Wood was the dominant structural material in early aviation, and it remains fully approved for use in certificated aircraft today — particularly in antique, classic, and homebuilt designs. For the Aviation Maintenance Technician (AMT), understanding which wood species are acceptable, how to evaluate grain quality, and how to identify rejectable defects is not just a written-test requirement. It is a genuine safety skill, because a flawed wooden spar or rib can fail without warning under flight loads. The FAA's Aviation Maintenance Technician Handbook — Airframe (FAA-H-8083-31) provides the authoritative guidance every technician must master.
This article walks through approved species, the physical properties that make aircraft-grade wood different from lumber-yard wood, the grain requirements that govern selection, and a systematic tour of defects that render a piece of wood unairworthy. It closes with the testable numbers, common exam traps, and a memory aid for the approval checklist.
Approved Wood Species for Aircraft Structures
Not every strong wood is an acceptable aircraft wood. The FAA recognizes a defined list of species whose structural properties have been tested and documented. Sitka spruce is the benchmark species — the standard against which all substitute species are measured. It offers an outstanding combination of strength-to-weight ratio, stiffness, workability, and availability. When a repair specification calls simply for "spruce," Sitka spruce is what is meant.
Several substitute species are approved when they meet specific strength requirements relative to Sitka spruce. The key approved substitutes and their required strength margins are:
- Douglas fir: Approved as a direct substitute for spruce in most structural applications. It is slightly heavier but has comparable strength. Grain must be straight and close. Douglas fir is more prone to splintering than spruce, which is a handling consideration during repair.
- Noble fir: Approved as a substitute for spruce, provided the piece meets the same visual and grain standards. It is lighter than Douglas fir.
- Western hemlock: Approved as a substitute, with properties very similar to Sitka spruce. It must be carefully inspected because it can contain hard knots more readily than spruce.
- Port Orford cedar (white cedar): Approved at a larger cross-section to compensate for slightly lower density and strength. Historically used in propellers and ribs.
- White ash: Approved for specific applications such as flooring and interior structural members. It is considerably heavier than spruce.
- Poplar (yellow poplar): Approved for non-critical interior components and some secondary structures.
The critical concept is that substitutes are not always interchangeable on a one-for-one dimensional basis. If a substitute species has lower allowable stress values than Sitka spruce, the replacement piece must be sized up in cross-section to carry the same load. Always consult the aircraft's manufacturer data or the FAA advisory circulars (particularly AC 43.13-1B, Chapter 1) for the specific substitution rules before making any repair.
Grain Requirements: Slope and Ring Count
Even a perfectly approved species becomes unairworthy if the grain orientation is wrong. Aircraft-grade wood is evaluated on two grain parameters: slope of grain and annual ring count.
Slope of Grain
Grain slope describes how far the wood fibers deviate from running parallel to the long axis of the piece. Wood fails dramatically along its grain, so a piece with steeply sloping grain may split at a load far below what the species would otherwise handle. The FAA standard for primary structural members (spars, longerons) is a maximum slope of grain of 1 in 15 — meaning for every 15 inches of board length, the grain may deviate no more than 1 inch to the side. For secondary or non-critical members, a slope of 1 in 12 may be acceptable, but always verify against the specific repair instruction. A piece with a slope steeper than 1 in 15 on a spar is rejected outright.
Annual Ring Count
Looking at the end grain of a piece of wood, you can count the annual growth rings. Aircraft-grade spruce and most substitutes must show a minimum of 6 rings per inch and a maximum of no upper limit specified — more rings per inch generally means stronger, denser wood. However, some sources cite an upper bound of around 40 rings per inch beyond which the wood may be excessively resinous (in some species). The key testable minimum is 6 rings per inch for the outer growth rings. Wood from the center (pith) of the tree is never acceptable for primary structure.
Identifying Rejectable Defects
Even wood that meets species and grain requirements can be rejected if it contains physical defects. Technicians must inspect every piece systematically before use.
