Truth In Icing

There are a lot of myths and rationalizations about airframe icing. Lets set some of them straight.


I received a call from the owner of a turbocharged, high-performance single who lives in the Great Lakes region, well-known for icing conditions in late autumn, winter and early spring. His airplane was equipped with an aftermarket TKS-style ice protection system and was not FAA-approved for flight in known icing (FIKI). The pilot wanted to discuss strategies for flight during the cold times of the year, including insights into conditions where icing layers are vertically thin and/or rates of ice accumulation are typically light (or even only a “trace”).

I kept circling back to the fact that his particular airplane was not FIKI-approved and that he does not have the authority to make a “quick climb” or “quick descent” through an icing layer, to take off when airframe ice is reasonably possible where he will fly, or to begin a descent and approach if the airplane ahead has reported even a trace of ice.

“Are you telling me I can’t fly my airplane in IMC at all in the winter?” the pilot fumed in response. “Yes, that’s exactly what I’m telling you,” I replied. The caller wanted me to support his airplane’s desired utility and teach him specific techniques for minimizing ice accumulation. I had to tell him that any ice at all is enough to cause him to divert, delay or cancel a flight, whatever it takes to remain away from “known ice.”

That call reminded me of rationalizations many pilots make about airframe ice they believe may legitimize flight in icing conditions in airplanes not certificated to do so. It also suggests some strategies of ice avoidance. Let’s look at some common ice rationalizations and seek the truth about airframe icing.

Keep Your Speed Up

The greatest hazard of airframe ice is the adverse and unpredictable effect ice has on aerodynamic surfaces. The wings and tail will be less effective with even small amounts of ice; further, since ice accumulation is not always symmetrical, there’s a strong likelihood the adverse effects will come unexpectedly, without warning. If airflow disruption is the biggest hazard, it seems reasonable that simply keeping your speed up will overcome the adverse effects and assure a safe landing with a load of ice. This rationalization is flawed.

In most airplanes, keeping the speed up with significant ice accumulation requires putting the airplane into a descent—sometimes a pretty dramatic one. One problem is the airplane may not have the control authority to recover from the descent when it comes time to land.

Secondly—and frequently overlooked in discussions of landing with a load of ice—forward visibility can be reduced to nil with only a very slight coat of frozen water. At night or in poor visibility, even small amounts of accumulation on the windscreen can distort visibility, making landing difficult. Regardless of the time of day, it’s very likely that you’ll have to resort to Lindbergh-style peripheral vision out the side windows to flare (such as it is with ice) and land.

Rationalizing that “speed saves” in ice is correct as far as keeping the wing flying. But keeping your speed up is not a reliable method of bringing an iced-up airplane in for a safe landing, especially on a typical GA runway. If ice is encountered, execute your preplanned “out” to remove it before you make an approach and landing requiring higher speed.

More Power Is Better

Rationalizing that it’s better to have extra power than to be able to remove ice, is correct…to a point. Extra power may allow you to climb through an icing layer (legally or not), but eventually you have to come back down. If you encounter icing in descent and/or on approach, the extra power won’t help.

Wet, slushy snow or volumes of clear ice can block an engine’s air inlet, reducing power. Alternate induction air sources typically are in the low-pressure area of an engine compartment. If the inlet air filter clogs with ice or snow, the alternate induction air pressure is low enough that the turbocharger may not provide full power. Some airplanes will experience reduced manifold pressure at full throttle when drawing induction air through the alternate air door.

Further, an iced-up propeller becomes less efficient at converting twist into thrust—it’s an airfoil, too—and the wings and tail become aerodynamic unknowns. What has been said about the effects of density altitude on airplane performance goes for airframe ice accumulation as well—you can’t turbocharge the propeller, wings or tail. Rationalizing that power overcomes ice does not hold up if the pilot is unable to escape to an area where ice is shed before descent and landing. By itself, turbocharging is not an ice-avoidance strategy.

