Icing diversions—an overlooked threat

Two recent accidents related to encounters with in-flight icing while operating an airplane not equipped or certificated for icing highlight some of the inherent risks associated with such encounters. In the first, a private pilot operating a Cirrus SR22 on an instrument flight plan encountered ice and elected to divert. He failed to capture the localizer on the first attempt, and during the subsequent radar-vectored maneuvering for a second attempt, appears to have become spatially disoriented and crashed. In the second, a commercial pilot operating a Beech G33 Bonanza on an instrument flight plan encountered icing and obtained a clearance to descend to 3000 feet. At this altitude, the pilot reported that he was below the cloud layer and he believed he was below the freezing level and that the ice was shedding. However, the airplane continued to descend until impact. Investigation at the site identified substantial chunks of ice conforming to the airfoil shape. No one survived either accident.

In a study of icing accidents that I presented as a paper for the American Institute of Astronautics and Aeronautics in 2006, I identified 142 events in which the pilot made the decision to land due to ice accumulation; in 84 of these, the decision was made before any aerodynamic consequences had been encountered. In only 23 of these 142 cases was a successful precautionary landing made. In the remainder, aircraft damage and/or personal injury or fatality was the result. This has to be tempered with the knowledge that there may be hundreds of cases in which a successful diversion was executed without any type of report generated, whether that be an official investigation or an ASRS report. That said, these results strongly suggest that the default strategy of simply diverting and landing when ice is encountered may not be as reliable as we commonly believe. In many of the cases that I have studied, the diversion ends with a hard landing and structural damage. In some, it results in an off-airport landing; in the worst cases, a loss of control.

ice chart
These charts are really important.

In the first accident cited above, the role that icing played could not be determined, beyond triggering the decision to divert. It is likely that the pilot was not able to program the aircraft flight management system either properly or quickly enough in the developing situation. It remains possible that the effects of ice accretion hampered the autoflight system, and/or the manual handling characteristics of the airplane with ice accretion created a significant distraction. At the very least, he was not prepared to execute the diversion.

In the second case, the pilot reported outside temperatures of 34 degrees F and later 39 degrees F to air traffic control after he had descended below the clouds. The freezing level was forecast to be pretty much right at 3000 feet, and the surface temperatures at the nearest reporting site, 43 miles away, were dropping and recorded at 3 degrees C around the time of the accident. The presence of ice at the crash site hours after the accident seems to indicate that the pilot’s belief that he had descended to warmer air was probably incorrect, and the accuracy of his OAT gauge must be considered. Sadly, the Board did not address this issue in the report; such an investigation could have yielded important information. In any event, when reaching air that is marginally above freezing, the shedding of ice may take some time. Indeed, for many years Boeing has required icing penalties applied to landing and go-around performance if ice has accreted on the airframe at any time during the flight and the forecast temperature at the landing field remains at or below 10 degrees C. When he was barely one thousand feet above the ground, the G33 pilot told ATC that he was “doing okay right now” and “waiting for this ice to dissipate…” three minutes after initiating the descent and less than two minutes before he crashed.

The first step in dealing with an icing encounter is usually to try to find warmer air. To do so, the pilot has to be intimately familiar with the atmospheric conditions. Had the G33 pilot reviewed the skew-T charts, forecasts of winds and temperatures aloft, or the Aviation Weather Center’s freezing level graphical presentation (no record could be found of a weather briefing), he would have perhaps realized that there was very little room between the freezing level and the ground. This is particularly important because of the time it may take the ice to shed. With diminished performance and not much altitude, that time may exceed the time available before a landing is required, intentional or otherwise. Knowledge of the forecast air temperatures and freezing level will also serve as a check against the OAT gauge; a couple of degrees variation would not be surprising in either the forecast temperatures or the OAT reading.

If the ice cannot be shed, an immediate diversion is absolutely essential. Unfortunately, one of the standard tropes of aeronautical mythology, namely the idea that the effects of icing are cumulative, can grossly distort the urgency and the need for aggressive action and precise execution.

For decades, the FAA published a cartoonish diagram showing how the four forces (lift, drag, weight and thrust) incur degradations which, when added together, result in a “cumulative” effect. The idea is fundamentally correct, and I’ll discuss that in a moment. However the word “cumulative” suggests linearity, in other words proportionality. In combination with the existing terminology used to describe icing severity (trace, light, moderate and severe) “cumulative” can be interpreted to mean that a little bit of ice is probably okay, a little more is not okay, a bit more than that is a problem, and any more beyond that is a serious threat. Many pilots interpret their own experience within this paradigm, i.e., their airplane “can carry a lot of ice.” This is almost universally incorrect.

