Definition
Wood cut from a log so that the growth rings meet the surface of the board at an angle of less than 45 degrees. The annual rings appear on the face of the board as wavy or oval patterns rather than as straight parallel lines.
Plain English
A board sawn from a log in a way that makes the tree's growth rings lie nearly flat across the face of the board, producing a wavy ring pattern on the surface.
Context Anchor
Seen in aircraft wood selection, wood repair instructions, and inspections of wooden aircraft parts.
Derivation
Called 'flat' because the growth rings lie nearly flat (parallel) to the wide face of the board, rather than standing on edge as they do in vertical (edge) grain wood.
Why Pilots Care
Flat grain is generally weaker and more prone to splitting under flight loads than quarter-sawn wood, so it is avoided in structural repairs.
Intuition Check
Flat grain wood does not mean the board is smooth or level. It means the growth rings meet the board face at a shallow angle, less than 45 degrees.
Example Sentence 1
The mechanic rejected the replacement spar blank because it was flat grain wood and the repair required vertical grain stock.
Example Sentence 2
Only vertical-grain spruce was approved for the new rib caps; flat grain wood was set aside for non-structural fairings.