Supercooled water is liquid even though its temperature is below 32 degrees Fahrenheit (0 degrees Celsius). Despite what you might have learned in school, water doesn't necessarily freeze at 32 degrees F. In fact, water can cool to around minus 40 degrees. (This happens to be the temperature where the Fahrenheit and Celsius scales are the same, but this is just a coincidence. Both scales were invented long before scientists learned how cold water can get without freezing.)
Water can freeze when its molecules slow down enough to lock into crystals, which happens at around 32 degrees. In addition to being cold enough for the molecules to slow down enough to hook together, the water needs templates to "teach" it how to form ice crystals. The templates are called freezing nuclei. These include tiny bits of clay and other natural materials, even some strains of bacteria. Some types of pollution can also do the job. As you might imagine, ice itself is one of the best freezing nuclei. For instance, snow falling from high clouds into lower ones made of supercooled droplets will turn the lower clouds into clouds of ice crystals, not supercooled water drops.
Supercooled water freezes when it hits something fairly large in comparison with the drops, such as a tree branch - or an airplane's wing. Enough drops hit such objects that some targets are bound to include freezing nuclei. Once the ice forms, other drops hitting the object will freeze.
Water supercools only in small amounts - the quantities found in cloud, drizzle, and raindrops. If there is enough water - the few tablespoons needed to make an ice cube in your freezer is more than enough - it will freeze when the temperature drops to around 32 degrees because the odds are high that it contains enough freezing nuclei to get things started.
The ice that forms on an airplane when it flies into drops of supercooled water is called structural icing. Such ice adds weight to an aircraft, but this isn't the primary reason it's so dangerous.
As ice builds up on an airplane's wings it creates random shapes that disrupt the flow of air over the wings. The airplane's designers, or course, had carefully calculated the wing shape needed to create lift. In minutes ice can, in effect, wipe out some of the designers' efforts. The loss of lift increases the airplane's stalling speed, which is why pilots are advised to land at a higher speed than normal if their airplanes have picked up ice.
While it's disrupting lift, ice is also adding to the drag acting on the airplane. Just when a pilot needs to increase airspeed to make up for the lift being stolen by ice, the added drag is making the extra speed hard to obtain. If a pilot doesn't escape the ice, the airplane will eventually be unable to maintain the airspeed needed to create the lift required to stay in the air. Ice also decreases the lift being produced by the the horizontal stabilizer, which is really a wing with the "lift" acting downward. You control pitch by changing the amount of downward lift that it produces. Tail plane icing, obviously, can rob pilots of some, maybe a lot, of pitch control.
Ice does more than decrease a wing's total lift; it is likely to create uneven amounts of lift along the wing. In other words, ice can make an airplane act as though it has sprouted ailerons with minds of their own. The flight of an EMB-120 Brasilia commuter airplane bound from Nassau in the Bahamas to Orlando, Florida, on March 19, 2001, illustrates what can happen.
The pilots were using the autopilot at about 17,000 feet when the airplane flew into a cloud, and the pilots saw ice beginning to form on the wings and propeller spinners, according to a National Transportation Safety Board report. The plane's airspeed fell from around 200 knots to 140 kt. When the autopilot was disconnected, the airplane rolled about 90 degrees to the left, and then about 120 degrees to the right, before the airplane did a 360-degree roll back to the right and went into a wings-level dive. The pilots pulled the aircraft out of the dive at about 10,000 feet.
They recovered and landed safely in Florida about 12 minutes later. The two flight crewmembers, the flight attendant, and the 25 passengers were not injured. But the elevators and horizontal stabilizer sustained substantial damage, the NTSB said.
In addition to showing how icing can wrest control of an airplane away from a pilot, at least temporarily, this story also illustrates that just because you are flying above shirtsleeve weather on the ground, you can't forget the icing danger.
Even so, pilots of small general aviation airplanes that usually fly closer to the ground are much more likely to encounter icing during cold weather.
