Updated: Jan 18
In this letter dated January 16, 2009, the FAA's Chief Legal Counsel clarified the definition of known icing conditions once and for all. This letter became necessary to clear up earlier confusion created by a "sloppy" interpretation offered by the Eastern Region Legal Counsel that was retracted by the Chief Counsel in September 2008. This newest guidance from the FAA has done a very good job eliminating some of the banter that has gone on in the pilot community for the last couple of decades. But there's no doubt that even this opinion will still generate conversation for decades to come. The new interpretation includes some very important language that is clearly stated regarding structural icing and definitely worth a review of a few of its salient points.
One of the most important paragraphs in this letter introduces a complete definition of airframe icing and leaves very little to the imagination. In the letter the author states,
"The formation of structural icing requires two elements: 1) the presence of visible moisture, and 2) an aircraft surface temperature at or below zero degrees Celsius. The FAA does not necessarily consider the mere presence of clouds (which may only contain ice crystals) or other forms of visible moisture at temperatures at or below freezing to be conducive to the formation of known ice or to constitute known icing conditions. There are many variables that influence whether ice will actually be detected or observed, or will form on and adhere to an aircraft. The size of the water droplets, shape of the airfoil, and the speed of the aircraft, among other factors, can make a critical difference in the initiation and growth of structural ice."
Therefore it is clear that flying in glaciated clouds (visible moisture that contain only ice crystals) is not considered known icing conditions. Also, clouds similar to the photograph above that likely contain tiny drops with little liquid water content will not adhere to the airframe.
Don't get your hopes up though...airframe icing will occur more frequently than not when saturation occurs at subfreezing temperatures. But for some parts of the U.S. and Canada during the cold season, glaciated clouds do occur in the presence of a very cold air mass during the winter. For example, let's look at such a case for an IFR departure in th