A Small Chance Of Icing Is OK

One of the most significant advances in winter-weather risk management in the past few years is the introduction of the current icing potential (CIP) and forecast icing potential (FIP) briefing products. These charts respectively show the likelihood and anticipated intensity of ice accumulation. Some tablet-based flight planning software packages enhance the charts and break them down by altitude to make them even more usable. Some examples from are reproduced on the right.

Weather information generally has gotten better in recent years, with major improvements in graphics. Icing observations and forecasts got that treatment, too, with the current icing potential (CIP) and forecast icing potential (FIP) charts being an example.

I’ve found the forecast likelihood of airframe ice makes some pilots consider flight where the anticipated accumulation is “light” and the probability of encountering ice is low (less than 25 percent). It’s true that these conditions are less likely to result in a life-threatening ice encounter, but they are still icing conditions—something to avoid in airplanes not certificated for ice, and to be escaped from in a FIKI aircraft if accumulation begins.

‘Known Ice’ Means It’s Safe

Even in a “known-ice” airplane, vision may be severely limited by ice. At its best, a windshield “hot plate” deicer opens only a small rectangle for forward visibility. Often this clear zone is in the center of the windshield, not directly in front of the pilot.

Graphical presentation of the freezing level also helps with preflight planning.

Certification for flight in icing conditions has definite limits. It generally does not include approval for flight in icing conditions resulting from supercooled water droplets (common in cumulus clouds) or in freezing drizzle/ freezing rain. “Known ice” in the airplane’s limitations therefore does not mean the airplane is safe (or legal) to fly in any kind of airframe ice. What it does is provide the pilot with the option of legally attempting a flight in areas of suspected rime or areas of very limited clear or mixed icing.

Prudence dictates that the pilot have a clear, preplanned escape route into ice-free air even (or especially) if he/she intentionally exposes the aircraft to areas where airframe ice accumulation is likely. Rationalizing that “known-ice” airplanes can fly in any ice-laden air is true to a point. But it does not hold true if the pilot does not escape icing conditions if they are found.

So, yes, an airplane not certificated for flight in icing conditions is essentially prevented from flight in IMC for that portion of the year when air temperatures are likely to be near or below freezing at the altitudes the pilot wishes to fly. Strategies for avoiding ice include:

  • Flying only in VMC (whether under VFR or IFR).
  • Flying at altitudes that remain below the freezing level.
  • Flying in the warm air of an inversion with no precipitation or a cloud layer below with temperatures conducive to ice formation.

It’s surprising how frequently wintertime flights can be made using these strategies. Yes, it’s yet another level of hazard that impacts your go/no-go decision-making and just like thunderstorms in warmer times of the year, often it takes only a short delay or diversion to avoid the risk of airframe ice.

What Is Known Icing, Anyway?

To the extent there’s an FAA definition of known icing, it’s in this paragraph from a 2009 letter from the agency to AOPA:

Any assessment of known icing conditions is necessarily fact-specific. Permutations on the type, combination, and strength of meteorological elements that signify or negate the presence of known icing conditions are too numerous to describe exhaustively in [the FAA’s] letter. Whether a pilot has operated into known icing conditions contrary to any [aircraft] limitation will depend on the total information available to the pilot, and his or her proper analysis of that information in evaluating the risk of encountering known icing conditions during a particular operation. The pilot should consider factors such as the route of flight, flight altitude and time of flight when making such an evaluation.

The FAA lists these products, at a minimum, as those a “prudent” pilot would evaluate to determine there is no likelihood of airframe ice accumulation:

  • Surface observations [METARs]
  • Temperatures aloft
  • Terminal forecasts [TAFs]
  • Graphical weather (supersedes the discontinued Area Forecast)
  • Pilot Reports

Flying A 150-Knot Ice Cube

Way back in 2008, the FAA published a discussion of the safety challenges posed by flying in icing conditions. The full four-page document (PDF) is available online. One aspect of airframe icing explored was airplane certification and handling.

