Definition
An imaginary line running through a structural member that is being bent, along which the material experiences neither tension nor compression. Fibers on one side of this line are stretched, fibers on the other side are squeezed, and fibers exactly on the line keep their original length.
Plain English
When a beam or panel bends, one side gets stretched and the other side gets squashed. Somewhere in the middle there is a line that does neither — it just sits there unchanged. That line is the neutral axis.
Context Anchor
Seen in airframe structure discussions, especially when describing how beams, wing parts, brackets, and repaired metal parts carry bending loads.
Derivation
Neutral comes from Latin neutralis, meaning 'neither one nor the other.' Axis is Latin for a central line. Together: the line that is neither in tension nor in compression while everything around it is one or the other.
Why Pilots Care
Knowing the neutral axis location lets technicians predict where stresses concentrate and choose safe places for repairs or fasteners.
Analogy
Bend a thick eraser between your fingers. The outer curve stretches, the inner curve bunches up, and a thin layer right through the middle stays the same length. That middle layer is the neutral axis.
Intuition Check
Neutral does not mean unimportant here. It means the part of the bending member that is neither being stretched nor squeezed.
Example Sentence 1
When a wing spar flexes upward in flight, the upper surface goes into compression, the lower surface goes into tension, and the neutral axis runs through the middle.
Example Sentence 2
Flight loads place tension above the neutral axis and compression below it in the wing beam.