Weather

Ice storms are often the most disruptive kinds of winter weather. Plows and shovels can move snow out of the way, and snow rarely breaks bare tree limbs.

In contrast, the freezing rain that falls during an ice storm instantly turns to ice when it hits anything that’s cold—such as tree limbs. A weatherwise pilot looking at the ice-encrusted tree limb that ripped down her home’s power lines could say: “Structural icing brought it down.”

This, in effect, is what the National Transportation Safety Board says after investigating many cold-weather aircraft crashes. Unlike with a tree limb, however, the weight of ice on an aircraft isn’t what causes it to crash.

In fact, even heavy ice on an airplane often doesn’t make up for the weight of the fuel that’s already been burned. Icing causes crashes by, in effect, redesigning the wings and horizontal stabilizer, which causes them to lose lift. It also increases drag.

Think of it this way: Your airplane might have been designed years ago by engineers using slide rules, or more recently with the aid of super computers running computational fluid dynamics programs. No matter what tools they used, the engineers focused on wing and horizontal stabilizer shapes that would supply the lift needed to keep the airplane in the air and give the pilot adequate pitch and roll control. Only a little ice is needed to randomly redesign these carefully shaped airfoils.

Secrets of supercooled water. Water does not always freeze when it cools to 32 degrees Fahrenheit (0 degrees Celsius). Ice does begin melting when it warms up to this temperature.

As the air and the water drops in a cloud cool below 32 degrees, some of the water molecules begin locking together more firmly to form six-sided ice crystals, but this doesn’t happen instantly at 32 degrees. Why? “Water below freezing wants to be ice,” says Charles Knight, an ice expert with the National Center for Atmospheric Research. “It just has to learn how.” The easiest way for a supercooled drop to “learn” how to become ice is to bump into an ice crystal. Supercooled water drops also freeze into ice when they run into freezing nuclei, which are tiny particles with a shape close to that of ice crystals. Ordinary air, even “clean” air, has millions of tiny particles that help water drops turn into ice. Most of these aren’t effective until the air cools to around 15 degrees F, however.

Water in relatively large containers—such as the sections of an ice-cube tray—turns to ice close to 32 degrees because slight imperfections in the metal or plastic ice tray’s surface act like freezing nuclei. When an airplane is flying though a cloud of supercooled water drops, some of them will freeze instantly when they hit something that “teaches” them how to become ice—such as a microscopic imperfection on an airplane wing. Once ice begins to form on an airplane, it supplies millions of ice crystals that convert more supercooled drops into ice.

Less lift. If you fly into icing conditions, the best-case scenario is if you notice that your wings are slowly losing lift, but as far as you can tell this loss is even on both wings. You have to add power to hold altitude. But, a suitable airport is near and you reach it and land safely before running out of power, lift, and good ideas.

Not only does ice on an airplane’s structure reduce lift and add drag, its random nature means that lift on one wing could be stronger than on the other, causing the airplane to roll toward the side with less lift.

Ice that forms on an airplane’s horizontal stabilizer is especially dangerous, because the horizontal stabilizer is a wing with the lift force acting downward. Increasing the lift that pulls the tail down brings the nose up, because an airplane pitches around its center of gravity. When a horizontal stabilizer or stabilator is iced up, pitch control is weakened or even lost, causing the tailplane to stall. The airplane can nose over into an uncontrollable dive. Tailplane stalls are most likely when the flaps are lowered for landing.

Icing under clouds. The conditions needed for in-flight aircraft icing are below-freezing temperatures and visible moisture, which includes clouds, rain, and drizzle. Drizzle refers to water drops falling from the sky that are 0.01 to 0.02 inches in diameter, while raindrops are slightly larger. Don’t let this make you think that you don’t have to be concerned about in-flight icing until you earn an instrument rating and begin flying in clouds—or flying in clouds with an instructor while earning your instrument rating.

Visual-flight-rules pilots rarely have to worry about icing unless they blunder into clouds because they didn’t turn back when they had a chance. Nevertheless, you could fly through very light rain, drizzle, or even snow while still maintaining the FAA’s requirements for visibility and distance from clouds. If the temperature is below 32 degrees F, rain or drizzle drops could be supercooled and freeze when they hit. Even a hint of ice beginning to form on your airplane means you need to land as soon as possible.

Beware mornings. If you’re ever driving to the airport on a clear, cold morning thinking, What a great day to fly, unlimited visibility and calm air, you should also say to yourself: “Frost on grass is beautiful; frost on airplanes is dangerous.”

Frost forms on grass, windows, cars, and airplanes on clear, calm winter nights when the clear sky allows heat to radiate away and no winds stir the air to mix the coldest air near the ground with warmer air a little higher up. Water vapor in the air deposits directly as ice crystals without first condensing into water and then freezing. When it forms on a white airplane, frost can be difficult to see.

Wind-tunnel tests have shown that frost on the wings that is no thicker or rougher than coarse sandpaper can reduce lift by 30 percent, and increase drag by 80 percent or more. The random wing redesign caused by the ice could keep the airplane from lifting off before you run out of runway.

Even worse, you might lift off in time, but unequal amounts of frost on the wings can cause the airplane to roll unexpectedly—too close to the ground for you to level the low wing before it hits the surface.