I’ve been thinking a lot about wintertime IFR flying in the mountain states, which is usually a no-go because of icing potential. But whenever these conversations come up, a common refrain is “always have an out.”
What does “an out” mean to you? What are some scenarios where you had to plan or use your “out”?
An out is usually VMC, either below you or off to one side. It can also be warmer temps below or (less usable) above.
Having an out is part of it, but another part is knowing the difference among different types of icing.
Clear or mixed icing means supercooled large droplets (SLD), which are terrifying, even in small amounts. Go nowhere near these in a piston aircraft, even with FIKI. Stay on the ground. Only turbine planes can handle SLD, and they do so by covering their wings with goop on the ground, then mainly by using their excess power to climb above it as fast as possible and their excess speed to create friction and warm the wing a bit. SLD is worst ahead of a slow-moving fall/winter/spring warm front (where it can still be well above freezing on the ground), but can also occur because of various types of lift (e.g. wind blowing over a ridge), or from lake effect (cold, dry wind, usually from the NW, blowing over open water like the Great Lakes). The one saving grace of lake effect is that it usually tops out by 8,000 ft with clear skies above, so you can overfly it if it's in the middle of your route; other types of SLD just mean stay home.
Rime icing is small-droplet icing, which isn't as bad, but you still don't want to be in it continuously unless you have FIKI. It occurs mainly in strato-cumulus-type clouds. If there's a thin SC layer around 3-6,000 ft, I don't worry about climbing through that without FIKI, as long as it's clear above and the forecast is for no more than light rime. There are usually clear breaks between those SC clouds, too, so there's a way down if you need it.
The potential trap with rime is when there are multiple layers of strato-cumulus, which is also common in winter in the northern US and southern Canada. You climb through the first layer, assuming that you'll find a clear space to cruise in between layers. That's usually true, but sometimes the bottom layer will rise and the one above you will come down until they join up, and that's when you need to have a good exit strategy. Flying IFR between layers in winter is an IFR equivalent of the VFR sucker hole, and I wouldn't recommend it without FIKI unless the layers are very far apart (e.g. a low SC layer, and a cirrus layer >10,000 ft above).
Rime won't generally accumulate fast like clear/mixed, but if you linger too long in it, you'll end up with an opaque, frosted-over windshield, which can be disconcerting. This is really the only place where FIKI in a piston plane improves your options, because you'll have a way to keep a spot on your windshield clear of rime, and pop off any that accumulates on the wings, so you can fly for longer in light/moderate rime icing.
My rule of thumb is to use my OAT probe (which is in the middle of the windshield in front of me, and painted black) as an ice detector. Thin surfaces ice up first, and as soon as I see rime starting to form on the probe, it's time to exit (different altitude, different route, etc). Don't wait around to see if things get better. Also, if I've seen
any ice, even just a trace, I tie something around my flap handle to remind me not to use any flaps on landing.
D (from Canada)