Knots
Knots are the remnants of branch attachment points within the tree. They interrupt grain continuity and create stress concentrations. Pin knots (very small, tight knots under about 1/8 inch) in clusters are rejectable. Any knot on the edge of a spar cap (the most highly stressed area in bending) is cause for rejection. Scattered, tight, intergrown knots of small size may be acceptable in some secondary members — but the FAA guidance and specific repair documents must be consulted. When in doubt, reject the piece.
Checks and Splits
A check is a lengthwise separation of the wood fibers along the grain, typically caused by rapid drying. Checks reduce the ability of the wood to carry shear loads. Any check on a spar that runs more than a superficial depth is cause for rejection. A split is a complete through-separation and is always rejected in structural members.
Compression Wood and Tension Wood
Compression wood (also called reaction wood) forms on the underside of a leaning or bent tree as the tree compensates for gravity. In softwoods, compression wood appears as abnormally wide, dense growth rings with a darker appearance on the affected side. It is brittle and fails suddenly in tension — exactly the wrong behavior for a spar. Compression wood is always rejected for primary structure. In hardwoods, the equivalent is called tension wood, which appears as abnormally light, woolly fibers and is similarly rejected.
Decay and Staining
Any evidence of fungal decay — including soft spots, discoloration, a musty odor, or a punky texture when probed — is an automatic rejection. Blue stain (a sapwood discoloration from a specific fungus) does not necessarily reduce strength but indicates conditions where true decay could exist; blue-stained wood should be carefully evaluated and is often rejected for primary structure. Brown or white rot visibly destroys wood fibers and is always a rejection.
Pitch Pockets and Resin
A pitch pocket is a well-defined opening between growth rings filled with liquid or crystallized resin. Small pitch pockets on flat-grain surfaces may be acceptable on secondary members, but pitch pockets on edge-grain surfaces of spars are typically rejected because they interfere with the critical shear-carrying grain structure. Excessive surface resin ("sap run") can also interfere with adhesive bonding.
Warp and Distortion
Cup, bow, twist, and crook are forms of warp that indicate internal stress and make the wood unsuitable for precision structural fabrication. Any significant warp in a spar blank is a rejection.
Key Numbers and Rules
- Minimum grain slope for primary structure: 1 in 15 (no steeper deviation).
- Minimum annual ring count: 6 rings per inch measured on the end grain.
- Benchmark species: Sitka spruce — all substitutes compared against it.
- Pith wood: Never acceptable in primary structure.
- Any decay: Automatic rejection, no exceptions.
- Compression wood: Always rejected; brittle in tension, catastrophic failure mode.
- AC 43.13-1B Chapter 1: The primary FAA reference for aircraft wood repairs and substitution guidance.
Memory Aid: "GRADS"
Use GRADS to remember the five main rejection triggers when inspecting aircraft wood:
- G — Grain slope steeper than 1 in 15 (primary members)
- R — Rings fewer than 6 per inch on end grain
- A — Any decay, rot, or fungal evidence
- D — Defects such as knots on spar caps, splits, or checks in structural zones
- S — Species not on the approved substitution list
Running through GRADS before accepting any piece of structural wood ensures you check every major rejection criterion in a logical order — useful both on the job and on the written exam.
Common Test Traps
- Douglas fir is not always a direct one-for-one replacement for spruce. It is approved as a substitute species, but its heavier weight and slightly different modulus must be accounted for in some designs. Test questions may imply it is always interchangeable — read carefully.
- A slope of 1 in 12 is not acceptable for primary structure. The limit is 1 in 15 for spars and main structural members. The exam may offer both values and count on you confusing them.
- Blue stain does not always mean the wood is weak — but it is not automatically acceptable. Questions may present blue-stained wood as either immediately rejected or fully acceptable; the correct answer depends on context, but blue stain in a primary structural member warrants rejection or very careful evaluation.
- Compression wood is harder to spot than obvious defects. It does not look rotten or cracked — it looks like denser rings on one side. Recognize the description: asymmetric ring density on one face of the end grain, darkened appearance in softwoods.
- Ring count is measured on the end grain, not the face. Students sometimes confuse grain slope measurement (done along the face or edge) with ring counting (done on the cut end). Each measurement uses a different surface of the wood sample.