Known or Observed or Detected Ice Accretion (fka known icing)
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It's that time of the year (or season) again. As the surface temps drop across the northern and mid section of the country, pilots start thinking about obstacles to their flying - short days, long nights, nor'easters, expansive overcasts, gusty surface winds, snow, and icing, among others.

And the first such question came in just today. Effectively, if flying through snow showers, is that considered known icing?

The definition to this thread title, and the answer to the question, is found in Table 7-1-7 of the AIM - “Actual ice observed visually to be on the aircraft by the flight crew or identified by on-board sensors”

Actual adhesion to the aircraft, rather than the existence of potential icing conditions, is the determinative factor in this definition.

This winter, take few extra minutes to study the current and forecast weather, icing charts, alternatives, pireps, and exit plan to make an informed go/no go decision regarding icing.

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Besides the FAA definition in the AIM, it makes sense. Snow showers are water that is already frozen before it hits the skin of the aircraft. Unless the friction of impact creates heat to melt the snow or the skin of the aircraft is warm enough to melt it and then refreeze it, no ice will adhere to the aircraft. There's no reliable way to tell the temperature of the skin in most GA aircraft but outside air temperature is a good indicator. If outside air temperature is close to freezing and there is visible, unfrozen moisture, that's the time to worry about airframe icing. When forecast temperatures are within 5C degrees of freezing along a planned route and there are forecast clouds, mist or drizzle, there's a good possibility of icing. Snow showers alone won't deter me unless they are so heavy that visibility becomes a problem. Flying IFR in winter usually means flying higher for radar coverage, especially in the mountains where I'm based. That often puts you in the clouds unless your aircraft has a high enough ceiling to climb above them. Mine has a ceiling of 12,500 but I've only managed to reach 11,500 with its carbureted, 4-cylinder, Lycoming O-360-C1F engine and that took a while. Since I don't have anti-icing or de-icing equipment on my airplane, my personal minimum is that I won't fly in clouds in the winter. If you are going to fly in clouds in the winter, there are several very good forecast icing weather products. However, forecasts don't always come true. Checking the outside air temperature frequently when you fly in clouds is always a good idea. If you are descending from a colder temperature through clouds, there's also a chance that your airframe is colder than the outside air and any unfrozen moisture in the air might hit the airframe and ice up. A less common but still dangerous situation is when you are climbing from colder air into a warmer temperature inversion where the moisture is close to freezing. If you do encounter icing, give ATC a PIREP to help other pilots.

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Let's not confuse the terms “known icing” and “known icing conditions".  Known icing conditions (conditions where icing is likely to occur) are far more prevalent than known icing (when icing is known to be occurring).  Flight into known icing conditions in airplanes not certified for icing is prohibited even when known icing isn't being reported.  See the FAA Chief Counsel Bell letter for a fuller explanation and details.