Definition
Atmospheric conditions in which the formation of ice is observed or detected on the aircraft, or in which a reasonable pilot would expect ice to form based on available weather information such as PIREPs, AIRMETs, SIGMETs, forecasts, and reported temperatures and visible moisture.
Plain English
Weather where ice is actually forming on aircraft, or where the available reports and forecasts make it clear ice is likely to form. It is not just about what you see out the window — it includes what the weather information tells you to expect.
Context Anchor
Seen in instrument flying, weather briefings, aircraft limitations, and decisions about whether an airplane is allowed to fly in icing weather.
Derivation
"Known" here does not mean "already happening to me right now." In FAA usage it means "information that a reasonable pilot has access to and should act on." Forecasts, PIREPs, and observed conditions all count as "known."
Why Pilots Care
Determines whether flight into the area is permitted for aircraft without approved ice protection systems.
Grounding Statement
If the air is cold enough and the airplane is flying through moisture, ice may start forming on exposed parts of the aircraft.
Intuition Check
Do not read “known” as meaning “I have already seen ice on my airplane.” In this context, it can also mean the available weather information makes icing a reasonable expectation.
Example Sentence 1
The pilot canceled the flight after the briefing showed known icing conditions along the route at the planned cruise altitude.
Example Sentence 2
Regulations require that an aircraft be certified for flight into known icing conditions before operating in such weather.