Icing: Altitude Strategy

Instrument meteorological conditions.

I’ve written about icing before, both here and here. Like storm clouds and scud running, icing is one of those things that pilots are supposed to avoid but occasionally stumble into anyway. The Canadian AIP contains some advice for pilots who end up in storm clouds (slow down, keep flying straight, and don’t worry about altitude, or something along those lines), and Rick Durden has written a good collection of scud running survival tips, but there is precious little out there to help us with dealing with icing on a day-to-day basis.

Fly high

OK, you’re planning to fly IFR on a day when there is no moderate or severe icing forecast, but there is cloud and possibly precip along the way, and the temperature at higher altitudes might be close to freezing. What do you do: fly low, to try to stay in the warm air, or fly high, where you might pick up ice?

My tip — and more experienced pilots reading this should feel free to correct me — is to fly high, even if you’re at or below the freezing level. Ice tends to accumulate at very specific altitudes: for example, you might pick up some light clear icing at 7,000 feet, but nothing at 9,000 or 5,000. In theory, then, you can either climb or descend to get out of icing. However, if ice should happen to accumulate quickly, and especially, if you should happen to be flying a heavily-loaded and/or weakly-powered plane, climbing might not be an option (for planes with boots, climbing too steeply is also dangerous, because the high angle of attack can allow ice to accumulate on the wing where the boots cannot reach it).

So, if your only choice is to descend, what happens? Assume that MOCA is 3,000 feet — if you’re already at 5,000 feet, the temperature drops a bit, and you pick up ice, you have only 2,000 feet left to descend safely in the hope that the ice will melt off; if you’re at 9,000 feet, you have 6,000 feet to descend to melt off the ice. That leaves you with a lot more choices. It may even be that 5,000 feet is the icing altitude for much of your route, but you’ll overfly it without ever knowing.

Of course, you don’t want to try this without some hope of warmer air underneath — if there’s an inversion causing freezing rain on the ground, descending probably is only going to make things worse. On a typical spring day or summer day, though, with some cumulus cloud and near-freezing temperatures at cruising altitude and a reasonably low MOCA below, I think that flying high makes more sense than flying low — like always, if you run into any trouble, your altitude is like money in the bank.

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3 Responses to Icing: Altitude Strategy

  1. Chris says:

    I don’t really agree because this avoids the question of how you get up there in the first place. If the cloud layer is thick, you really don’t want to have to climb through thousands of feet of possible icing to get up there. Second point is, assuming that you do climb and you pick up ice at e.g. 5000 and the temperature is well below zero at 9000, it will take a while for the ice to dissappear by sublimation. In addition, you’ll be nicely cold when you eventually have to descend, so you might pick up ice even more easily. Also, since icing varies locally, you guarentees you that you won’t be picking up ice all the way down from 9000 to say 5000 at your destination?

  2. david says:

    What I left out is that I stop climbing immediately if I hit an icing layer — fly at the highest altitude possible only if you do not hit ice on the way up. You have a good point about the descent at the end.

  3. Adam says:

    Yea, I agree with the descent at the end. But, I can also see Chris’ point.

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