Plane & Pilot
Saturday, December 1, 2007

Icing Folklore

Avoid flying by rules of thumb

icing folkloreIcing is already a terribly complex topic without the many old wives’ tales and rules of thumb making it even more difficult. Rules of thumb generally plead ignorance. Ignorance often leads to bad decisions. When the weather is on its worst behavior, rules of thumb rarely apply and can actually be dangerous. Here are a few of my pet peeves when it comes to icing folklore.
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Estimating the freezing level using the standard lapse rate is like dividing 19 by 95 and getting 1⁄5 by crossing out the 9s. The method worked well for this particular example, but it’s less likely to work when applied to other situations.

When it comes to icing, there are very few rules of thumb that a pilot can apply successfully. However, one of my personal favorites is called a “glory.” As you fly over clouds, you may see your airplane’s shadow cast on the top of the cloud, assuming the sun angle is perfect and there are no clouds above you blocking the sun. If there’s liquid water in the cloud, you may also have witnessed a glory. A glory is a halo (or rainbow) around your aircraft’s shadow. A glory is an excellent indicator that the cloud is dominated by liquid water. When the OAT is below freezing, it may be an excellent indicator that there’s icing potential in the clouds below. If you see the shadow of your airplane and you don’t see a glory, you may see what look like sparkles around the shadow. This typically means that the cloud is glaciated. In other words, the cloud is dominated by ice crystals.

My best advice with respect to icing is to put aside old wives’ tales and avoid using rules of thumb. If you want to know the freezing level, for example, look at a temperature profile of the atmosphere using a forecast temperature sounding instead of guessing with a formula that works only when the atmosphere fits the formula. If you want to know if a snow-producing cloud is an icing threat, look for the presence of AIRMET Zulu or examine the Current Icing Product (CIP) probability analysis. If you want to know if you’ll encounter freezing drizzle aloft, use the CIP SLD (supercooled large drop) analysis. In other words, attack the problem directly by utilizing weather products that depict the current icing environment instead of relying on folklore.


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