Definition
A longitudinal separation of the wood fibers in a piece of timber, usually running between or through the annual growth rings, caused by stresses in the tree before it was cut. In aircraft structural inspection, a shake is considered a defect that weakens the wood and generally renders it unacceptable for use in load-bearing wooden components such as wing spars, ribs, or propellers.
Plain English
A natural crack or split inside a piece of wood that formed while the tree was still alive. It makes the wood weaker, so wood with shakes is rejected for aircraft parts.
Context Anchor
Seen during inspection or selection of wood used in aircraft structures and repairs.
Derivation
From Old English 'sceacan,' meaning to shake or move violently. The term reflects the idea that the wood fibers were pulled apart, as if shaken, by stress in the living tree.
Why Pilots Care
A shake reduces the strength of wood used in aircraft construction and can lead to structural failure under flight loads.
Intuition Check
“Shake” does not mean vibration here. In this maintenance context, it means a split or separation inside the wood.
Example Sentence 1
During inspection of the spruce spar stock, the mechanic found a shake running along the grain and rejected the piece.
Example Sentence 2
During pre-cover inspection, any shake found in a rib must be cut out and spliced.