Updated: May 1, 2019
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to clear up earlier confusion created by a sloppy interpretation offered by the Eastern Region Legal Counsel that was retracted by the Chief Counsel in September 2008.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. The new interpretation includes some very important information 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 structural 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, flying in glaciated clouds (visible moisture that contain only ice crystals) is not considered known icing conditions. Also, clouds similar to the image above that likely contain very small drops and little liquid water content will not adhere to the airframe.
Don't get your hopes up though...structural icing will occur more frequently than not when saturation occurs at subfreezing temperatures. But for some parts of the U.S. 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 the afternoon on February 4, 2007 out of Benton Harbor, Michigan. Benton Harbor (KBEH) is located close to the southeast shore of Lake Michigan.
A strong cold front passed through the region and ushered in a very cold air mass over the Upper Great Lakes as well as the Upper and Middle Mississippi Valleys dropping surface temperatures to below 0°F at some locations. As is often the case during the winter with a westerly to northwesterly flow, the area downwind of Lake Michigan was experiencing light to moderate snow as shown in the image above. The automated surface observation (METAR) at Benton Harbor valid at 1945 UTC (below) was reporting a quarter-mile visibility in moderate snow with a ceiling of 700 feet. The temperature was a very chilly -16°C (3°F). While there may be many other reasons why this IFR flight should not be taken, would a departure out of Benton Harbor at 2000 UTC constitute a flight into known icing conditions?
KBEH 041945Z AUTO 28013G23KT 1/4SM SN FZFG BKN007 OVC021 M16/M18 A3017
Given such cold temperatures at the surface and even colder air aloft, the clouds producing the snow only contain ice crystals. This can be easily confirmed by looking at several weather products. As shown on the above the Aviation Weather Center (AWC) had issued a couple of AIRMETs for moderate icing, but all these were located well to the south in Kentucky, Tennessee and southwest mid-Atlantic where the environmental temperature was much warmer. The AWC wasn't concerned about the potential for structural icing anywhere in the Upper or Lower Great Lakes or the northern Ohio Valley region due to the colder temperatures.
In addition to the lack of AIRMETs for moderate icing, the sounding analysis (not shown) had a classic glaciated signature. Moreover, the cloud top temperatures (not shown) were in the range from -25°C to -30°C also implying glaciated conditions in these clouds.
The Current Icing Product (CIP) clearly indicated a low probability of light and trace icing in the area over Benton Harbor. It was a bit surprising that CIP was giving any credence to icing potential in this region, but a comparison of the CIP analysis to the radar signature (NEXRAD) clearly shows the two match. CIP was likely giving more weight to the precipitation that was falling not ruling out a small chance of light or trace icing in the region.
There were several pilot reports in the area as shown above. This included one pilot report from an Embraer ERJ-145 (below) that was departing near South Bend, Indiana. The pilot report at 7,000 feet indicates the the tops were reported at 6,800 feet MSL with clear skies above. The temperature aloft was -21°C and the icing was negative during the climb through the layer.
SBN UA /OV SBN330015 /TM 1856/FL070 /TP E145 /SK OVC-TOP068/SKC /TA M21 /IC NEG /RM DURC
While there may have been a very small risk of trace icing in this cloud deck according to CIP, the expectation from the other composite information was pretty clear that the environment was NOT conducive for airframe ice despite the overcast cloud deck. Therefore, in the spirit of the Chief Counsel's interpretation, a departure out of Benton Harbor would not likely be construed as known icing conditions given that visible moisture was not likely to adhere to the airframe.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS research meteorologist