Thursday, June 12, 2008

The Season of Ice

It's official almost summer, so for me this can only mean one thing: annual ice awareness training. Yeah, my company training is out of sync with the real world. It's just the way it is. Instead of just having me watch the Transport Canada When in Doubt video on deicing again, they had me do this NASA video course on icing, too. It's excellent. Canadians who only fly in Canada will find some of the weather products unfamiliar, but you can skip those parts. If you do, or might fly in the US, it's a good how-to-use for their icing products, which are different from the Canadian GFAs. And the advice is very practical, not just recitation of examination factoids, like "contamination the texture and thickness of sandpaper decreases lift by 30% and increases drag by 40%."

Here are some examples of academic information transformed to simple, practical advice.

Weather basics tell us that stratiform clouds form in layers, usually not more than a few thousand feet thick, and and cumulus clouds develop vertically. The NASA course says to escape ice in stratiform clouds, change altitude by 3000', but navigate around cumulus clouds.

Weather theory tells us that an air mass loses moisture from crossing a mountain range, and that all else being equal, more moisture in a cloud means more icing. So NASA says, plan to fly on the leeward side of a mountain range.

Fronts are the places where air masses meet, named according to which air mass is advancing. At a warm front, warm air is advancing over a wedge of cold air. At a cold front, a wedge of cold air is forcing warm air up. So the NASA advice is to fly on the warm air side of a front, i.e. traverse a warm front perpendicular to frontal movement and behind the front and traverse a classic cold front perpendicular to movement and in front of it. This is probably good advice everywhere in the US but Alaska, but I'd have to reverse it for the north in the winter, because then the cold air is down below minus thirty, too cold for icing, but if the warm air is up around minus ten it will contain the more dangerous icing.

We already do this for engine failures and depressurization events, so why not develop an icing escape plan for each point along your route.

The NASA course does treat North America wide phenomena. They mapped of the continent for icing potential, illustrating that icing is more common around large bodies of water. At first I blinked at it. Why did it not show massive icing around the northern lakes and bay? Then I noticed the caption "November to March." The northern bodies of water are frozen solid then, so are not a source of moisture for icing until spring.

They acknowledge that their audience already knows things about icing, some of which are myths. NASA is wonderful because they don't just pool the advice of experts and regurgitate it back. They go out and do research, some of which involves flying actual airplanes around in actual icing conditions, and can back up what they say.

Pilots who have flown regularly in ice have all looked out the window and confidently assured their terrified copilots that they've seen worse. NASA research, and you get to trust them more than the grizzled captain because they really do research what they write about, reminds us that it is nearly impossible for a pilot to visually distinguish between an accretion that is flyable and one that threatens survival. I am reminded every time I preflight of how little it takes to change the flyability of an airplane. That's because my airplane has a vortex kit, eighty-eight little metal tabs on the wings and tail. They're tiny, each about the thickness of a dime and the area of the first joint of my thumb. But lose three and the maximum gross weight of my airplane drops by six per cent. I mentally compare that to how an ice accretion could affect airflow over the wing. By how much will it decrease the maximum flyable weight of the airplane?

And no icing video is complete without a nod to the old advice on deicing boots. In the old days, before I learned how to fly, probably before I was born, manufacturers told pilots to wait until there was a significant accretion on the wing before inflating the boots. The theory back them was that if the boots were inflated when the ice was very thin, inflation of the boots could result in bridging, creating a boot-sized airspace surrounded by ice the boots couldn't meet. NASA points out that while this may have been possible with the old, slower inflation boots, it is not with modern boots and that no one has ever shown ice bridging to occur with any boots. "Ho hum," I thought at that part of the presentation. Old news. Everyone has heard that debunked by now. The pilots I have flown with who advocate waiting are not delaying activation to avoid ice-bridging, which they don't believe in either. They are delaying it because they are waiting for the "skin" of ice to be thick enough to pull off the runback ice as the boots pop. This video, however, addresses this too. It admits that studies confirm that larger amounts do shed more cleanly with one inflation. But that research shows that although ice may remain between cycles, the ice will ultimately clear as well as it would have, had you waited for a large build up, and by activating the boots as soon as you have ice, you reduce the maximum amount of ice you are ever carrying. This makes more sense than decreasing the minimum amount.

I have notes on a couple of things I hadn't met before, too.

Freezing drizzle (small supercooled raindrops that freeze on contact with your airframe) is according to NASA most commonly formed by collision coalescence. That is, by even smaller supercooled water droplets bashing into one another and becoming big enough to drift down as drizzle. I had thought that it was usually formed the way textbook freezing rain is formed at a warm front, with rain falling out of the warm air aloft, and becoming supercooled by its passage through the colder air below. I can diagram it and everything. Useful as my original knowledge is for passing Transport Canada exams, there's an important difference: as the NASA course points out, I should not assume that the presence of freezing drizzle indicates warmer air above, into which I could climb to escape the ice.

The other new-to-me thing I remember is electro-expulsive deicing where the shape of the actual wing is mechanically altered to dislodge ice, like flexing an ice cube tray.

It's very usable, multi-media with exercises to do, and so many videos available to watch you can miss a lot of them by not looking at the 'related information' section. The only spelling error that distracted me was the term "full blow thunderstorms" where I would have expected "full blown," but perhaps that's a regionalism. The course is something you have to dedicate a day to. There's that much information.


Tina Marie said...

But that research shows that although ice may remain between cycles, the ice will ultimately clear as well as it would have, had you waited for a large build up, and by activating the boots as soon as you have ice, you reduce the maximum amount of ice you are ever carrying.

Boy, the pilots of the ATR in Roselawn would have been interested to know that.

From the NTSB:
"The loss of control was attributed to a sudden and unexpected aileron hinge moment reversal that occurred after a ridge of ice accreted beyond the deice boots."

I can't find a reference now, but why do I remember that they were over-cycling the boots? The final report blamed the boot design, but I know I saw something around the time of the accident that said that if they had used them as the ops manual said, they wouldn't have gotten the runback ice to begin with.

majroj said...

Thank You! As a groundling I appreciate learning more about what's "up there" from a pragmatic denizen's view.
Vortex kit..dang. Wonder if I can get one for my Honda Accord.

PS: nice to hear about NASA's non-human cannonball work for a change.

Anonymous said...

Vortex kit? I thought the standard terminology was vortex generators?

Aviatrix said...

Vortex kit, vortex boundary layer system, them vortex thingies ... it all does the same job.