Plane & Pilot
Tuesday, October 21, 2008

The Go/No-Go Decision In Winter

The rules change when the weather turns cold

It had been a long day. It was January 2003, and I’d departed Reykjavik, Iceland, in a 58 Baron; destination Iqaluit, Nunavit, Canada, with stops in Greenland, where it was clear and cold—in this case, minus-20 degrees C. I’d landed on the gravel runway at Kulusuk in the dark of noon, refueled as quickly as possible to avoid having the engines cool down, and leaped back off across the ice cap for the old U.S. air base at Sondre Strom Fjord, well above the Arctic Circle. The weather remained perfect as I spanned the cap at 14,000 feet in smooth, frigid air.
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IMC becomes notably less clement in the gap between about 0 and minus-25 degrees C, then often begins to improve as it gets really cold. Below minus-25 degrees C, the air can become so dry that it may not support the formation of clouds. In summer, the worst possible weather system is a thunderstorm, an unquestionably formidable opponent. The only good news about thunderstorms is that they’re often predictable and usually fairly visible, and most pilots don’t have to wonder if it’s safe to fly into a huge cumulonimbus cloud with lightning sparking out the sides.

Winter weather is less predictable and more insidious. The question is often how bad is too bad? Overcasts are sometimes lower and generally thicker, weather systems are often larger, and in-flight icing can represent a definite no-go situation. The wind tends to be stronger both on the ground and in flight, sometimes bad enough to compromise safety all by itself. In parts of the Midwest and East, winter weather can go from manageable to deadly in a heartbeat.

The big question that most pilots must answer is how to handle airframe icing. If icing is forecast and your airplane isn’t fitted with approved deice equipment, either pneumatic or liquid, your decision is made for you. Technically, you can’t just go up and “take a look.” Depart into known icing without proper equipment, and you may get away with it, but if you don’t and survive the encounter, the FAA may have some embarrassing questions.

In flying the ocean several times each winter, icing is always one of my biggest challenges, and I start considering an exit strategy the instant it appears. Ice is to winter what thunderstorms are to summer. Airframe icing is caused by supercooled water droplets that adhere to the airplane when they contact the surface. If the limit of your deice equipment is pitot heat, or even if you have a full set of approved pneumatic boots, the rule is, don’t go there.

So what do you do if you encounter inadvertent icing? The type of ice dictates how you handle it. Climbing, descending (if the terrain below will allow) or a 180 are the options. Freezing rain is the worst and demands the most respect and immediate action. The rate of accretion can be unbelievably fast.

Several years ago in Anchorage, Alaska, the crew of a corporate King Air 200 from North Carolina was briefed on the possibility of freezing rain on a flight to Sitka. The captain had never flown in significant icing conditions before and took off anyway, assuming his Super King Air was super enough to handle anything. It wasn’t. Ten minutes later, he staggered down the ILS back into Anchorage under nearly full power, barely able to make the runway because of a load of ice that deformed the wings and tail and nearly turned the big corporate turboprop into an icicle.

Clear ice, often most evident in cumulus clouds and other types with vertical development, is the second most serious. Clear ice is tougher to remove with pneumatic boots. Rime is perhaps the most common and least severe form of icing, often filled with air pockets and generally the easiest to crack off with boots. These days, more and more airplanes are being fitted with a TKS system, laser-drilled titanium panels fitted to the leading edge of wings and tail that exude a special glycol-based liquid.

I have a good friend in the Northeast who flies his Mooney 201 all year round with his aftermarket TKS system constantly enabled to protect him from evil, even in severe icing conditions. He spends a fortune on TKS fluid, but in 10 years of operation during some nasty winter storms, he’s never seen any significant accumulation of ice.

For a more in-depth analysis of icing, Robert N. Buck’s excellent book, Weather Flying, has the best advice I’ve read. Avoidance is the best remedy.

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