January 1, 2007
By Thomas A. Horne
As I write this, I'm stuck in Birmingham, Alabama, trying to fly AOPA's Win a Six in '06 Piper Cherokee Six back to its home base at AOPA headquarters in Frederick, Maryland. The lowering ceilings and freezing temperatures attending last night's ILS approach into Birmingham foreshadowed today's troubles. Icing airmets are posted across a wide swath of the Appalachians, freezing levels range from 2,000 feet to the surface all along the route, and METARs and TAFs advertise cloud layers ranging up to 12,000 feet. To top it off, 30-knot headwinds would slow the trip and yield a bumpy ride. So, here I sit. Thinking about the potential outcomes of making a "go" decision.
This would be a 600-nm trip, with a fuel stop midway. Although some stations are reporting VFR conditions, that doesn't interest me. What gets my attention is the chance that I'd end up flying on top of cloud layers, or in and out of clouds. The cloud bases are below the minimum en route altitudes along the two routes I've planned for the IFR flight. So, although airports may be reporting VFR weather, I'd be flying up where the clouds are. Do I want to run the risk of icing encounters along a route this long? Do I want to risk picking up ice on the descent to my fuel stop? I don't think so, especially with AOPA's name on the airplane registration. Even the smallest ice buildups can dramatically reduce lift, compromise hand-ling characteristics, and increase drag.
The Cherokee Six is a relatively slow, draggy airplane to begin with. Under the best of conditions, small power reductions can produce big airspeed losses. You have to wonder: What would happen if even a small amount of ice should adhere to the airframe?
If vital airspeed was to be preserved, a descent would be mandatory. If that descent should happen to be through more clouds, more ice could collect. That could very well mean descending below the minimum en route altitude. And if the descent was for a landing at a nearby airport, then any ice would remain on the airplane — right through to the landing. Temperatures at the surface were at or slightly below the freezing point.
There's a paradox here. The Win a Six is fitted out with datalink weather, and has a wealth of equipment to boost situational awareness. But operationally speaking, it's still a very limited airplane, like most piston singles. All that gear is for avoidance of adverse weather, and anyone who thinks datalink weather capability — or airborne weather radar — somehow makes flying in bad weather an acceptable option had better think twice.
We've said it before, and we'll say it again. Avoid ice if at all possible, and take immediate evasive action should you blunder into icing conditions. That goes for pilots flying typically equipped piston-powered airplanes, as well as those lucky enough to fly turbocharged or turbine-powered airplanes with ice-protection equipment that's certified for flight into known icing conditions.
Which brings up another paradox, highlighted in the icing accident story featured in this month's "Flying Seasons" article (see page 83). Many icing accidents happen to high-time pilots flying well-equipped, high-performance airplanes. Were they lulled into complacency? Overconfident? Speculation is risky, but debates over icing accidents can be lively — and educational.
Airframe icing is always bad, but different icing types occur under different conditions.
Rime icing is typically associated with stratus clouds, and most often happens in the minus-10-to-minus-20-degree-Celsius temperature range. It first shows up on small airframe projections, such as OAT (outside air temperature) probes, rivet heads, wingtip strobe/nav light blinders, and other small, sharp projections. Because it's frosty white, you may not notice it as it first builds on wing leading edges painted white. Eventually, though, a rough, milky accretion will be seen on leading edges and windshields as this type of ice progresses. Rime ice is a product of the smallest supercooled water droplets — held in suspension as an unfrozen liquid — until your airplane flies by. Then the supercooled droplets freeze on contact with airframe surfaces.
Clear icing usually happens in cumulus clouds, in the zero-to-minus-10-degree-C range. It's created by supercooled water droplets larger than those associated with rime icing. It's a tenacious, transparent coating that can accrete rapidly. Because it's clear, you may not notice the onset of this type of icing — unless you're vigilant. All the more reason to avoid any clouds when icing OATs prevail.
Mixed icing, as the name implies, is a combination of clear and rime. This can mean a rough, pebbly-surfaced accretion that robs you of lift and radically increases drag — immediately. Some of the worst mixed-icing shapes I've ever seen were over the North Atlantic, in damp clouds at an OAT of minus 8 degrees C. The leading edges took on a classic ice shape called the "double-horn." This is a scoop-shape formation that projects into the relative wind. But this one had an unusual feature: feathery appendages that looked like hoarfrost. Luckily, I was able to descend to warmer air, where the ice, feathers and all, slipped away.
Supercooled large-droplet (SLD) icing is a relatively new discovery, made during the accident investigation of an American Eagle ATR 72 commuter turboprop airliner in Roselawn, Indiana, in October 1994. Findings indicated that supercooled droplets many times larger than conventional, clear-icing droplets existed at the time of the accident. These droplets happen in a narrow band right around the freezing point. SLD icing is bad news because its huge droplets splatter on the airframe, then run back before freezing. In the case of the ATR 72, this frozen runback flowed aft of the wing deice boots, then formed a ridge of ice that helped cause a fatal stall and rollover. A redesign of the boots — to extend them farther aft on the wing chord — helped deal with the problem on this type airplane. But with other airplanes? You're on your own. SLD usually happens in areas around the Northeast, the Great Lakes, and the Pacific Northwest. Large droplets, with snowy or slushy contents, splattering on the windshield, running back, and causing ice accretions at the aft corners of the windshield are your warning signs.
Freezing rain is the most dangerous type of icing, and it's common near warm frontal boundaries in the colder months. Rain falling from warm air aloft becomes supercooled as it falls into the colder air below, and the stage is set for any passing aircraft to become flash-frozen in a sheath of all-enveloping ice. Because freezing rain is a form of clear ice, and is also a large-droplet icing event, runback ice is a concern. Valleys in the Appalachian mountain chain sometimes experience freezing rain when warm air aloft yields precipitation. In this situation, cold air trapped in the valleys provides the medium for supercooling the raindrops.
Frost, it can be argued, is another type of icing. Caused by moist air being cooled to dew points below the freezing mark, frost typically happens in the early morning — when temperatures are the lowest. Frost usually consists of a thin coating, but it's the surface roughness that increases drag and stall speed, so all frost must be removed before takeoff.
In October 2006's " Wx Watch: Ice Surprises," I talked about some of icing's traps. I called these unexpected turns of events "sucker punches," and asked readers to send in any stories of their icing surprises. Here are a few memorable ones.
At AOPA Expo last year, I gave a presentation on sources of aviation weather available via the Internet. Before the convention I was of two minds about the topic: Either the audience members would be old pros at surfing weather Web sites, or they'd be near-neophytes looking for ways to expand their knowledge.
An overflow crowd showed up — so big that the 300 handouts I'd made were gone in a flash. The group members were a mixture of the two types I'd anticipated. I came to several conclusions about Internet-based aviation weather. One is that it can serve as an excellent visual adjunct to the information we receive during our "official" briefings from flight service personnel, or from DUATs. In many ways, the Internet information is clearer and more informative than that obtained from conventional briefing sources. But with that information comes a duty to better understand weather fundamentals. A picture may be worth a thousand words, but first you have to understand completely what the graphics are showing. A show of hands revealed that most of the audience were more comfortable getting the bulk of their preflight weather from flight service than from the Internet. They needed the hand-holding — and the official briefing status! — that FSS personnel can provide. The attendance showed an increasing curiosity about Internet weather sources. And why not? The more information, the merrier, and safer, we are.
E-mail the author at firstname.lastname@example.org.
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.
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