Ice: gotcha… Where is the ice?

Where? Simple. Ice is where you find it. As pilots we have to accept the fact that ice will be forecast when it is cold and there are clouds but if we are to get any utility out of our airplanes in the wintertime, we have to develop the weather wisdom to recognize the times when ice is likely and when it is not. Then we can decide whether or not to go and have a look, with an escape plan always ready.

In Part One I mentioned that between us, Mac McClellan and I had only a few significant ice encounters. Mine were both over 30 years ago with one over 50 years ago. Both were in airplanes without ice protection and I learned the same lesson from both encounters: I needed to know more about ice.

Low pressure
The lifting action east of a low usually causes the worst icing.

I’m not going to bore you with the science of what causes supercooled water droplets but I am going to remind you that they are caused by instability lifting warmer moist air up into colder temperatures. The more unstable the air, the larger the lifted droplets, which means more ice more quickly as we move through those cold clouds, splattering the droplets so that they can freeze.

A characteristic of stationary fronts is that they can actively promote ice. Waves can form on these fronts. Viewed from above, they look like ocean waves in the sense that they undulate. What they are is low pressure trying to form but not quite making it to a complete circulation because of not enough upper level support.

Air flows counter-clockwise around a low and inward. This is trying to happen with low pressure waves. The result is areas along the front with more lifting. You might fly along for a while with no ice and then get zapped. I’ll tell you about one of my encounters. I have written about it before and it always offers pertinent ice characteristics.

It was night. Russell Munson and I were headed from New Jersey to Oklahoma City to fly and photograph a Rockwell Commander 700, an airplane that met with no success.

We stopped at Springfield, Missouri, for fuel. This was before the days of computerized weather briefings so I was briefed by an FSS person.

There was a strengthening low pressure area to the west but no fronts were shown on the map. The available weather radar information showed little precipitation. It was cloudy and cold and ice was discussed. As I recalled, the specialist thought there would be some ice-free air out there between, I think, 4,000 feet and 8,000 feet. I was flying my Cardinal RG so not a lot of vertical options were available.

I don’t do night VFR so we headed out into the dark sky IFR. Russell had a flashlight and word to tell me the minute he saw any ice forming on the wing.

We had been cruising along for a bit when Russell said there was some ice forming but he added that it didn’t look like much. I think I remember trying a different altitude but ice was still forming and it still didn’t look too bad according to Russell. I looked at it when he shined the flashlight out there and I had to agree.

There might not have been a front on the map, but I strongly suspected we were flying in the area of a stationary front with waves along the way.

The airplane had a different interpretation of the icing condition. It was getting sluggish and sloppy and the indicated airspeed was decreasing at a rate that got my undivided attention. I had little ice experience in this airplane so I was in the dark, literally and figuratively.

It was time, if not past time, for action. I told the controller that I wanted to divert to Joplin, Missouri, which was off the right, and that I wanted the lowest possible altitude because of ice. I did know at the time that in many cases, ice would be less pronounced lower in the clouds.

The ceiling was such that we were beneath the clouds with reasonable visibility to enter on a downwind leg. I also knew at the time about not using flaps and adding a lot of airspeed to the normal amount for an approach.

The small tires on the Cardinal RG protested the higher than normal touchdown speed but everything was fine and we were soon parking the airplane.

How much ice was there? I will let the lineboy tell you, in his own words, “Holy @#$%, you guys must be crazy.” A perceptive lad.

Ice tends to build on this airfoil more atop the wing, just aft of the leading edge and after we got out and looked at the ice, it was hard to see why the airplane was still flying at all.

We had apparently run afoul of a wave moving along a stationary front. Actually, that wave did make the grade and become a full and rather vigorous low later that night. When we looked out the next morning there was a lot of snow out there. At the airport, we got the airplane into a heated hangar to rid it if the snow and the big load of ice from the night before.

When we were ready to launch for Oklahoma City, the FSS briefer was waving the ice flag mightily and Russell was wondering if I was crazy enough to challenge the clouds that had caused trouble the night before. I didn’t see a problem because the winds where we would be flying were now out of the north, there was no lifting, and it was cold enough that the age of the clouds and the temperature meant that the clouds were comprised of ice crystals and not supercooled water droplets.

We encountered no ice on the way to OKC. There was some snow but that doesn’t cause airframe icing. It’ll put a white line down the leading edge and, in extreme cases of wet snow, it can clog things up and impede the flow of air in important places. I saw that once in my P210 when heavy wet snow caused the engine to revert to the alternate air source. The item in the POH saying that can cause a drop in manifold pressure is an understatement.

CIP icing map
Pilots today have vastly better tools for avoiding ice than they did 20 years ago.

On the trip from Joplin to Oklahoma City the next morning I think it was a matter of the forecasters making up for the lack of an icing forecast the night before by waving red (white?) flags. I would hasten to add that they have much more information available to them now and that forecasts, while never perfect, have improved. As pilots, we can access some really good ice products on the National Weather Service’s Aviation Digital Data Service. These are not perfect but they are sure a lot better than what we had back in the good old days.

Because of the way lifting mixes up temperature and moisture, I learned that any ice is worse east of a low or a developing low, it isn’t as bad north of the low, and west of the low it is likely to be ice crystals, or snow. There are, of course, always exceptions but I flew on for over 30 years after that encounter and used that experience, and a similar one about 20 years earlier, to form better opinions about where ice might be found.

In flying my ice-protected P210 for 28 years and almost 9,000 hours I didn’t often use the deice boots in anger. I think the most ice I got on that airplane was intentional, to develop icing video for Air Facts programs.