In approximately 90 events in my research, I was able to find estimates of ice thickness, either from the pilot or from the investigators on the accident site shortly afterwards. These estimates are highly subjective and not necessarily accurate for the actual time of the event. However, within that set of 90 events, the median of minimum thicknesses was one half inch, and the median of maximum thicknesses was five eighths of an inch. Bear in mind that the term median means that fully one half of the events exhibited lesser thicknesses.

Pilatus ice
“My airplane can carry a lot of ice.” Probably not.

We have known for decades that very small accretions of ice can have serious aerodynamic effects. The most commonly discussed effect is the loss of lift and increase in stall speed. However, the increase in drag is often poorly understood, particularly how it may literally skyrocket when the angle of attack is increased during the landing flare. These effects can be experienced with very thin ice accretions; their precise aerodynamic location on the wing, surface roughness, and even horn angle are crucial factors that cannot possibly be estimated by visual inspection during flight.

Even less commonly discussed is the loss of propeller efficiency. The prop is a thin surface; it is likely to accrete ice earlier than the wing. You can’t see the ice on the prop, but in the end it is still an airfoil and ice will destroy its ability to generate thrust no matter what the RPM or manifold pressure. In this way, these effects are additive (a word I much prefer over cumulative), but individually, none of these effects are linear. Aerodynamics is instead a nonlinear science. Turbulent flow is actually the mother of all nonlinear problems. Small changes in ice shape, roughness, angle of attack and/or load factor can have very non-proportional effects.

What often is linear are the degradations you experience with ice accretion up to the “cliff,” the unknown, almost impossible-to-predict point at which you encounter dramatic nonlinearity and the lift or drag curve simply goes straight south. This can further reinforce the idea of linear effects, particularly if you have never had the pleasure of sailing over the cliff. You notice a speed decay, but have no idea that the angle of attack associated with the next two knots of speed decay will result in a stall without warning. You’re doing fine on the approach, and have no reason to believe that you will fall out of the sky when you flare; but many pilots do. Even Cessna managed to prang a 208 when landing following a test flight with artificial ice shapes attached to the wing. (Meanwhile the idea that ice increases weight is simply absurd unless you are a Zeppelin commander; the amount of weight added by a good coating of ice is almost certainly less than the weight of the fuel you have burned since you took off.)

The next formidable problem to be faced when diverting due to ice accretion is the additional ice you will accumulate during the diversion. A little bit of ice can be quite dangerous, but more ice is never better. You may feel that you have made a good, proactive decision to divert; all of the cases I cited in the 2006 study fell into this category. By the time you actually get to your diversion airport, you may be in much more trouble than you had been to start with.

The next problem is setting up and executing the approach. Even in the large jets that I fly with a two-man crew, a diversion is a very task-saturated event. When diverting an ice-contaminated airplane, you are going to land an under-performing airplane at an unfamiliar airport, probably in very much less-than-optimal weather conditions. You will be aware that time is of the essence, and you may be prone to rushing. In many general aviation cases, you may be faced with a need to circle after executing the approach, and a circling maneuver with ice on the airplane can be a potential death sentence. One of the earliest comprehensive investigations of an icing accident involved a United Airlines DC-3 that crashed while circling to land at Chicago Midway Airport in 1940. Nothing has changed since, and many of accidents I have studied involve an attempted circle to land. Managing the angle of attack and the load factor is absolutely essential.

The key here is to be prudent, and assertive, in placing the airplane in a position from which a stabilized approach to landing can be made. It won’t do to be high, or sloppy on the localizer, or not configured until the last second. Note that a stabilized approach can be flown at higher speeds, and at any flap configuration, so you have options to fly a faster-than-normal approach due to ice. Indeed, you absolutely must be aware of and utilize the manufacturer’s recommended speed additives, and configurations, for ice on the airframe. The NTSB commonly publishes the section of the POH describing the minimum icing speeds, as a supplement to the report in which the pilot is quoted, from his or her statement, as having flown ten or fifteen knots slower than those minimum speeds. But the airplane has to be stabilized by the final approach fix, or one thousand feet during a visual approach.

Ice on wing
Just because you divert doesn’t mean the ice will disappear.

That said, some marginal instrument approach procedures, even when flown perfectly, may leave you in a position from which substantial maneuvering is required to get to the runway. This opens you to the same threats as a full blown circle-to-land approach. Manage the load factor.

