You are about to go on a helicopter training flight at a local airport. There’s an Airmen’s Meteorological Information (AIRMET) Zulu posted for icing inclusive of your area of flight. Your helicopter has the following limitation written in the Rotorcraft Flight Manual (RFM): “Flight into known icing conditions prohibited.” Can you fly?
Whether you are flying an airplane or a helicopter, weather is one of the most poorly taught subjects in basic flight instruction. Did you like that? “Whether” and “weather.” Moving on.
It’s an often poorly taught subject because it can be confusing and sometimes contain information we see as irrelevant. I was lucky enough to be forced to take weather classes in college, so I got a bit more exposure to the subject than that contained in pilot ground school requirements. I learned a lot from that experience, and highly recommend all helicopter pilots spend extra study on weather.
Basic Icing Science
From a scientific perspective, we know that the accumulation of ice on an aircraft requires (1) visible moisture, and (2) the temperature of the collecting surface being at or below freezing.
Typically, most aircraft icing occurs in supercooled clouds, as the visible moisture present consists of “liquid drops…at outside air temperatures (OAT) below 0 °C (32 °F) in these clouds.”1 As helicopter pilots, this is typically not encountered as most of us are flying for Visual Flight Rules (VFR) operations, which would prevent flight inside clouds. However, we can essentially encounter the same conditions in Freezing Fog—fog is just a cloud on the ground after all.
Additionally, we are concerned with icing from Freezing Rain and Freezing Drizzle, both of which involve supercooled water droplets striking an aircraft, transforming into ice, and adhering to said aircraft. While flight in dry snow is not an icing hazard—the moisture is already frozen and thus can’t adhere to the aircraft—wet (or “mixed”) snow can be hazardous as it will include supercooled water droplets. If “snow” or “snow and mist” are reported and temperatures are only slightly below or above freezing, icing could certainly occur.1
There are a few more considerations, but that will turn this article into a whole lesson just on icing itself. What we are concerned with in this article is determining whether we are compliant with limitations attributed to “known icing conditions.”
Known Icing Conditions
The limitation you read in the RFM used the phrase “known icing conditions.” What exactly is that? Here’s the definition from the Aeronautical Information Manual (AIM):
Known Icing Conditions
Atmospheric conditions in which the formation of ice is observed or detected in flight.
Note: Because of the variability in space and time of atmospheric conditions, the existence of a report of observed icing does not assure the presence or intensity of icing conditions at a later time, nor can a report of no icing assure the absence of icing conditions at a later time.2
The problem with this definition is that it requires observed or detected conditions in flight. If this definition were followed to the letter, you’d be legal to fly in freezing rain all the way until icing began to form, completely disregarding the fact that you intentionally flew into conditions that were absolutely sure to produce icing.
On the positive side, the note attached to the single-sentence definition of known icing conditions indicates that a single report of observed icing does not necessarily mean that we will encounter icing conditions at a later time nor vise versa. Just because a pilot encountered icing 30 minutes ago doesn’t mean we will right now, in the same place.
Determining The Presence Of Known Icing Conditions
I mentioned an AIRMET Zulu in this scenario. Does an AIRMET for icing mean that there are known icing conditions and you can’t fly? AIRMETs cover thousands of square miles and multiple states, and merely report that conditions in its effective area are conducive to icing, usually indicating just an altitude range. Obviously, with such a large area being discussed, we won’t get icing in every single location it includes, nor can an AIRMET predict the timeframe such conditions would even exist with perfect accuracy. An AIRMET for icing does not automatically equal the presence of known icing conditions, however, it is one of many factors to consider in the process of determining the presence of known icing conditions.
The Federal Aviation Administration (FAA) recognized the confusion on this issue. In a 2009 response to a letter written by the Aircraft Owners and Pilots Association (AOPA), the Office of the Chief Counsel indicated that the FAA does not “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.”3 They recognized that various factors come into play.
They further went on to explain that if we actually started accumulating icing on an aircraft restricted for flight in such conditions, the determination of whether or not we violated the RFM’s limitations—and thus violated 14 CFR 91.9—depends upon an evaluation of the pre-flight and in-flight actions taken by the pilot.
Basically they ask and answer the question: Were the pilot’s actions reasonable?
“The FAA will specifically evaluate all weather information available to the pilot and determine whether the pilot’s pre-flight planning took into account the possibility of ice formation, alternative courses of action to avoid known icing conditions and, if ice actually formed on the aircraft, what steps were taken by the pilot to exit those conditions.”3
Remember 14 CFR 91.103? It states that “(e)ach pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight.” Our pre-flight planning must include evaluating all available meteorological information relevant to our flight, including our own surface observations, Aviation Routine Weather Reports (METARs), Winds & Temperatures Aloft Forecasts (FD), Terminal (TAFs) and Area Forecasts (FAs), AIRMETs, Significant Meteorological Information (SIGMETs), and Pilot Reports (PlREPs). I like to also include a review of HEMS Tool, which has some good icing probability/severity overlays available.
After evaluating these reports, if we choose to fly, we still must have an exit strategy if we unexpectedly encounter these conditions. I have encountered unexpected icing twice. In both instances, I already had an exit strategy planned and executed successfully. These could include altitude or course changes, or even landing and waiting it out. Helicopters certainly offer more flexibility in safe response to such encounters.
The FAA says it best: “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 ice to adhere to the aircraft along the proposed route and altitude of flight, then known icing conditions likely exist.”3
All pilots should take the time to learn as much as they can about icing conditions, and I highly recommend sitting down and reading the excellent Advisory Circular AC91-74B: Flight in Icing Conditions.
Proper Planning Prevents Poor Performance,
- Federal Aviation Administration. (October 8, 2015). AC91-74B, Pilot Guide: Flight in Icing Conditions. Retrieved January 16, 2017 from https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-74B.pdf
- Federal Aviation Administration. (May 26, 2016). Aeronautical Information Manual, Table 7-1-9. Retrieved January 16, 2017 from https://www.faa.gov/air_traffic/publications/
- Kerry B. Long, Chief Counsel, Federal Aviation Administration. (January 16, 2009). Bell-AOPA Letter of Interpretation. Retrieved January 16, 2017 from https://www.faa.gov/about/office_org/headquarters_offices/agc/pol_adjudication/agc200/interpretations/data/interps/2009/bell%20-%20(2009)%20legal%20interpretation.pdf