What Are Known Icing Conditions?

However, 14 CFR § 91.9 (a) states, “…no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.” Therefore, if your aircraft has an operating limitation and/or placard that states, “flight into known icing is prohibited,” then you must adhere to that limitation.

Moreover, a pilot must have an aircraft that is certified for flight in icing conditions, meaning it must have functioning deicing or anti-icing equipment on critical surfaces like wings, propellers, and windshield, and the pilot must be adequately trained on the aircraft's ice protection systems and procedures for operating in icing conditions. This often includes specific training on how to identify and avoid severe icing situations, as well as how to manage the aircraft's performance when encountering ice buildup. These are the requirements.

Now let’s examine the specifics of what constitutes known icing conditions.

In a 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to clear up earlier confusion created by a "sloppy" interpretation offered by the Eastern Region Legal Counsel that was retracted by the chief counsel in September 2008.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 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 the 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 contains only ice crystals) is not considered known icing conditions. Also, thin clouds similar to the photograph above will likely contain tiny drops with little liquid water content and 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 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 cold air mass over the Upper Great Lakes as well as the Upper and Middle Mississippi Valleys, dropping surface temperatures to below 0 degrees Fahrenheit 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 METARs above (asterisks to the left of the station model indicate snow). The automated surface observation (METAR) at Benton Harbor valid at 1945Z (below) was reporting a quarter-mile visibility in moderate snow with a ceiling of 700 feet. The temperature was a very chilly minus-16 degrees Celsius (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 20Z 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 will only contain ice crystals. This can be easily confirmed by looking at several weather products.

As shown above, forecasters at the Aviation Weather Center (AWC) 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 airframe icing anywhere in the Upper or Lower Great Lakes or the northern Ohio Valley region due to the much colder temperatures aloft.

In addition to the lack of AIRMETs for moderate icing, the sounding analysis (below) near South Bend, Indiana (KSBN), 25 miles south of Benton Harbor had a classic glaciated signature. Moreover, the cloud top temperatures were about minus -28 C, also implying glaciated conditions in these clouds.  

The current icing product (CIP), not shown, 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 products align quite well. 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 that was departing near South Bend. The pilot weather report at 7,000 feet indicates the tops were reported at 6,800 feet msl with clear skies above. The temperature aloft was minus-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.