Definition
Flight conducted in atmospheric conditions where structural icing is reported, forecast, or otherwise known by the pilot to exist along the intended route. An aircraft may legally enter known icing conditions only if it is certified for flight into such conditions and equipped with operative ice protection systems appropriate to those conditions.
Plain English
Flying through air where ice is currently forming on aircraft, or where reports and forecasts say ice is likely to form. Only aircraft built and equipped to handle ice are allowed to fly into it.
Context Anchor
Seen in weather briefings, IFR go/no-go decisions, aircraft operating limitations, and instrument flight planning.
Derivation
The phrase hinges on the word 'known.' In aviation regulatory language, 'known' does not mean the pilot has personally seen ice forming. It means icing is reported by another aircraft, forecast by official weather products, or otherwise reasonably expected based on available information. This broader meaning of 'known' is what trips up many pilots.
Why Pilots Care
Most light aircraft lack certification or equipment for known icing, and entry can cause rapid loss of lift, increased weight, and control problems.
Grounding Statement
Picture flying through wet cloud air near freezing: ice can start collecting on the airplane even though the runway below may only look cold and damp.
Intuition Check
Known does not mean you personally saw ice on your airplane. It can also mean reports, forecasts, temperature, and visible moisture make ice something a reasonable pilot should expect.
Example Sentence 1
The pilot canceled the flight because the aircraft was not approved for flight into known icing conditions and an AIRMET Zulu covered the entire route.
Example Sentence 2
The aircraft was not equipped or certified for flight into known icing conditions, so the flight was postponed.