Definition
A surface that curves in two directions at the same time, such that no straight line can be drawn along it in any orientation. In aircraft sheet metal work, a compound curve cannot be formed by simple bending alone — it requires stretching the metal in some areas and shrinking it in others to achieve the double curvature.
Plain English
A shape that bows or domes in more than one direction at once, like the outside of a bowl. You can't make it by just bending a flat sheet — the metal has to be stretched and squeezed to take the shape.
Context Anchor
Seen in airframe maintenance when describing aircraft skin, covers, and other shaped parts that must match the aircraft’s rounded surfaces.
Derivation
Compound' comes from the Latin componere, meaning 'to put together.' A compound curve is literally two curves combined into one surface — curving one way and another way at the same time.
Why Pilots Care
Compound-curved skin panels are harder to fabricate and repair than flat or single-curved panels. A technician working on these areas needs forming techniques like shrinking, stretching, or hammering over a form block — a straight bend won't produce the correct shape, and an incorrect repair will not fit the aircraft contour.
Analogy
A sheet bent around a can has one simple curve. A piece shaped over a ball has a compound curve because it bends in more than one direction.
Intuition Check
Do not read “compound” here as chemical or merely complicated. In this context, it means multiple curve directions combined in one aircraft shape.
Example Sentence 1
The wingtip fairing has a compound curve, so the technician used a form block and a shrinking tool to shape the replacement skin.
Example Sentence 2
Repairing damage on a compound curve section of the fuselage requires shrinking and stretching tools to avoid wrinkles.