What Are Known Icing Conditions?

A pilot must have an aircraft certified for flight in icing conditions and be adequately trained on its ice protection systems.

Scott Dennstaedt, Ph.D

Wednesday, January 01, 2025 at 02:00 PM ET

Verified

If your aircraft has an operating limitation and/or placard that states, “flight into known icing is prohibited,” then you must adhere to that limitation. [Credit: iStock]

Key Takeaways:

Operating in known icing conditions is prohibited for aircraft not certified with de/anti-icing equipment and for pilots not trained for such conditions, as per aircraft operating limitations (14 CFR § 91.9(a)).
Structural icing fundamentally requires both visible moisture and an aircraft surface temperature at or below 0°C.
However, not all visible moisture at freezing temperatures constitutes known icing; specifically, glaciated clouds (containing only ice crystals) are explicitly not considered known icing conditions.
Pilots should utilize comprehensive weather data, including AIRMETs, soundings, and PIREPs, to confirm glaciated conditions where structural ice is unlikely to form, even with visible precipitation.

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What are known icing conditions?

Answer: First of all, let’s address the question of what prevents you from flying your aircraft into known icing conditions? After all, there’s no specific Federal Aviation Regulation (FAR) that directly prohibits this.

However, 14 CFR § 91.9 (a) states, “…no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.” Therefore, if your aircraft has an operating limitation and/or placard that states, “flight into known icing is prohibited,” then you must adhere to that limitation.