Most of what the average pilot thinks they understand about “known icing” is either confused or wrong – there is no prohibitive reg. under part 91 ( icing is an airframe limitation). Many misconceptions are implanted by well-meaning educators who are also mistaken or confused – sorry but let’s fix this now. I discover this ignorance continuously on flight tests, where applicants are supposed to understand this information (for good reason). I also witness confusion when discussing winter flights in the clouds with other pilots. It is not uncommon to see fear, misunderstanding and also incriminating accusations. Many well-meaning pilots condemn any flight in a “cold cloud” between September and June. A good place to start is with this Chief Council Opinion .
Flight into known ice is not directly referenced in part 91 and known icing conditions are only referenced in subpart F, which applies to large and turbine-powered multiengine airplanes and fractional ownership program aircraft. However, there are provisions in other subparts within part 91 that require a pilot to consider the consequences of flying in such conditions.
This is also an excellent “fire starter” for an exciting hangar discussion. Certainly, one sure way to create safety in aviation is always to just stay on the ground. But unfortunately, we have created a “boogeyman” that protects the innocents but also might keep every other pilot without a FIKI (flight into known icing) airplane from flying IFR (or even VFR) all winter long. This “cold cloud avoidance club” also points an accusing finger at any pilot who goes flying in the winter weather implying they are crazy and unsafe. This is an unfortunate situation that creates more heat than light so let’s dig in and find some middle ground.
Obviously, caution is a good thing, and I would be the first to admit there are definite, definable risks in winter IFR (as there are everywhere). But ignorance, denial, fear, and finger-pointing are not good strategies for successful aviation. As in any risk-management situation, we need to acquire definite skills, knowledge and training to fly safely in the winter weather. There are clouds that are safe and strategies to mitigate the icing risk. Please stay with me here and let’s poke the bear a little.
The legal prohibition – “operating limitation” – is in your POH or AFM, and you might have to do a little digging to find it. In addition to “approved” and “prohibited” operations for Flight Into Known Icing (FIKI), there are several levels of “semi-FIKI” aircraft, so the plot thickens. (Read your data carefully, you are legally bound to these restrictions). The FAA’s legal “gotcha” is actually CFR 91.13 prohibiting “careless and reckless operation” – which is included in every pilot violation or sanction. As always, you are PIC and you choose your conditions and tools to conduct aviation as you see fit; just do it legally and safely.
The definition of “known icing” has been changed numerous times by the FAA over the years. To find a solid legal definition you need to consult several good sources: Chief Counsel Letter of Interpretation, the Federal Register, the Advisory Circular and the AIM. Reading and understanding all of these in detail is the first step in flying safely in potentially icing weather; know the rules and cautions. (Also a good idea for flight tests)
“If the composite information indicates to a reasonable and prudent pilot that he or she will encounter visible moisture at freezing or near-freezing temperatures and that ice will adhere to the aircraft along the proposed route and altitude of flight, then known icing conditions likely exist.
Most recently – and what I hear often on most flight tests is – “known icing” is indicated (and flight prohibited) by “a current PIREP reporting icing.” For years this was regarded as the sure arbiter of “known icing,” and it certainly might be a time not to fly. Recently, the FAA, in the AIM, seems to even be backing off from this criterion as a definition or legal justification for pilot violation:
“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.”
The definition of “known icing” migrated from overly permissive to excessively restrictive in the late 1990s, limiting flight in any visible moisture below freezing. This draconian interpretation got the “cold cloud avoidance” started. As such, “known icing” became the boogeyman everywhere and always in winter clouds and the only legal IFR solution for non-FIKI pilots was “park it till June.” (Sorry for friends south of the equator). BTW, the ancient history of the “known icing” legal debate can be found here in AvWeb.
The current interpretation, issued in 2009, allows for pilot discretion in evaluating and choosing a “reasonable and prudent” course of action in most conditions: CFR 91.3 rules: (PIC). And if you examine most “textbook” icing accidents, you will see some really terrible conditions pilots failed to avoid and usually bad decisions made with partial information or “emotional planning tools.” In giving pilots discretion, the FAA is also providing enough rope to hang themselves. But “legal” does NOT mean “safe” and as in all aviation decisions, be comprehensive in your planning and cautious in your decisions.
how does a safe pilot mitigate risk and fly IFR safely in the winter?