Most of us understand the idea of avoiding known icing conditions in airplanes not certificated for flight in icing. Most of us also realize that, if you are going to fly IFR in such airplanes, sooner or later you will encounter some ice. The upshot to this is that you must be thoroughly familiar with the exit strategies; know the freezing level and know your enroute alternates. Study the skew T’s (and learn how to read them, and consider how long it has been since that last sounding). Study the winds and temperatures aloft; study the graphical forecasts for icing and freezing level available on the Aviation Weather Center site or in whatever weather service you subscribe to. Study the approaches to your possible enroute alternates; know which ones you don’t want to try with ice and which you do.

As soon as you make your decision to descend to warmer air, or to divert, you need to be on your toes. Get way ahead of the situation and stay there. Know your automation and flight management system cold, and know how to make it do what you need it to do. As a general rule of thumb, it is wise to disconnect the autopilot as soon as you start to accrete ice. The autopilot will mask handling issues until it simply gives up and hands you a very squirrelly airplane. This means that you must be proficient in hand-flying, bearing in mind once again that you will be in a very high-workload environment. Everything you can do to reduce the workload, in terms of planning, review and familiarity with systems, terrain and airport facilities will be of enormous help.

Manage the angle of attack and load factor very cautiously. Go for the stabilized approach, and be prepared to land with power, as the drag rise in the flare may be quite significant. Put the airplane on the ground by the thousand foot markers. Be prepared to land off the airport while you still have control if the airplane just ain’t doin’ it. Always remember that the safety of yourself and your passengers is all that matters; never try to protect your airplane when performance is marginal and disappearing.

In January of 2016, a flight instructor giving instrument dual instruction to a PA-46 pilot executed a forced landing off the airport after encountering freezing rain following a practice missed approach. Despite correctly operating all of the aircraft ice protection systems, they were unable to maintain altitude enroute back to their home airport. They walked away.


    • The AoA vane should be heated and ice free. Assuming it is, it will measure AoA just like it always does; the problem is that the critical alpha for the ice contaminated wing has changed, and the aircraft system has no way of knowing what it has changed to. The manufacturer may require a manual or automated speed bias to be added to such things as stall warning, and they may require a speed additive for maneuvering and/or landing. These are educated guesses that will cover 90% of the cases, but not all. In the data that I discussed for the article, it is very common to identify clear indications of a stall in the report, and yet find that the crew either made no mention of artificial stall warning or actually stated that the artificial stall warning did not operate.

      • Skew-T charts??? Who in the GA world has knowledge of skew-T charts? I’m not absolving the GA guys from obtaining a proper weather brief but I think your audience is better served by not throwing in charts that are NEVER taught or used by the average bug smasher pilot. (like the ones in your examples G33, SR22)
        Unlike the airlines, the weather isn’t presented to GA pilots upon arrival to their aircraft. There are many sources of approved aviation weather utilized by GA pilots and to my knowledge none of the “normal sources” contain or offer Skew T Charts.
        The FAA doesn’t take adequate steps to find out if weather was obtained or not and I seriously doubt the NTSB does either unless it involves JFK JR type folks.
        A good friend of mine was killed in icing when it wasn’t forecast or observed at the time of departure. She obtained an extensive brief just prior to flight and was a very cautious and professional pilot. (she was flying FAR 135 in a single engine airplane)
        I flew professionally for 21 years in FAR 135 and 91 operations ending my career as the chief pilot for an aircraft manufacturer. Corporate aircraft, not airliners. Not once in my career were Skew T charts ever mentioned or used.

        HS-125, ….. 14000+ hours, 10000+ in turbine

        • Glider pilots use Skew-T charts. They can be found on aviationweather (dot) gov -> search (enter “Skew” in search box) -> results. One extra click and a few keystrokes beyond the normal sources of information found on that page. Many, many well-written articles and tutorials online regarding using and understanding the info in Skew-T charts.

          Student Pilot

        • Despite the fact these icing charts aren’t featured in primary training, anyone dueling with clouds and moisture should have familiarized themselves with available information, especially freezing levels. To be instrument flying without knowing what a skew-T chart is . . . just hasn’t done their homework.

  • A good cautionary discussion.

    I wholeheartedly agree with your endorsement of the skew-T diagram for flight – and diversion – planning in potential icing conditions. Of all the icing products available to GA, skew-T provides the information most useful for extracting oneself from difficulty.

    In some conditions the sensible diversion may be climbing into overlying, warmer, air rather than descending to land if you have a good picture of the vertical atmosphere.

