Does snow really count as "known icing conditions?" We talked to the Aviation Weather Center in Kansas City to find out more. Here's what we found.....
FAA's Revised "Known Icing" Definition
In 2006, the FAA published a letter of interpretation that stated, "known icing conditions exist when visible moisture or high relative humidity combines with temperatures near or below freezing." This definition grounded many general aviation pilots from flying on days with high humidity and low temperatures, even if no visible moisture was present.
After years of lobbying from AOPA and other groups, the FAA released a revised definition in a 2009 letter of interpretation that left much of the decision making up to individual pilots. No longer was there a humidity value to determine known icing conditions. The pilot's analysis of available weather products and forecasts in comparison to flight route, altitude, and time would now determine whether a flight was safe and legal.
The letter also clarified that instead of the FAA specifically defining "known ice", the FAA instead defines "Known or Observed or
Detected Ice Accretion" in AIM Paragraph 7-1-22 as:
"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.
But that doesn't mean you can go flying through anything you'd like. The FAA goes on to say that in the event of an investigation...
"The FAA will specifically evaluate all weather information available to the pilot and determine whether the pilot's pre-flight planning took into account the possibility of ice formation, alternative courses of action to avoid flight into known icing conditions and, if ice actually formed on the aircraft, what steps were taken by the pilot to exit those conditions."
It goes without saying, but it's incredibly important that your icing knowledge is up-to-date, so you can make safe and informed pre-flight decisions.
That brings us to the snow question. If you see snow in the forecast, or you see a snow shower ahead of you, what should you do?
Wet Snow vs. Dry Snow
When snow is completely frozen in crystallized form, it generally does not pose a risk for icing. According to the FAA's Pilot Guide To Inflight Icing Conditions, dry snow does not contain liquid, and it's unlikely to adhere to your aircraft. In this case, it's unlikely to pose an icing risk. And the colder the OAT, the more likely it is you'll encounter dry snow instead of wet snow.
Wet snow is a much more hazardous condition. If the outside air temperature sits around freezing, you're much more likely to encounter liquid within the snow.
According to the FAA, "If wet snow does begin to stick, it should then be treated as an icing encounter because ice may begin to form under this accumulation of snow." Snow may fully mask a layer of clear ice, like the type in the picture below:
Flying Below The Clouds
Temperatures between 0 degrees and -5 degrees Celsius are most prone for water and wet snow combinations in the air. Snow gains mass much faster than small liquid droplets, and subsequently precipitates out of the cloud first. If liquid droplets remain suspended in the cloud, you probably won't encounter much risk for ice. However, if the liquid does begin to precipitate and the temperatures are around 0 to -5 degrees, you're putting yourself at risk for accumulating substantial ice. In this case, the colder the air temperature below 0, the safer you'll be.
But don't let the dry snow below the clouds fool you. There could still be plenty of super-cooled liquid within the cloud itself, which is the perfect recipe for icing conditions. If the water droplets in the cloud gain enough mass to precipitate before they fully freeze, you might run into freezing drizzle or rain, which is the worst case scenario you could find yourself in.
Flying Through Clouds With Snow
If you fly through a cloud with snow in it, you'll likely be flying in icing conditions unless temperatures are well below -20 degrees Celsius. Snow forms from moisture accumulating and sticking together, so if you fly through that formation process you're likely to encounter liquid that will turn to ice on your airplane. Don't expect to find just dry snow inside a cloud.
So, Is Flying Through Snow Safe?
The answer is, you guessed it...it depends. If you can determine that you're flying through an area of cold, dry, crystallized snow below the clouds, the risk for airframe ice developing is relatively low.
However, flying through clouds (2C or colder), or flying through snow below the clouds when the temp is between 0C to -5C puts you at higher risk.
Where You'll Find The Most Hazardous Conditions
According to the Aviation Weather Center, the West/Northwest side of low pressure systems behind a cold front contain some of the worst mixed precipitation. Dry air aloft cuts across moist air near the cold front, resulting in moisture remaining right around the supercooled temperature range of 0 to -20 degrees Celsius. The supercooled liquid isn't quite cold enough to freeze on its own, but it's certainly cold enough to pose the risk of ice accumulation when it impacts your airframe.
Pre-Flight Planning Tips
Since it's your responsibility to determine if and where icing conditions exist, make sure you're using the right weather products to determine the potential for icing on your flight:
Snow doesn't always equal known icing conditions. If you fly through a light, dry snow shower, you're unlikely to see ice accumulation. But if the temperature is warm (roughly 0C to -5C), wet snow mixed with liquid water could to stick to your airplane. If you think it's warm enough for icing conditions to exist, play it safe and stay on the ground.
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