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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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.

Folklore: It’s possible to collect ice at a temperature greater than zero degrees Celsius.

It’s common to hear pilots say that they’ve picked up ice at temperatures above freezing—even as warm as 5 degrees Celsius. Induction icing, sure; but structural icing—it can’t happen. Typically this is attributed to a faulty or inaccurate outside air temperature (OAT) probe that’s reading a few degrees too warm.

Assuming the OAT probe is accurate, the other possible explanation is that the pilot descended into a supercooled liquid cloud while the temperature just above the cloud was a degree or two above freezing. It’s not uncommon to see a temperature inversion (an increase in temperature with altitude) immediately above a cloud deck. The pilot may notice that the OAT is 2 or 3 degrees Celsius just before entering the cloud and jump to the quick conclusion that the remainder of the descent has to be above freezing. He or she is then astonished to witness ice form on the wings, being completely unaware that the temperature was actually colder in the cloud than above it.

Cold soaking is another possibility, though it’s rare. Imagine an aircraft quickly descending from very cold conditions into a cloud deck where the temperature is slightly warmer than zero degrees Celsius. There may be just enough thermal momentum to keep part of the aircraft’s surface below freezing, causing ice to accrete briefly.




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