    • Arthur is right about the lack of emphasis and instruction regarding skew-T diagrams by the FAA and by flight schools. He is also right that it isn’t a silver bullet that will guide you to the best decision in potential icing conditions.

      What it does do is show where you’re likely to encounter visible moisture, what ceiling and thickness to expect for the layer and its temperature profile. It can be an asset when used in conjunction with traditional icing forecast products (CIP FIP plots). The chart is a little intimidating at first glance but there are tutorials on the web.

      This is the NOAA site for the current chart: https://rucsoundings.noaa.gov/gwt/
      A form titled “Choose a sounding” will appear in front of the chart. The first line of the form asks for your site. You can enter an airport designation (most are recognized). The second line asks for a start date. Just click the “latest” check box. The third line asks for the number of hours. If you want to see the forecast 10 hours out, put that in the box. Then hit the “Load” button.

      • Indeed, the concept of a skew T chart is slyly hidden in AC 00-06B and AC 00-45H under the guise of radiosondes and weather balloons, with zero mention of what you might do with that information. I suspect that is an artifact of back in the day, when despite all the banging teletypes and creeping fascimile machines, there wasn’t a skew T to be found. Alas, the digital age has resolved that. There are quite a few articles in Air Facts that discuss the skew T in more detail than I’d want to try. My personal preference is the presentation by the severe storm folks in Norman, which I like because it has a nice display of CAPE, lifted index and precipitable water as well:


  • I have a built in anti icing program. My job. When it snows, i am on call to plow it off the roads. So since i am tied to said job during the winter, my flying is pretty much pattern and local flying during that time frame. Also, being VFR helps. No flying anywhere near clouds for me, and certainly not going VFR on top.

  • I would only use icing forecasts as an opportunity to be pessimistic, not to optimistically “manage” the risk. Besides aircraft handling being unpredictable, icing intensity varies greatly within an event on a much smaller scale than any forecast resolution…that can vary from apparently “benign” to lethal in minutes.

    On a technical note, large radius stagnation points accumute the fastest, watch those closely, prop may be impacted more by icing but don’t think it’s shape that makes it accumulate faster, more that it is effectively traveling a greater distance through the icing.

    I remain amazed by those who think that aerobatics in a suitable aircraft by a trained pilot is more dangerous than their autopilot/automation dependent flights in a non or minimally ice equipped airplane with insufficient excess power/thrust to climb through icing. No such thing as a “little icing” if you can’t immediately climb clear (and I mean turbofan rocket ship rates, not piston death climbs!)

    • A wealth of information shared in the article and the numerous comments that followed. As a VFR only pilot, I found the explanations involving ice shaping of airfoils, the propeller’s march through the air, and the AOA linear and non-linear moving to a stall events to provide additional awareness. I suspect there may be occasion for even me where I remain more aware of the physical forces affecting my flight maneuvers. Thank you!

  • Mr Green, this is a great and timely article (I spent 20 minutes yesterday with pre-solo student discussing icing while removing heavy frost & ice before a PA-28 flight).
    My understanding is ice accumulates quickest on tight radius areas, like antennae, propeller, and empennage (I used to see this ice buildup on the leading edge of old style auto radio antennae during winter driving). Therefore, if you see a very small amount of ice on the main wings, You might have a significant amount of ice (that you cannot see) on the tail.
    You mentioned “You’re doing fine on the approach, and have no reason to believe that you will fall out of the sky when you flare; but many pilots do.” I believe this “falling” can more likely result from a tailplane stall than main wing stall. And, since lowering flaps typically increases the angle of attack over the horizontal tail, landing with ice (or suspected ice) should be accomplished without flaps.
    I request your comments.

    • Bravo! Tail plane icing in our small GA airplanes is not mentioned near enough! The tail feathers are thinner and therefore will likely collect ice quicker than the typical fat Hershey bar wing. To keep myself focused I assume the tail has twice as much accumulation as the wing.
      I too suggest a no flap landing with corresponding speed increase anytime icing is observed on non known icing aircraft.

    • Eric, I wrote about the hard landing issue a couple of years ago here: https://airfactsjournal.com/2016/11/in-flight-icing-hidden-threat-landing-flare/

      I wouldn’t say that ice contaminated tailplane stall (ICTS) is more likely to lead to a hard landing than a premature stall of the main wing; in most of the reports that I have studied, it is pretty clearly a plain old stall of an iced-up wing. However, ICTS is not well studied in GA accidents, and I can think of a handful in which the airplane’s behavior looked a bit like the loss of longitudinal stability that is an early indication of impending ICTS. The fact that at least some manufacturers recommend a no-flap landing after accreting ice on the airframe suggests that ICTS is a threat in their minds. Insofar as most GA airplanes don’t really need flaps for a reasonably routine landing, there is almost no downside to landing with the flaps up, and in that configuration there is very little chance of a tailplane stall.