Stall Warning

“Prior to 1973, small airplanes were not…tested to the icing condition standard used today…. Small airplanes certificated prior to 2000 were given certification credit for natural aerodynamic stall warning (buffet) even if the airplane was equipped with a stall-warning system. Since ice buildup on the wing lowers the stall angle of attack, the stall-warning sensor might not provide warning in icing conditions.”


Pilots should treat their POH/AFM “recommended” minimum icing airspeeds as limitations. “If your POH/AFM does not have minimum icing airspeeds, add 15 to 20 knots indicated airspeed to your normal operating airspeed. This goes for all phases of flight, including approach and landing, where most small airplane icing accidents occur. Also, treat any buffet or vibration as an impending wing stall, and limit maneuvering in icing conditions.”


“Pilots should consider the climb performance of the airplane and the route’s minimum altitude when determining routes and exit strategies in preflight planning. The airplane’s climb performance will be degraded in icing conditions.” Changing altitude to warmer or drier air may get you out of icing. But “don’t assume you can climb with ice on your airplane.” Stay at or above the minimum icing speed and consider a 180-degree turn. You may need to enter a slight descent—trade altitude for airspeed—to remain above that speed.


Of course you wouldn’t knowingly take off with snow, ice or frost on the airframe, if for no other reason than it hasn’t been tested. “No airplane manufacturer, nor the FAA, has an established procedure for polishing frost, and the lift and drag penalties are unknown…. Make sure critical surfaces such as lifting surfaces, control surfaces, propeller blades, and engine inlets are free of contamination.”

This article originally appeared in the December 2019 issue of Aviation Safety magazine.

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  1. I don’t have a lot of faith in icing forecast predictions. The probability of icing is 0 or 100 percent. I have had ice in conditions when there should not have been any and sailed through clouds where I was sure I was going to get ice and got nothing.

    FIKI for smaller piston aircraft is missed named, it should stand for Finding Ice Knot Ideal. If your flight doesn’t have an out where you can escape ice then you should not be going even in a FIKI aircraft.

  2. That same pilot who wants to fly through ice is probably the same type of person that wants to overload his vehicle, then attach a trailer.

    Buy the right tool for the job, or don’t do it.

  3. T’was 1969, Parkersburg, WV.
    Learning the ropes of charter flying in BE-18s. A freezing rain one afternoon and in quick succession an old Aero Commander and a Piper Twinkie taxied to the ramp after landing. Slack jawed I gawked at the obscene ice formations projecting forward from all surfaces. As I recall the Aero Commander had boots yet was still an ugly mess. The engine air inlets on the Twinkie were almost closed over with ice. Early that spring pregnant bride and I drove the overloaded VW Bug south to where the sun -does- shine. Never looked back.

  4. This is required reading. No ice means just that ! And do not be fooled by the ones who got away with ‘a little ice’ – they were just lucky. I’ve had ice trouble in heavy turboprops and jets which were certified for icing, so have learned to have huge respect for the phenomenon.

  5. This is a great article. For folks who want to learn more, one of the best resources I’ve found for icing theory is a book by Scott Dennstaedt that is primarily about how to read Skew-T charts but has a few chapters at the end about SLD and icing in general. Google his name and Skew-T book if you have any trouble finding it!

  6. Well, this is why I enjoy this blog. If you’re not continually learning while outside the flight deck, regardless of experience level, this flying business will teach you lessons while you’re at the controls. I hate those unexpected lessons.

    Ice is the sneaky neighborhood felon of this gig. Icing and bravado is really a disregard for human life.

    My inexperienced self loved the power of the BE-1900’s PT6’s and I absolutely wanted to punch the guys who invented de-ice boots. Waiting to “build up” enough ice before popping the boots, in my mind was a ridiculous, nerve rattling concept.

    I never understood how the designers couldn’t steal some bleed air, or electrically heat the leading edges. They figured out how to heat the props.

    I’m pretty convinced even with the leading edge clean, the dang stuff could build up on the air frame, in a shockingly short period of time, straining the mighty PT6’s enough to have to get the heck out of whatever innocuous conditions I found myself “filed” in.

