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Does AIRMET Zulu always imply known icing conditions?

Updated: Oct 6, 2023

In a word, no.  Although AOPA's Air Safety Institute (ASI) would make you believe that AIRMET Zulu is considered known icing conditions based on their Precipitation and Icing interactive online course.  As shown on the right, the ASI clearly states in this course that known icing conditions exist if icing AIRMETs/SIGMETs exist for your route.  If you are familiar with the legacy AIRMET you understand that AIRMET Zulu all by itself does not imply known icing conditions as suggested by the ASI course. Here's why.

AIRMET Zulu is an advisory for widespread moderate structural icing.  So it would seem plausible that an AIRMET for moderate icing would be a forecast for known icing conditions. However, AIRMETs are what forecasters at the Aviation Weather Center (AWC) refer to as time-smeared forecasts.  That is, they are forecasts valid over a six (6) hour period.  In fact, the AWC provides a good explanation on their website.  They say,   

"These AIRMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3,000 square miles at any one time.  However, if the total area to be affected during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time."

This is very clearly stated.  Icing conditions that are expected to develop, move or dissipate within the six hour forecast period must be included within the bounds of the AIRMET. Consequently, AIRMETs can cover large regions of airspace because of their time-smeared nature.     

Let's take a look at a specific example. Assume that you are proposing a short 52 nautical mile IFR flight at 7,000 feet leaving shortly after 1500 UTC from Fitchburg Municipal Airport (KFIT) in north-central Massachusetts to Taunton Municipal Airport (KTAN) located in northern Rhode Island (shown below by the red arrow).  You receive a standard briefing from Lockheed Martin Flight Service and briefer advises you of an AIRMET (shown to the left) that was just issued at 1445 UTC for moderate ice below 12,000 feet that covers your entire proposed route. According to the online ASI course mentioned above, an AIRMET is one of the parameters that triggers known icing conditions.  Assuming you are not flying an aircraft with a certified ice protection system (IPS), are you effectively grounded because of this AIRMET?

Not in this case.  As of 1500 UTC the air was completely free of clouds with excellent visibility between your departure and destination airports as shown by the surface observations (METARs) above (and text below) that are valid shortly before 1500 UTC (a green square implies clear below 12,000 feet).

KFIT 071452Z AUTO 27015G24KT 10SM CLR M01/M11 A2937 RMK AO2 KORH 071454Z 26021G31KT 10SM CLR M03/M10 A2932 KSFZ 071455Z 29009G15KT 240V300 10SM CLR M01/M09 A2936

Also notice that the METARs for Fitchburg and Taunton above report a fairly large dewpoint depression (temperature-dewpoint spread) of 10 and 8 degrees Celsius, respectively, which implies very dry conditions near the surface.  Keep in mind that surface observations only show the conditions near the airport.  So it plausible that clouds and potential icing conditions could exist between these reporting stations.  Or perhaps this is just a case of a bad forecast by the Aviation Weather Center? 

Possibly.  But weather was anticipated to move in from the west with time.  Looking at the latest visible satellite image shown above, the air between Fitchburg and Taunton is actually clear of any significant clouds.  Moreover, there were no pilot reports (PIREPs) for icing anywhere near this route as shown below.     

So based on the ASI's definition contained in the online course, known icing conditions would exist even if the sky were clear over your departure airport and along your route of flight as long as an AIRMET for icing existed. Of course, this does not make common sense.

Perhaps the ASI is just following the guidance provided by the FAA?  In letter dated January 16, 2009 (see below), the FAA's Chief Legal Counsel clarified the definition of known icing conditions once and for all.  In that letter sent to AOPA, it states,

"Known icing conditions involve instead circumstances where a reasonable pilot would expect a substantial likelihood of ice formation on the aircraft based upon all information available to that pilot."

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In this proposed flight, would a reasonable pilot would expect a substantial likelihood of ice based on all available information, not just a single AIRMET. Given the METARs, PIREPs and visible satellite image, the likelihood of airframe icing on a flight from Fitchburg to Taunton at 1500 UTC was certainly not substantial. Perhaps if the departure was several hours later when moderate icing had moved in, then you can be sure the NTSB/FAA will use the legacy AIRMET as evidence that the pilot should have known better even if there are no pilot reports for icing.

Most pilots are weatherwise, but some are otherwise™

Scott Dennstaedt

Weather Systems Engineer

CFI & former NWS research meteorologist

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