      • Thanks, Steve. I just read your other article. Very informative. I’ve found very few pilots really understand the rapid increase in AoA within a couple degrees of stall on a clean wing, let alone one with ice. All, or nearly all, training articles and ground schools only show the straight line curve of Cl vs AoA, but not the near-parabolic shaped curve of AoA vs airspeed.
        Keep teaching, remember, you will never know when you saved someone’s life.

  • Great article.
    As a Mooney owner who lives in Seattle, ice is a threat for about half the year as the MEAs are typically at or above the freezing level from late Oct through April. The Mooney wing does not tolerate ice well. Although I upgraded to a FIKI equipped model, I have found that under certain conditions (moderate+)the TKS can be overwhelmed by ice. FIKI just buys you time to get out of it or to get on top of or descend to warmer air. It should not be used to fly in significant icing conditions for an extended period.

  • Good article and timely. However, putting the flaps down in a Bonanza can and has been deadly. Ice changes the airflow and may hide the elevator resulting in a nose down condition for which there is no recovery. Unfortunately some have been lost on the glideslope at the very last moments carrying enough ice. No flaps whatsoever!

    • Thomas, as I mentioned in comments above, the ICTS threat has to be taken seriously even though it has been studied very little in the GA fleet. That said, the other thing to consider is that ice contamination is almost certainly going to increase drag, and adding flaps on top of that may be all it takes to push you over the cliff; the L/D goes south, you increase AoA just a bit to increase the lift and offset the increased drag, the wing stalls and down you go. There’s just no downside to a no-flap landing for most non-ice-protected GA airplanes.

    • Thanks to Steve for the useful article and to those who contributed all the good comments. My advice is complimentary and came from my instrument instructor many years ago: When you first notice Ice accumulation, notify ATC and turn around. You know the atmospheric conditions immediately behind you… This way the ice stops accumulating while you calmly assess your options and replan accordingly, e.g. diversion, return to departure airport, or safely climb or descend to further mitigate the ice accumulation.

  • Really interesting article and emphasizes the “not out of the woods yet!” mindset that is required once out of icing conditions. The problem after all has not gone away.

  • Years ago I flew my ice-protected turbo Aztec Toronto to Detroit City on a lousy Great Lakes winter day. I was in and out of strato-cumulous clouds at 6000 ft., running boots, prop and windshield heat, thinking all was well until ATC called attention to my groundspeed, which had dropped 15 kts. I climbed quickly to clear air 1500 ft. above, and finished the flight. At the FBO a small crowd gathered around the nose of my plane. There was a grapefruit- sized ball of ice on the nose you could not chip through with car keys. It took an hour to melt off.

  • A very good article, and comments added all have relevance. Early on in my flying career In Australia we had 2 fatal accidents involving the MU2. In both it was concluded that airframe icing had caused a rapid decrease in airspeed leading to a stall at high altitude followed by a spin that was not recovered from. Attached is the link for anyone who wishes to read the Official reports. Those two accidents have followed me through my 20000 hour of general aviation and Airline career and have served to heighten my “spidy senses” whenever planning or encountering a flight were visible moisture is present…


  • Interesting and thought provoking article.
    One has to wonder how these stats were obtained? Is there a record of diversions because of ice, and as mention, there could be hundreds of successful diversions that we don’t know about.
    However, the message is clear. Don’t divert. Well… that doesn’t sound good so perhaps we should say divert sooner.
    How soon? And how?
    If we’re over freezing precip we might not want to divert until we have a option without the freezing precip.
    If we’re in no ice conditions and our destination is a mins with temps below freezing (or even close) we might want to divert early instead of after a miss.

    As for planning, use EVERYTHING. I like the ADSB icing forecasts, backed up by pireps when enroute. Skew T is nice also.

    And, the biggest takeaway I get is to always have a safe plan B, and in some cases a plan C.

    The vast majority of us have flown ice successfully and just don’t push that last part of the envelope…. and accomplish 95% of our flights without issue.

    Also, one needs to know their equipment and personal limitations. Some planes handle ice better than others, and some pilots know how to deal with ice better than others.

