November 2004Departments

The Weather Never Sleeps

The ice man cometh

Respect this cold-weather staple

To get by in the real world we sometimes have to forget things "everyone knows" and come to a more realistic understanding of how things work. Ice is a good example.

Any pilot who believes "water freezes at 32 degrees Fahrenheit" is going to have a hard time understanding how ice can form on an airplane's wings, tail surfaces, and windshield. Tiny water drops such as those that make up clouds or even the much larger ones that fall as rain can cool well below "freezing" -- sometimes as cold as minus 40 degrees Fahrenheit -- and still remain liquid. Once you grasp this concept, you're on your way to understanding aircraft icing.

When liquid water turns to ice, water molecules that have been moving around at random begin locking into six-sided ice crystals. As anything cools, its molecules slow down. When water cools below 32 degrees Fahrenheit (0 degrees Celsius), movement of the water molecules slows enough that the molecules can by chance come together to form a tiny crystal called an ice embryo. But the jiggling of surrounding molecules can break up small ice embryos until the water cools more, maybe close to minus 40 degrees in a small water drop.

If there's more water, such as the amount in the dog's water dish that you left outside in the winter, the odds are much higher that larger ice embryos will form. When this happens the water quickly turns to ice at temperatures close to 32 degrees.

When water is colder than 32 degrees but not frozen, it's known as supercooled water. This is the most dangerous kind of water for pilots because tiny drops of supercooled water instantly turn into ice when they hit something, such as an airplane's wing.

If you live just about anywhere but the very southernmost parts of the United States, you've probably encountered the ground version of aircraft icing -- an ice storm. Every few winters an ice storm will bring life to a halt in some Southern city or another, like Atlanta or Dallas. When supercooled raindrops fall they can coat roads, sidewalks, trees, and power lines with ice when the rain freezes on contact.

You can imagine what it would be like to fly an airplane into freezing rain, which leads to an important point about avoiding it. If sleet -- tiny ice particles -- is hitting the ground, don't even think about taking off. While sleet will bounce off your airplane without sticking, it's a sign that freezing rain is above you. Sleet is freezing rain that's fallen through a thick enough layer of cold air near the ground to freeze before it hits something. In the same way, a weather report of ice pellets -- this is how weather observers report what most people call sleet -- means there is freezing rain aloft above the weather station.

Pilots who aren't qualified to fly under instrument flight rules (IFR) usually shouldn't have to worry about aircraft icing in the air as long as they follow the rules and stay clear of clouds.

You could run into icing in VFR conditions if light freezing rain or drizzle were falling, but not reducing visibility to less than three miles. Flying through snow that doesn't reduce visibility too much can be dangerous -- if the stuff falling from the sky is all snow, the flakes or crystals will bounce off the aircraft without sticking. But, snow can have freezing rain mixed in.

The basic rule for airframe icing is that the temperature has to be below freezing and you have to be in "visible moisture," like cloud drops or rain drops. Note that this rule applies to airframe icing.

Carburetor icing is a completely different animal, and it can happen on a hot summer's day if the air is humid. As air goes through a carburetor's venturi it's forced through a smaller opening, which speeds up the air and lowers its pressure. At this point, fuel is added to the air and evaporates into it.

Since both lowering the air pressure and evaporation cool the air, it's possible for the air's temperature to drop maybe 50 degrees, instantly turning the water vapor in the air -- its humidity -- into ice that can choke the air-fuel mixture flowing to the engine, making it run rough or even stop running. The cure: Apply carburetor heat as recommended by the pilot's operating handbook -- details vary for different kinds of airplanes.

Another kind of "icing" that can occur without visible moisture is frost. Frost can be especially dangerous when it forms on an airplane because it looks so harmless.

Frost is ice that forms on an object, such as a blade of grass or an airplane, directly from water vapor in the air. It can cover the grass, your car's windshield, and your airplane's wings. An airplane that is covered by white frost would be hard to miss but easy to ignore; you might assume that the thin layer won't affect the airplane's performance.

But frost is rough enough to disturb the smooth flow of air around the wings, reducing lift. An airplane with frost-covered wings might not be able to lift off before running out of runway. Frost can also form on an airplane that's been flying in very cold air and descends into warm, humid air. The frost will reduce lift and increase stalling speed. As the airplane warms up, the frost will melt.

The most dangerous ice accumulations, however, are those on the wings and horizontal stabilizer as the airplane flies through supercooled, liquid water. In both cases, ice changes the shape of the wing or stabilizer, which reduces lift. Obviously, reducing the lift of the wing makes keeping the airplane in the air more difficult.

Changing the shape of the horizontal stabilizer can be even more dangerous. You use the tail-down force of the horizontal stabilizer to control the airplane's pitch. Ice that changes the shape of the horizontal stabilizer can degrade pitch control.

As ice accumulates, things can quickly grow worse. For instance, ice is not likely to affect both wings the same, which can lead to one wing losing more lift than the other, which means the airplane is trying to roll in one direction or the other. In some cases, ice has affected control surfaces like the ailerons, sending airplanes into uncontrollable rolls.

Trying to fly an airplane as ice builds up on it is more excitement than any rational person wants. Student pilots or private pilots who have not earned an instrument rating won't run into icing conditions as long as they follow the rules, stay out of clouds, and avoid flying in even light rain or snow (even if the visibility is good) when it's cold. This underscores why you should have a good idea of what the weather is likely to do before you take off, and make sure to turn around and land early in the game if you encounter unexpected clouds.

Once you obtain an instrument rating and start flying into clouds, weather decisions become much more difficult. Winter instrument flying requires deeper study of icing and how your aircraft reacts to a buildup of ice. For instance, icing accidents have prompted changes in operating procedure for some commuter aircraft that prohibit using the autopilot in potential icing conditions. An autopilot compensates for things such as loss of lift and consequently can hide the effects of icing from a pilot until it's too late.

Meteorologists have been working for years with some success to improve icing forecasts, but icing conditions remain difficult to forecast. Factors including temperatures at various altitudes and locations, and the speeds of upward air movements -- all clouds are in places where air is rising because if the air were not rising even the smallest cloud droplets would fall. Researchers have found that dangerous icing is more likely at certain speeds of rising air.

A big factor is how much water is in a cloud. The more supercooled water there is, the more ice can form on an airplane. Clouds that form in air which has been forced upward -- flowing over mountains, for example, or because an advancing front has pushed it up -- tend to have more water in them.

Obviously, clouds that form in places near large bodies of water such as oceans or the Great Lakes are more likely to coat airplanes with dangerous amounts of ice than clouds in drier places. This is why the northwest and northeast corners of the United States, and areas around the Great Lakes, offer some of winter's most dangerous flying. If clouds form above a layer of warm, humid air, they can have more water because warm air can contain more water vapor than cooler air. This is one reason that icing is included in the many hazards of thunderstorms.

Most pilots avoid thunderstorms no matter how skilled they are and what kind of aircraft they are flying, and we hear of few accidents caused by icing in thunderstorms. In the winter, however, clouds that are usually benign can be made of supercooled but still liquid water droplets.

Knowing how ice forms on airplanes is just the beginning of what any pilot who plans to fly in winter clouds needs to learn about the weather, forecasts, and his or her aircraft.

Jack Williams is the weather editor of An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.