Plain English Meaning
A wing surface that has nothing stuck to it — no ice, no frost, no snow — so the air flows over it the way it is supposed to.
Full Definition
An airfoil whose surface is free of ice, frost, snow, or other contamination, allowing airflow over the wing to behave as designed by the manufacturer.
Context Anchor
Seen in icing discussions when comparing how a normal wing or tail performs before and after ice forms on it.
Derivation
Clean' here is used in the engineering sense of 'uncontaminated' or 'unaltered from designed shape' — the same way mechanics speak of a 'clean' fuel sample. It does not refer to dirt or washing.
Why Pilots Care
A clean airfoil produces the lift and drag values shown in the aircraft's performance charts; any contamination reduces lift and increases drag, often dramatically.
Intuition Check
Clean does not mean washed or shiny here; it means free of anything that changes the airfoil’s shape or surface. Airfoil does not mean only the main wing; it can also include tail surfaces and propeller blades.
Example Sentence 1
Before takeoff in winter conditions, the pilot inspected the wings carefully to confirm a clean airfoil.
Example Sentence 2
The handbook performance numbers assume a clean airfoil; any ice requires using the more conservative figures.