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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Folklore: Climbing is usually the best option when flying through freezing rain or freezing drizzle.

If you’re collecting ice and you’re below the clouds or between layers, then you likely are flying in freezing rain or freezing drizzle. Pilots are taught to climb to a higher altitude if this happens. That’s making a dangerous assumption that there’s an altitude above you that contains air that’s warmer than zero degrees Celsius.

In the case of freezing rain, there might actually be a warm layer above. Freezing rain is produced when snowfall from a cloud enters a layer of air that’s above freezing. The warmer air in this layer melts the snow into rain drops, which then fall into a subfreezing layer below and deposit on your aircraft. In this particular case, a warmer layer aloft does exist.

For freezing drizzle, this often isn’t the case. There likely will be warmer air above you, but often the temperature aloft doesn’t rise above the freezing mark. Freezing drizzle is normally the result of an all-liquid process as small drops in the cloud collide and coalesce into drizzle-sized drops large enough to fall out of the cloud as freezing drizzle.

The best strategy in this case is to turn around and go back to where you were not accreting ice. If you don’t know the temperature profile above you, it may not be wise to attempt a climb.

Folklore:
Using the standard lapse rate is a good way to estimate the freezing level. For example, if the temperature at the surface is 4 degrees Celsius, the freezing level is 2,000 feet above ground level.

Many pilots apply the standard lapse rate of 2 degrees Celsius per 1,000 feet. This means that the temperature decreases 2 degrees Celsius for every 1,000-foot gain in altitude. Therefore, with a temperature of 4 degrees Celsius at the surface, flying in the clouds at, say, 5,000 feet AGL would be a bad idea. Correct?

Actually, you can’t rely on these estimates, especially when Mother Nature is at her worst. In an unstable situation, such as that discussed with stratocumulus clouds, the lapse rate near the surface is often greater than standard—often reaching 3 degrees Celsius per 1,000 feet. At the other extreme, in a stable environment, there often is a temperature inversion that may put the freezing level as high as 10,000 feet AGL.




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