    Ice is for drinks.

  • I heard about Skew T diagrams on my aircraft club FB page many years ago, and I use them all the time now, great tool. They were not available, to my knowledge when I had my training many decades ago, but neither was GPS or ADSB. I think we as pilots have a responsibility to constantly improve our skills using whatever new technologies become available. Aviation is much safer now thanks to many of those new technologies.

  • Another overlooked threat is the possibility of engine induction icing of the air filter. During cruise in icing conditions the manifold pressure may slowly diminish and the common reaction is to move the throttle forward (thinking it might have crept backwards) to maintain the desired manifold pressure. Note the position of the throttle. If the mp continues to diminish at a fixed throttle position then it’s time to activate the alternate induction air to recover all available mp before the engine is starved of air and quits. This may have been the situation of the Beech G33 Bonanza and the PA46 in your article. The G33 and the PA46 both have manual alternate engine air controls. Don’t forget about them and exercise them at least once a month prior to takeoff. I have had induction icing a few times in piston twins during a period when I was flying over 900 hours a year throughout the 48 contiguous states. One of our Cessna 401’s was lost when it experienced dual engine failure in severe icing conditions. Induction icing was suspected. Remember the acronym GAS: Gas-Air-Spark. It takes all three to keep the engine running.

    • Mike, that’s a great point and a good technique for monitoring. In the research I do, I carefully weed out all of the carb icing and induction icing events because they are not germaine to icing aerodynamics that we are working with. That said, there are a very large number of them (although many are speculative simply based on the carb icing probability chart). And similarly, it is hard to know what role induction icing may have played in many, many events from which there were no survivors. Occasionally, there willl be a report describing the air inlet being iced over, but most of the time, any induction or carb ice is long gone by the time anyone looks.

  • The DeHavilland DHC6 Twin Otter flight manual says to limit flap to no more than 10 degrees if carrying any ice. It also gives landing distance graphs and speeds for that configuration. It is the only light aircraft I have ever flown which had that information. As Canadians are very aware of icing I would expect that they have done testing to have brought this out in the flight manual.

    This 10 degree performance chart was very useful to us in Australia and it did not involve ice. It allowed us to maintain a reasonable speed on final when operating into capital city airports with long runways without impeding following traffic by having to slow to around 90 knots for flap settings greater than 10.

      • Kim and Bill,

        I suspect the 10 degree limitation was well established before NASA started their ICTS research. The earliest Twin Otter ICTS event I have in my data was at Barter Island, Alaska in 1985. I flew the NASA Twin Otter back in the late 90’s with Rich Ranaudo, and got a rather educational workout with artificial ice shapes glued to the horizontal stablizer. That was where I learned how much we take longitudinal stability for granted; the airplane is quite squirrelly when the it becomes longitudinally unstable. The worst event they encountered during the flight testing was a full-blown tailplane stall, with Rich applying 170 lbs of back pressure on the yoke to keep the airplane from swapping ends…which is interesting when you consider he only weighed about 145 lbs…needless to say, they didn’t do that again.

  • Steve, thanks for your outstanding article! Though I have flown aircraft that are certified
    FIKI over my 53 year career, my ice experience has taught me that no GA aircraft is immune from the ill effects of ice. The strategy? Exit ASAP!
    One additional comment: early in my flight instruction in multi-engine aircraft, I was taught that AFTER ice is encountered, don’t adjust the flaps from their pre-ice position. If the flaps are partially extended (such as in the approach regime) and ice
    is LATER encountered, don’t touch the flaps! Do you concur?

    • Andy, we get very, very accustomed to operating within a certification envelope with a clean, stable airplane. The point your instructor was making is that once you ice up the wing, you really don’t know where you are in that envelope, if you are even still in it. So it is wise to maintain the configuration, since you really don’t know where you are with respect to the cliff, and changes to configuration may push you over it.

      That said, drag is a serious threat in icing…I certainly wouldn’t add flaps once ice has been accreted…and it may be appropriate to retract flaps in a worst-case scenario if you are out of excess power and you think the drag is taking you into the trees.

      A completely different scenario is the ice contaminated tailplane stall that we discussed in comments above. That is very unlikely, and certainly not unless you have full flaps extended. Retracting the flaps one notch is part of the response to ICTS. But then, if you know you have ice, follow your thinking and don’t extend the flaps in the first place, and that eliminates any chance of an encounter with ICTS. Some manufacturers may even have a restriction on flap extension with ice, which is a not-so-subtle clue that they think ICTS is a possibility.

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