Parking the plane until June in the North is super safe, but totally ruins any utility and efficiency in aviation (cold cloud club). Launching without care or preparation to “get ‘r done” is a really bad expedient. Between these two polar forces of excessive caution vs. efficiency is where we negotiate safety in aviation. Flying on the east side of the Great Lakes for 40 years, I have seen pilots on both sides of this caution equation. I think the best answer is to prepare more carefully, fly with more caution (acknowledging the known hazards) and allow a greater margin of safety for escape in the event of surprises. Guidance for known ice approved operations is here in FAA AC 91.74B
So first, safety in potentially icing conditions requires a more comprehensive and careful preflight analysis, and there are amazing new tools especially from the NWS Forecast Icing Potential, FIP and Current Icing Potential: CIP CFR 91.103 (all available information). Knowing weather theory is also essential since this process is in motion. (Scott Dennsteadt’s excellent weather book is a great start for pilot education)
Second, obtain (and issue) PIREPS 
for any and all changes (especially for tops and temps.) PIREPS are the best real-time peer-to-peer information sharing system we currently have. During dynamic winter conditions where a few degrees of temperature make all the difference, information sharing is essential.
Lastly, always assure a safe escape route if you suddenly encounter icing, since it is impossible to forecast ice precisely and the consequences of unforecast icing are both terrifying and dangerous. Clear air below (but above the MEA) is best, but lateral diversions and big-picture awareness are essential for safety.
The FAA’s newest icing AC 91-74B is well written with lots of good information. And the NASA Glenn Research Website has a very good online training for pilots I highly recommend – knowledge, information and caution. Finding a savvy CFI with lots of experience in winter weather is your best educational resource (as always) since you need a mentor to explore any potentially-hazardous phenomenon safely. Stay warm and fly often!
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The FAA interpretation you mention actually contradicts itself and doesn’t answer the question fully, as usual. The relevant quote is, “‘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.” However, just prior to this statement the FAA states, “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.” This interpretation contradicts the most updated Advisory Circular on icing, AC 91-74B Pilot Guide: Flight in Icing Conditions (dated 10/08/2015), which states, “Pilots can reasonably expect inflight icing when flying in clouds with temperatures at or below 0 °C (+32 °F).”
The overwhelming likelihood is that no one but you and the others aboard your aircraft will ever know if you actually encountered icing. Unless you go blabbing to the ramp inspector or they see you taxi in with 1″ of ice on the wing, OR, more likely, you have an accident due to icing, the FAA cannot and will not take enforcement action against you simply for flying somewhere that maybe, possibly had ice. Especially if you have valid arguments as to how you avoided icing or why you did not encounter it despite there being forecast conditions.
That said, if you are a conservative pilot, I think the “cold cloud club” is correct. As AC 91-74B says (and in my personal experience flying in the winter up north), simply being in clouds at or below freezing can cause ice to accumulate on the aircraft. If ice does not adhere or melts immediately after contact, I do not personally consider this known icing, but I have seen cloud layers with no precipitation, but below freezing temps–typically behind frontal activity–cause moderate icing on an aircraft. Clouds and cold temps are undoubtedly “known icing conditions” per the FAA definition even if there is not “known icing.”
An example of an important practical versus legal distinction is the Cessna 208 (which ostensibly is equipped for FIKI). The C208 is now restricted in its icing operations due to multiple accidents that occurred due to ice accumulation. Despite being a turboprop–with all the utility that provides–the marginal performance of the aircraft when loaded makes icing particularly dangerous if the de-ice equipment fails, runback/SLD occurs, or de-icing equipment cannot keep up with the rate of accumulation (which, in my experience flying the aircraft, is frequent enough to be concerning). I say this to illustrate that a combination of aircraft performance along with anti-/de-icing capability is really needed to operate in known icing with a high safety margin.
Additionally, due to the way the FAA and NTSB prosecute violations of the FARs, no one legal interpretation is sufficient to make sure you are protected legally. The FAA and NTSB will consider you guilty until proven innocent in the civil case against your certificate or for fines to be imposed (it is not a criminal trial). And rest assured they will have well beyond preponderance of the evidence to throw at you for your supposedly unsafe behavior if they actually send you a letter of enforcement action. The FAA can violate you on any of 91.3, 91.9, or 91.103 for this sort of operation, so an interpretation of “known icing conditions” isn’t even necessary to find you at fault if they can prove via other evidence that you violated the FARs. It’s better to not tempt the FAA into enforcement action by flying in conditions for which most people would agree a light GA aircraft is not equipped.
My advice is: unless you fly an aircraft with a significant performance margin to reach high altitudes quickly and/or an aircraft that cruises at high airspeeds to exit icing quickly or give you longer to identify the effects on the aircraft associated with significant ice accumulation, I would not attempt even “possible” icing conditions such as clouds + freezing temps without full anti-ice/de-ice equipment that is certified for moderate icing conditions. Another important risk factor is aircraft performance versus terrain height. An aircraft may perform adequately below 10,000′, but in mountainous terrain have little climb margin to exit conditions or maintain altitude as ice accumulates.
Ultimately, slow training aircraft such as the venerable Cessna 172 do not have the performance margin to exit icing conditions in a safe manner. Often pilots of these aircraft find their options have quickly run out and the only choice left is to descend below the Minimum IFR Altitude and risk obstacle or terrain collision to find clear air–and we know how those flights often end up.