A faint line running across the grain of a wood spar usually indicates which type of failure?

A faint line running across the grain of a wood spar typically indicates a compression failure. This occurs when the wood is subjected to forces that exceed its ability to support the load. In wood, compression failure often manifests as visible lines or fractures, which run perpendicular to the grain direction, reflecting the structural compromise caused by compressive forces.

During the load-bearing process, the fibers of the wood may buckle or crush under excessive pressure, leading to the formation of these faint lines. It is essential for aircraft constructors and maintainers to recognize signs of such failures, as they relate to the integrity and safety of wooden components in aircraft structure. Proper evaluation and understanding of failure types ensure better maintenance practices and structural reliability.