by david.megginson » Tue Mar 12, 2024 3:41 am
I spent a lot of my 19 flying years around ice, so here goes:
There are two basic types of airframe icing, clear and rime. Where there's a combination, it's called "mixed".
Clear icing is the killer. It's caused by supercooled large water droplets, which come from either lifting action (e.g. near the top of a TCU) or warm, moist air overrunning cold air (e.g. a winter warm front) — also sometimes in freezing fog from cery cold air over open water or in lake-effect weather southeast of the Great Lakes. Even with boots, you stay away from this stuff at all costs in a piston aircraft, and as much as possible in a turbine aircraft (ditto for mixed icing).
Rime icing is typically fairly harmless unless it gets very thick, which takes time to accumulate. You'll pick up a bit of light rime climbing through a typical 2,000 ft winter stratus or stratocumulus layer, but it sublimates off quickly in the sunlight on top, even in very cold air. Light rime just a fact of life for winter IFR in Canada or the northern US. We try not to spend too much time in it, but in Canada we don't worry about a trace of rime even in basic planes with no deicing equipment, like a C172 or PA-28. The main rule is always to have an out ready.
Airframe icing is most common between about +1c and -10c outside air temperature. Below that, the likelihood falls off quickly. By -20c, most clouds will consist mainly of ice crystals, and nothing will stick to the airframe. In stratus clouds, that's pretty much universally true; however, if there's a lot of lifting action (for example, in a TCU or with ridge lift), it's possible the supercooled droplets could be pulled up into the colder air and not have frozen yet, so you really don't want to go through that kind of cloud even at -20c.
Finally, icing forms on thinner surfaces like antennas and temperature probes before it forms on wings. I had a black OAT probe sticking out of my front window. If there was just a tiny trace of frost on it, I didn't worry. If it got a lot of frost on it, then it was time to calmly execute my exit strategy. If I saw any drops of clear ice start to form, then it was an urgent situation and I had to ask ATC for an expedited altitude change. Canadian ATC is used to that, but US ATC kind-of freaks out — different attitudes, I guess.