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"Known Icing, or is it?”



Fellow pilots, “tis the season” as they say.  With the holiday season behind us I’m not talking about presents, lights, and cheer.  I’m referencing the season of cold, dreary weather in which moisture in the form of aircraft structural icing occurs.  Hopefully every pilot who is reading this remembers the FAR 91.103, “Preflight Action”, which states “each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. Specifically, subsection (a) “for a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecast”.  As a result, we all should be checking the weather to some degree prior to blasting off into the wild blue.  But what if the current weather products make your Spidey Sense tingle just a bit?  What if that wild blue isn’t so blue but rather a dark grey?  I like to teach the mentality of “can we take off, can we land, what is it like in between?” As pilots, how do we know if we are going to encounter icing conditions?  What is known icing anyways? Have you ever noticed that most aircraft flight manuals or operating handbooks and other related documents use the term “known icing conditions” vs “known ice”. Sadly, unless you’re reading a PIREP submitted by another pilot who encountered actual icing conditions leading to structural icing, the answer of what is known icing isn’t that clear.


The FAA defines known icing in the AIM, Chapter 7-1-20 as follows: “Actual ice observed visually to be on the aircraft by the flight crew or identified by on-board sensors". The AIM further goes on to describe the different types of icing (AIM Ch 7).  Actual adhesion to the aircraft, rather than the existence of potential icing conditions, is the determinative factor in this definition.  Well, what about an AIRMET Sierra on a cold winter day or even an AIRMET Zulu?  Surely this must mean there are icing conditions at play.  Not so fast, once again the FAA defines these in the AIM as a “concise description of the occurrence or expected occurrence of specified en-route weather phenomena that may affect the safety of aircraft operations”.  I want to draw your attention to the part about “expected occurrence”.  One can assume with cold temperatures and the presence of visible moisture that aircraft structural icing will occur.  Does that mean if I fly an aircraft high into the flight levels and fly through a cirrus cloud, I’ll pick up icing?  The answer is you could but highly unlikely.  For icing to occur there must be two elements: 1) the presence of visible moisture, and 2) the aircraft surface temperature at or below zero degrees Celsius. The FAA does not necessarily consider the presence of clouds (which may contain only ice crystals) or other forms of moisture at temperatures conducive to the formation of ice or to constitute known icing conditions.  Remember, AIRMETs are intended to inform pilots of potentially hazardous weather phenomena.  An AIRMET Zulu still doesn’t mean known icing. There are many variables that determine whether ice will be detected or observed. The size of water droplets, shape of an airfoil, route of flight, altitude, and speed of the aircraft to name a few.  Luckily for us the FAA has shed a bit of light on the topic in a 2009 Letter of Interpretation sent to the Aircraft Owners and Pilots Association (AOPA) in response to a request for clarification on the topic (read here).  In the letter the FAA addresses several key factors regarding “known icing” and “known icing conditions”.  Known ice being a situation where ice has been detected (aircraft systems) or observed (pilot) and known icing conditions as circumstances where a reasonable pilot would suspect the likelihood of ice formation on the aircraft. 

At an early age in our pilot careers were taught the grave dangers of flying an aircraft that wasn’t certified for flight into known icing (FIKI) into such conditions.  My hopes are no one reading this has intentionally done so. If by chance you did your due diligence per 91.103 and inadvertently experienced structural icing while flying an aircraft not FIKI certified, you know how humbling it can be.  One of the benefits of being a Designated Pilot Examiner (DPE) is the ability to test pilot applicant knowledge on the topic of icing.  For the most part the answer is unanimous, “visible moisture and freezing conditions”.  As we have learned, there is some truth in this.  However, as pilots we must exercise due diligence and look at the bigger picture.  There are several excellent approved resources at our disposal to help pilots make an informed “go/no go” decision.  Even if an aircraft is FIKI certified, it would likely be a poor decision for a pilot with little to no icing experience to fly in such conditions.  As an option, fly with another more experienced pilot or make the wise decision not to go at all.  A wise pilot once said, “it's better to be on the ground wishing you were in the air than to be in the air wishing you were on the ground”.


Fly safe my friends.


Andrew Reischauer (ATP/MEI/DPE)

Asst. Chief Instructor

Specialized Aero Works

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