Definition
In aircraft wood, a knot is a localized defect in a board or plank where the cross-section of a branch was embedded in the trunk of the tree. Knots interrupt the continuity and direction of the wood grain, reducing the strength of the piece. In aircraft-grade lumber, knots are restricted by size, location, and condition (sound and tight versus loose, dead, or with checks), and they are not permitted in critical structural areas such as spars.
Plain English
A knot is the round, often darker mark in a piece of wood where a branch used to grow out of the tree. The wood fibers have to bend around it, so the wood is weaker there. For aircraft parts, only small, tight knots are allowed in non-critical spots, and none are allowed in highly stressed parts like wing spars.
Context Anchor
Seen when inspecting, selecting, or repairing wood used in aircraft structures, especially older or wood-built aircraft.
Derivation
The English word knot for this wood feature comes from Old English cnotta, meaning a tied lump or hard nodule. The same root gave us the knot in a rope. In wood, it describes a hard, round lump in the grain that interrupts otherwise straight fibers, much the way a knot in a string interrupts a smooth line.
Why Pilots Care
Knots weaken wood and can cause structural failure if they are large, loose, or located in load-bearing parts.
Grounding Statement
Picture a tree branch growing out of a trunk; when that trunk is cut into boards, the place where the branch joined the trunk appears as a knot.
Intuition Check
A knot is not just a cosmetic spot in aircraft wood. If it changes the fiber direction or is loose or damaged, it can affect strength.
Example Sentence 1
The mechanic rejected the spruce plank because a large loose knot fell within the area planned for the wing spar.
Example Sentence 2
Small tight knots were acceptable in the non-structural fairing but not in the load-carrying longerons.