It is often said that minus 15 or 18 degrees C is when it starts getting too cold for ice. This has to be taken with a grain of salt because the temperature can and does change rather quickly when you are flying in unstable air. It could be too cold for ice right before you fly into an area where warmer and wetter cold air is being lifted. We have long heard war stories about sudden accumulation of ice when flying over mountainous terrain. It is likely found in the updrafts and can really splatter on an airplane.

Another clue to ice is cold and turbulent clouds. You can find ice in perfectly smooth clouds but it would accumulate rather gradually. If ice is forming in bumpy air the condition could become serious more quickly.

It is generally true that ice increases as you go higher in certain types of clouds and reaches a peak right before you get to the cloud tops. This has foiled many an attempt to climb on top of ice, which was what the pilot of the ill-fated TBM 700 in Part One was trying to do. This is also true in the widespread stratocu decks that develop behind cold fronts that are becoming stationary. A normally-aspirated airplane can run out of vim and vigor before reaching the tops of these clouds, which are often between 5,000 and 10,000 feet.

The lake effect clouds that form downwind of the Great Lakes offer interesting cloud top challenges, too. The height of these tops depends on the temperature of the lake water and the air and the strength of the wind flow. However, as a generalization, it has been my experience that the tops of these clouds are usually around 18,000 feet. Move toward the upwind side of the lake and the tops go down dramatically.

Any time you are flying through a front, only one thing is certain: the temperature will change. It could change for the better, as when flying from cold air through a warm front and toward warmer air. The reverse would be true when flying toward the cold air behind a cold front. The important thing about ice and any front is to visualize how the temperature will likely change with the passage of distance and recognize that conditions, and the potential for ice, won’t stay the same.

Freezing rain can create real problems and the conventional wisdom is that if you climb, you will fly into the air from which the warm rain is falling into the below freezing temperature beneath. That means there is a temperature inversion with the air becoming warmer instead of colder with height. What causes this is warm air overrunning cold air at the surface. The most likely weather synopsis is a cold front that stops or cold air damming where a surface circulation traps cold air at the surface, as when it is moved up against mountains and held there.

I didn’t consider my most notable freezing rain encounter as “significant” because it didn’t have a substantial effect on my Twin Comanche’s performance.

It was night and I had made one basic mistake. I was running out of options on where to land for fuel because all the places within reach while maintaining a one hour fuel reserve were reporting freezing rain. In other words, I knew full well I was going to get some ice before I landed.

I decided I would land at Indianapolis because it was a big airport with excellent approach and runway lighting systems and an ILS. As I was maneuvering to get on the ILS and down, I could hear the pitty-pat of rain on the windshield and I could see the view of outside becoming far less clear quite rapidly as ice accumulated on the windshield.

I never made good landings in that Twin Comanche but the one that night wasn’t bad, considering that the only view was through ice except to the side where I had the little storm window open to hopefully enhance my sense of height. The landing was with no flaps, which made it a little easier.

The challenge came after I landed. I managed to find my way off the runway but then didn’t think I could see to safely taxi. I asked ground control if they could send a follow-me truck out for safe passage to the ramp and they did. The air activity at that airport had just about stopped for the duration of the ice and they didn’t have much else to do.

I offer this next paragraph as an aside to show how in freezing rain, it isn’t over until your head hits the pillow.

I got a motel reservation a few miles from the airport and arranged for a rental car. All that was available was a Chevy Corvair. It had a rear air-cooled engine and rear wheel drive. I had never driven one but had a VW bug, with the same engine/drive configuration, and knew it did well in inclement weather. Not the Corvair. It was like driving an eel and, to boot, the defrost system didn’t make much heat and I had no follow-me truck to help me get to the motel. It was a long few miles.

In the airplane, I had a plan and followed it. In the car I had no plan. A bad result in the car might have been a ditch. In the airplane it could have been much more serious.

Pilot reports on icing are viewed by some as almost holy but, to me, they weren’t too useful most of the time. They are an insight into what is experienced in one place at one time. For every pirep made there are hundreds, if not thousands, of pilots in the air who don’t make pireps. Almost nobody makes a “no icing” pirep.

Where pireps are useful is in extreme conditions like those encountered by the TBM pilot in Part One. It is wise to pay close attention to pireps like the ones from that area at that time because they described conditions that are truly dangerous.

I have said it about other areas of the relationship between flying and weather and I’ll say it again about ice. The pilot who flies with the FAA-mandated minimum knowledge of weather is many times more likely to be surprised by a weather development than is the pilot who studies the subject. Weather wisdom is simply one of the better safety devices available and it doesn’t cost much other than a little effort.

3 Comments

  • Thanks for a great weather post, Richard – if you have any more, please don’t hold back.

    Over a short 10 years of IFR flying, I have scared myself early on with clearice once — no excuse but overconfidence combined with underexperience — but I’ve also learned that we use the term “icing” to cover too many different things, ranging from light rime to severe clear icing. Once a new pilot goes up and finds out that light rime is no big deal (at least on a draggy plane with a dirty wing, like a Cherokee or 172), (s)he can get overconfident and blunder into real trouble in SLD icing. It’s the same as when parents use the same blanket warning for all drugs — if kids try pot and find out nothing bad happened, will they still believe your (more-accurate) warnings about the hard stuff?

    OTOH, north of the Canada/US border, I’ve sometimes found flight services go pretty far the other way. Once, early in my IFR flying, the London FIC specialist told me not to worry too much about a moderate mixed forecast during a night flight — it was clear at my departure point, so just get on top, and descend as fast as possible through the clouds at destination (in the end, there was a nice big hole over the airport). I wouldn’t make that flight now, with 700 hours more experience.

  • Richard,

    Thanks for sharing your weather wisdom. The way you explain the complicated subject of icing is very helpful. Please keep it coming.

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