Definition
The direction, size, arrangement, and appearance of the fibers in a piece of wood or other fibrous material. In aircraft construction, grain direction is critical because wood and similar materials are strongest along the grain and weakest across it.
Plain English
The way the fibers run inside a piece of wood. Wood is much stronger when a load pulls along the fibers than when it pulls across them.
Context Anchor
Seen in aircraft maintenance, wood-structure inspection, wooden propeller inspection, and repair instructions for wooden parts.
Derivation
From the Latin granum, meaning 'seed' or 'small particle.' The word came to describe the small fibrous lines visible in cut wood, since they resemble rows of seeds or threads running through the material.
Why Pilots Care
Wood is strongest when loads run along the grain; cutting or stressing across the grain greatly reduces strength and can cause failure.
Analogy
Wood grain is like the direction of threads in a piece of cloth. Pull with the threads and it behaves one way; pull across them and it may behave differently.
Intuition Check
Do not read grain here as grain like wheat or rice. In this aviation use, it means the direction and pattern of the fibers inside wood or another material.
Example Sentence 1
The mechanic inspected the wooden spar for any cracks running across the grain, since those would weaken the structure.
Example Sentence 2
Drill mounting holes so the fasteners load the wood along the grain rather than across it.