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Understanding known icing and forecasts

Posted by Scott Dennstaedt 
Understanding known icing and forecasts
July 09, 2014 01:31PM
It won't be long and summer convection will morph into winter icing. Discussion will turn from avoidance of thunderstorms to avoiding airframe ice. If there's no forecast for airframe ice, does that imply it is safe and legal to fly through visible moisture when the temperature is below 0 degrees Celsius?

Here's a short snipit from a pilot about known icing conditions and icing forecasts...

I first noticed something different because I was losing altitude (had to trim), then I looked up and I had a layer of ice all over the leading edges of the plane, windshield etc. The ice wasn't forecast so I didn't break any rules. They put an AIRMET for icing for that area because of my report.

Do you agree with this pilot given the statement highlighted in red?

Here's my take on this. Just because ice wasn't forecast doesn't imply there were no rules broken. AIRMET Zulu is the primary source for the official icing forecast. However, a forecast or the lack of a forecast doesn't imply flight into known icing as I discuss here. In fact, in this FAA Chief Counsel interpretation, it says the following...

"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."


"Pilots should also carefully evaluate all of the available meteorological information relevant to a proposed flight, including applicable surface observations, temperatures aloft, terminal and area forecasts, AIRMETs, SIGMETs and pilot reports (PIREPs). As new technology becomes available, pilots should incorporate the use of that technology into their decision-making process. If the composite information indicates to a reasonable and prudent pilot that he or she will be operating the aircraft under conditions that will cause icing to adhere to the aircraft along the proposed route and altitude of flight, then known icing conditions likely exist. If the pilot operates the aircraft in known icing conditions contrary to the requirements of FAR 91.9(a) the FAA may take enforcement action."

So it is pretty clear that a forecast or lack of a forecast isn't the determining factor; it's based on a review of all available information. Even hearing other pilots reporting ice on the same frequency during a flight may constitute known icing conditions. Moreover, unless it is very cold, the simple knowledge of flying at an altitude that is forecast to be below freezing with visible moisture present may be enough to be considered known icing conditions. So you can't just say, "well, there's no forecast for ice, so I can test the waters and fly through saturated air at subfreezing temperatures and be considered legal even if ice accretes on the airframe."

To comment on the pilot's last sentence above, while PIREPs are very important to forecasters, a single pilot weather report doesn't necessarily cause a forecaster to issue an AIRMET. There are many other factors the forecaster must consider. Is the icing widespread and expected to be moderate? If not, then a forecast won't likely be issued. Is it convective? If so, then an AIRMET won't be issued since AIRMETs are for non-convective icing. There are times where there could be dozens of reports of light icing and maybe even a moderate or severe mixed in, forecasters must carefully consider their relevance before issuing an AIRMET. The fact may be that the forecaster was issuing the next round of AIRMETs and icing was expected to develop in that area.

Scott Dennstaedt
CFI & former NWS meteorologist
Administrator of AvWxWorkshops.com
Most pilots are weatherwise, but some are otherwise™

Edited 6 time(s). Last edit at 07/10/2014 05:57PM by Scott Dennstaedt.