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Rate of Accumulation

Four factors influence the rate of ice accumu-lation on an aircraft: (1) the amount of liquid water, (2) droplet size, (3) airspeed, and (4) the aircrafts size and shape.

AMOUNT OF LIQUID WATER. Ice formation is more rapid in cloud formations that are thick and continuous. Icing increases as the amount of supercooled liquid water in the air increases.

DROPLET SIZE. As aircraft move, they push or deflect the air. The faster they fly, the stronger is the airflow across their surface. When aircraft pass through clouds or precipitation, the small water droplets tend to move with the deflected airstream, and not to collect on airfoils or structural parts. Larger droplets resist the deflecting influence. More large supercooled droplets will strike an aircraft, thereby increasing the rate of ice accumulation.

AIRSPEED. The rate of ice formation is increased, to a point, by an increase in airspeed. But at very high speeds, such as those attained by jet aircraft, the situation is reversed. At high speeds, friction creates enough heat on the skin of jet aircraft to melt structural ice. Icing is seldom a problem at airspeeds in excess of 575 knots. However, the airspeed at which frictional heating prevents ice formation varies with aircraft (type, configuration, surface characteristics, etc.) and the ambient air temperature. Helicopter cruise speeds will control the rate of ice accumulation. Blade and rotor speeds of 570 to 575 knots are common at normal cruise speeds; therefore, frictional heat precludes ice buildup at the out-board portion of the main rotor blades. The chance of ice buildup increases inboard toward the rotor disk.

AIRCRAFT SIZE AND SHAPE. The size, shape, and smoothness of aircraft surfaces and airfoils also control the rate of ice accumulation. Ice accumulates at a faster rate on large, non-streamlined aircraft with rough surface features than it does on thin, smooth, highly streamlined aircraft. However, even on the best surfaces, once ice forms, it presents a larger surface area on which freezing droplets can collect, and the rate of accumulation is accelerated.

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