before joining to equalize moisture content.
• Exceeding the open-assembly time for gluing can cause joint failure.
• Proper clamping pressure forces air out, brings surfaces together evenly, and strengthens the glue line.
• Clamps, nailing strips, power presses, brads, nails, and screws can apply pressure to glue joints.
• A minimum curing temperature of 70°F (or manufacturer's specifications) is needed for adhesives.
• Stains and discoloration may indicate decay or rot when inspecting wood structures.

Wood Repair Techniques

• Remove and splice in a new section or replace the entire spar to repair elongated bolt holes in a wooden wing spar.
• A scarf joint is used to splice a solid or rectangular wood spar.
• Splices are prohibited under attachment fittings for wing roots, landing gear, engine mounts, or struts.
• The maximum number of splices allowed for a single spar is two.
• Scarf joints have tapered or beveled pieces with a slope of 1 to 10 or 1 to 12.
• Matching pieces are essential for a tight glue joint in a scarfed repair.

Plywood Use and Repair

• Plywood is primarily used for gusset/reinforcing plates and aircraft skin.
• Splayed patches, surface patches, plug patches, and scarfed patches are examples of plywood skin repairs.
• A splayed patch can repair small plywood skin holes (less than 1/10th inch thick) if the hole is less than 15 times the skin's thickness in diameter.
• Waxed paper or vinyl plastic prevents a plywood patch and pressure plate from sticking together from extruded glue.

Hardware and Honeycomb Structures

• Lightweight steel bushings prevent wood crushing when tightening bolts.
• Large washers provide additional bearing area to prevent wood damage when tightening hardware.
• Stainless steel, titanium, magnesium, plywood, glass nylon, and cotton cloth are used for facings and cores in bonded honeycomb structures.
• Bonded honeycomb structures have solid facings bonded to either side of open six-sided cells.
• Damaged areas in bonded honeycomb structures must be completely removed before repair.
• A drill bit for composites should have a 135-degree included angle.

Composite and Plastic Repairs

• Sonic vibration, internal moisture expansion, liquid leakage, and manufacturing defects cause composite structure delamination.
• The potted repair method fills holes and repairs some defects in bonded honeycomb structures.
• Exposure to ultraviolet light, stress, solvents, and improper handling cause crazing in transparent plastics.
• Back up Plexiglas with wood and feed the drill slowly to avoid damage when drilling.

Wood Assemblies and Species

• A laminated wood assembly has two or more layers glued together with parallel grains.
• Aircraft plywood consists of an odd number of thin plies with grains at 90-degree angles.
• Spruce is the standard wood species for strength comparison.
• Mineral streaks are acceptable if inspection reveals no decay.

Aircraft Fabric Covering

• Aircraft covering fabrics are made of organic and synthetic materials.
• Fabric used for re-covering must be of at least the original quality and strength.
• Cotton and linen are two types of organic fibers used for covering aircraft.

Fabric Terminology and Application

• Warp is the fabric's length, weft is across the fabric, and bias is 45 degrees to the warp and weft.
• The selvage edge prevents unraveling.
• FAA-PMA stamp is on the selvage edge, with the warp parallel to the line of flight.
• The original manufacturer determines fabric type with strength based on never-exceeding speed and wing loading.
• Fiberglass and heat-shrinkable polyester fibers are synthetic fibers used for covering aircraft.
• Fiberglass is unaffected by moisture and mildew.
• Reinforcing tape prevents lacing cord from cutting the fabric.
• Surface tape is used over rib stitching, sewed seams, overlapped edges, corners, and leading/trailing edges.
• Plastic, aluminum, or brass grommets reinforce edges of drainage, inspection, and ventilation holes.
• Punch testers (Maule or Seybolt) and laboratory pull-testing assess fabric condition.
• Fabric is unairworthy if it deteriorates to less than 70% of its original strength.
• Apply a protective coating of paint or varnish to a structure prior to covering it with doped fabric.
• The envelope method for covering wings has almost all seams machine-sewn by the supplier.
• A tie-off knot is a modified seine knot used on all stitches except the starting stitch.
• Plain overlap, folded-fell, and French-fell are common machine-sewed seams.
• Anti-tear strips are used under wing rib stitching when the never exceed speed is greater than 250 mph.
• Consult AC 43.13-1B to determine rib stitching spacing if the original spacing is unknown.
• The fabric warp is the direction along the length of the fabric.
• The envelope method involves sliding pre-sewn fabric envelopes onto the structure, with seams in the proper place.