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Aircraft Wood Species

Sitka spruce is the yardstick for aircraft wood. Every approved substitute species is rated as a percentage of spruce's strength. You can only use a substitute where the structural repair data specifically allows it, and only at the listed percentage of the original spruce member's dimensions, or with an equivalent strength adjustment.

Grain and Defects

• •Slope of grain: in highly stressed members, the grain can't run off the length of the piece by more than a set slope, commonly given as no steeper than 1:15. Too much slope cuts the strength a lot, even when the wood looks sound.
• •Compression wood: dense, brittle wood that forms on the underside of leaning trees. It's unsuitable for aircraft structure. Look for an unusually dense band and a lack of luster.
• •Compression failures: wrinkles running roughly across the grain, where the wood was crushed, often by wind stress while the tree was growing or by rough handling. This is a serious defect that means rejection, because a failure plane already exists in the fibers.
• •Shakes: separations along the grain, usually between growth rings, most often caused by seasoning (drying). Check them against the allowable limits before using the piece.
• •Knots and pitch pockets: the specification limits their size and location. A knot in a highly stressed spot, like near a fitting, is far more serious than the same knot somewhere lightly loaded.
• •Decay and rot: always rejected. There's no allowable limit. You'll often spot it by a change in color or texture, or by a dull sound when you tap it instead of the sharp ring of sound wood.

Aircraft Plywood

Aircraft-grade plywood is bonded with a waterproof resin adhesive, unlike ordinary construction plywood. The direction of the face plies' grain matters too. Line it up with the structure's main load direction and the panel is strongest where it needs to be.

Glue and Bonding

Only use adhesives specifically approved for aircraft wood structure. Modern waterproof synthetic resin glues, such as resorcinol-based adhesives, are the standard.

Casein glue was used in the past, but it doesn't resist moisture, and it isn't an acceptable substitute in modern wood repairs. Moisture breaks down a casein joint over time, even if it looks fine at first.

Inspecting Wood Structure

• •Sight down the piece: look along its length to check grain slope and spot visible defects.
• •Tap test: tap the wood and listen. Sound wood gives a clear, sharp tone. A dull thud can mean decay inside, or delamination in a plywood panel.
• •Moisture content: check it with a moisture meter. Wood aircraft structure has a specified acceptable range. Too wet and it's prone to decay and glue joint failure. Too dry and it gets brittle.
• •Glue joints: look for any separation, discoloration or a joint line you can open with light probing. A failed glue joint is a structural defect, however sound the wood looks.

Wood Repairs

The standard way to splice new wood into an existing structural member is a scarf joint: a long, tapered joint at a shallow angle. A butt joint alone can't carry load across the grain properly.

How gradual the taper has to be, the slope ratio, depends on the load the member carries. A shallower, longer scarf transfers load better, but it takes more material and more precision to get right.