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.
Plain English
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.
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.