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 an aircraft into known icing conditions is primarily restricted by its operating limitations (per 14 CFR § 91.9(a)), requiring certified de/anti-icing equipment and adequately trained pilots, rather than a direct FAA regulation.
Structural icing, which defines known icing conditions, fundamentally requires two elements: the presence of 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 generally not considered conducive to structural ice formation.

See a mistake? Contact us.

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.