    Anyway, now I’m Googling and going to read up on Skew-T charts (something I grasped enough decades ago, barely enough to pass a final exam).

    A lot of times the comment section here gives me as much information as the articles.

    Good stuff here folks, thanks.

    • Check out a Piaggio Avanti. It has a bleed air heated wing for ice protection, along with an electrically heated forward wing, and moveable internal ice vanes for the inlets. Props get heated by the exhaust. No ice protection on the tail, none needed. The biggest problem with the heated wing is that it was almost impossible to keep the leading edge painted, due to erosion, if the plane flies a lot in moisture. The Piaggio engineers had to go with rubber inflated boots on the exterior of inlets due to lack of electrical power available. Even with all of this ice protection there was one time while flying in icing that I noticed that I was adding power to keep speed up due to the accumulation of ice on unprotected surfaces.

  7. Departing Taos, NM one July morning, I arrived at the aircraft at 0530 on a clear, star sparkling morning, to find the airframe completely covered in ice! The black body radiation had cooled dew drops to the point that they froze! It took quite some time to clean the air frame of the majority and wait for the rising sun to do the rest.

    Back in the 70s I was in cruise flight, in rain, over Tennessee at altitude in a 172 when I began losing power. After checking the mags, mixture, and fuel selector, I decided to try carburetor heat. That was it! I’d never heard of carb ice at cruise power settings before.

  8. I got my Commercial, Airplane Multi-Engine (Limited to Centerline Thrust), Instrument Airplane ratings in Jan 1974 and, at the time, could read and understand a Pirep.

    So, today, with MegaBytes/GigaBytes/TerraBytes of computer power, can someone tell me why a pirep looks like this: PIREP: LBF UA /OV LBF/TM 1519/FL060/TP CL35/SK BASE003 TOPS 040/RM ZDVFDXV.

    What the hell is that? Why can’t a pirep be written in English or Swahili? At least I wouldn’t need to dig out some obscure reference to tell what it says.

    BTW, I’ve flown many a PAR to 100 and a quarter and as many ILS to 200 and a half without a flight director or an autopilot, in a single engine jet, many times covered in ice. And, while I’m ranting, just when and why do me need LIFR, IFR, MVFR, and VFR? As far as I’m concerned, 1000 and 3 is VFR, lower is IFR.

    • “ Why can’t a pirep be written in English or Swahili?”

      Because that’s the way it was, and we liked it!

      Seriously, that’s one of my pet leaves too. Long gone is the teletype and Morse Code, and we simply can’t transition to plain English (or Swahili).

  9. I agree with Dexter above that deice boots seem to be a good way to get yourself killed. Having to wait until just the right time to inflate them is a pretty dicey situation. I guess they were the best we had way back then, but it seems there are better alternatives now. I have only had a few encounters with icing, but it has always amazed me how quickly the ice will accumulate under the right conditions. We picked up over a half-inch of rime ice one day just flying through a cumulus cloud over the Rockies. Summer day and we were in the cloud maybe 30 seconds.

  10. Ice protection on most normal and turbo-prop driven transport category planes buys you time to get out of icing, not continuously stay in icing. Until the icing AD that came out in 2006 on Caravans, there was at least one pilot a year getting in over what the plane could handle, and not surviving the end resulting crash. The author makes a valid point about the legalities of the phrase “certified for known icing conditions”. On the other hand pt135 makes no mention of “ certified for know ice” All the pt135 regs say is that to fly in icing the plane must have ice protection on the wings, windshield, inlet or intake, prop, tail surfaces, or other critical surfaces. Most jets can handle much more ice than prop driven planes because they have much more excess power at lower altitudes. This does not mean jets are immune. In the late 1980’s a B1 bomber crashed due to ice. B1’s do not have ice protection on the wings or the inlets. I know this because I worked for a company trying to design an ice protection system for the inlets before the project was dropped in the early 1990’s.