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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

EVEN A THIN layer of frost on an airplane's leading edges and control surfaces can spoil lift (right). Remove all traces of ice from the aircraft before you launch.

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. Neverthe-

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Flight Training - November 2012

Table of Contents for the Digital Edition of Flight Training - November 2012