A short review of known icing
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interpretation offered by the Eastern Region Legal Counsel that was
retracted by the Chief Counsel in September 2008.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
some of the banter that has gone on in the pilot community for the last
couple of decades. The new interpretation includes some very
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,
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 (clouds that contain only ice
crystals) is not considered known icing conditions. Also,
clouds similar to the image above (click here to view a larger image)
that likely contain very small drops and little liquid water content
will not adhere to the airframe.
Don't get your hopes up
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. (Regular
and Elite members of AvWxWorkshops.com can click here to view a more detailed
workshop describing this event.) Benton
Harbor (KBEH) is located close to the southeast shore of
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
westerly to northwesterly flow, the area downwind of Lake Michigan was
experiencing light to moderate snow as shown on the Aviation Digital
Data Service (ADDS) METARs Java tool display above and to the left (click here
to view a larger image). The automated surface observation
Benton Harbor valid at 1945 UTC (below) was reporting a quarter-mile
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
KBEH 041945Z AUTO
Given such cold temperatures at the surface and
even colder air aloft,
the clouds producing the snow only contain ice crystals. This
be easily confirmed by looking at several weather products.
shown on the right (click here
to view a larger image) 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
anywhere in the Upper or Lower Great Lakes or the northern Ohio Valley
In addition to the lack of AIRMETs for moderate
Rapid Update Cycle (RUC) sounding analysis (not shown) had a classic
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) found
on the ADDS website (not shown) clearly indicated a low probability of
trace icing in the area over Benton Harbor. It was a bit
surprising that CIP's 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
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 to the
left (click here to view a larger image).
This included one pilot report from an Embraer ERJ-145 that
departing near South Bend, Indiana. The pilot report shown
for 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
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
in this cloud deck according to CIP, the expectation from the other
composite information was pretty clear that the environment was NOT
conducive for known ice despite the overcast cloud deck.
Therefore, in the spirit of the Chief Counsel's
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.