Definition
A reference line drawn on an airfoil section from the leading edge to the trailing edge, passing through the point of maximum camber (the highest point of the upper curve). It is used in airfoil geometry to describe and compare the shape of a wing's cross-section.
Plain English
A line drawn across the side-view of a wing that runs from the front edge to the back edge and passes through the highest point of the wing's upper curve.
Context Anchor
Seen in aircraft maintenance drawings and sheet-metal layout for cone-shaped parts such as fairings, ducts, or transitions.
Derivation
From Latin apex, meaning 'tip' or 'highest point.' In airfoil geometry, the line is named for the way it touches the apex — the highest point — of the wing's upper surface curve.
Why Pilots Care
Pilots usually do not draw apex lines, but correctly laid-out parts fit better and help keep repairs or replacements accurate and airworthy.
Intuition Check
Do not read apex line as just any line near the top of a part. It is specifically a line from the pointed end of a cone-shaped part to its wide edge.
Example Sentence 1
When comparing two airfoils, the engineer drew the apex line on each cross-section to show how the maximum camber differed.
Example Sentence 2
Maintenance crews referenced the apex line when inspecting the leading-edge structure of the swept-wing jet.