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:

Pilots must comply with aircraft operating limitations (e.g., "flight into known icing is prohibited") as per 14 CFR § 91.9(a) and operate only FAA-certified aircraft with appropriate deicing/anti-icing equipment and training for such conditions.
Known structural icing conditions, as clarified by the FAA, require both the presence of visible moisture and an aircraft surface temperature at or below zero degrees Celsius.
Crucially, the mere presence of visible moisture at freezing temperatures does not automatically constitute known icing; factors like water droplet size, airfoil shape, and aircraft speed are critical, and flight through glaciated clouds (containing only ice crystals) is specifically not considered known icing conditions.

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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.