The basic rule to remember is that visible moisture and below-freezing temperatures are needed to produce structural icing. Visible moisture refers to clouds, rain, or drizzle, but not the humidity in the air. Obviously, the starting point for avoiding structural icing is to stay out of clouds when the clouds are in air that's below freezing.
If the air is very cold, clouds will be made of ice crystals. In fact, the air doesn't have to reach minus 40 degrees F for this to happen. When ice crystals begin forming in a cloud they will grow at the expense of water drops. But this is of concern only to pilots with the training and equipment to fly in clouds under instrument flight rules. For visual flight rules pilots, the danger of icing is one more reason to avoid clouds.
Staying out of clouds, however, won't guarantee that you'll avoid ice if anything is falling from the clouds. Dry snow won't stick to an airplane, unless it's pretty cold. But you can't always be sure that the snow isn't wet, with water on the snow crystals, or with freezing rain or drizzle mixed in with the snow. The fact that the snow isn't heavy is no guarantee that it can't build up dangerous ice, and you don't have to be in the air for this to happen.
In 1995 the National Center for Atmospheric Research reported on a study of five airline take-off crashes that occurred when wet snow was falling. In all of the cases, the precipitation was reported as "light" with visibility ranging from a quarter-mile to two miles. Since the precipitation was light, it wasn't perceived as dangerous. In fact, it was more dangerous than heavier snow might have been because the wet ice crystals and water drops are smaller than dry snowflakes, which reduce visibility more than the wet mixture does. In the cases studied, temperatures were between 25 and 32 degrees F, which is the danger zone for wet snow or snow mixed with freezing rain.
Sleet - tiny ice pellets that bounce when they hit-at ground level is a warning not to take off since it's a sure indication of freezing rain aloft and maybe on the ground. Weather stations report what most people call "sleet" as ice pellets. If you obtain an en route briefing or a weather report for your destination airport and ice pellets have been observed, don't even think about trying to land there. In fact, you should immediately start looking for an alternate landing place that is well clear of reported ice pellets.
Sleet forms when rain falls from air that's warmer than 32 degrees into frigid air. As the rain falls it cools below 32 degrees, becoming supercooled. If the layer of cold air is deep enough, the freezing raindrops will turn into ice pellets before hitting the ground. Often, sleet at the ground will have freezing rain mixed in. Remember, where there's sleet, freezing rain is lurking somewhere near by.
Freezing drizzle can mislead you. Drizzle does not mean "light rain." The term refers to falling drops of water that are less than 0.02 inches in diameter. Bigger drops are rain, by definition. Drizzle can become heavy enough to endanger airplanes. And, researchers have found that some clouds contain drizzle-sized drops of supercooled water, making such clouds one of the most dangerous kinds to fly into.
All of icing's dangers aren't in the air. Freezing rain or drizzle or wet snow could leave a coat of ice on an airplane that's sitting on the ground. Any ice should be removed from the airplane before it's flown. Frost on the wings shouldn't be ignored. Researchers have found that as little as 0.8 millimeters of frost on a wing - a little more than a hundredth of an inch - can reduce lift by 25 percent as well as increase drag. The roughness of the frost disrupts the flow of the air that creates lift.
Since frost forms on clear nights, you might not be expecting any ice as you begin a winter morning's preflight inspection. Frost isn't precipitation; it forms when humidity in the air turns directly into ice without condensing into water. It's most likely to form on clear nights because such nights are colder - everything else being equal - than cloudy nights.
Moving an airplane into a warm hangar is the easiest way to remove frost. If you do this, be sure to wipe the wings and other surfaces dry so the water won't refreeze when you move the airplane back outdoors.
When cold air moves in, pilots have to get into the habit of being alert for ice, not only in the air but also on the ground - even when no visible moisture in the form of drizzle, rain, snow, or even a cloud is to be seen.
Jack Williams is the weather editor of USAToday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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