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Page Title: Factors Necessary for Structural Ice Formation
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Factors Necessary for Structural Ice Formation

Factors necessary to produce structural icing on aircraft in flight are (1) the ambient tempera-ture must be at or below freezing and (2) visible liquid moisture in the form of clouds or precipita-tion must be present.

AMBIENT AIR TEMPERATURE. Ice can form when saturated air flows over stationary air-craft with ambient temperatures as high as 4C (39F). Aircraft on an airfield or flight deck are subject to such icing. When an aircraft is stationary and the ambient temperature lowers to near freezing (1C to 4C), the aircrafts structural surfaces may freeze because of evapora-tion and the pressure changes caused by the moving air.

When an aircraft is flying through saturated air, its structural surfaces are heated by friction and the impact of waterdrops. Because the heating and the cooling of moving objects tend to balance, structural ice does not form unless the outside air temperature is at or below freezing (0C). The most severe aircraft icing occurs in layers of air whose ambient temperature is between 0C and 10C.

CLOUDS AND PRECIPITATION. Clouds are the most common form of visible liquid moisture. However, not all clouds with below-freezing temperatures produce serious icing conditions. Serious icing conditions are rare in temperatures below 20C. Pilots must be made aware, however, that structural icing is possible in even colder temperatures.

Rain and drizzle are forms of precipitation (liquid moisture) that can form ice when conditions are right. When rain and/or drizzle fall through layers of air whose ambient temperature is at or below freezing, the droplets become super-cooled. When the supercooled droplets strike an object, such as an aircraft, they freeze. Freezing rain can build hazardous amounts of ice in a few minutes, and the ice is extremely difficult to break loose from aircraft structures.

Types of Aircraft Structural Ice

Aircraft structural icing may be clear, rime, a combination of clear and rime, or frost. The type of ice that forms on a moving object normally depends on (1) the ambient air temper-ature, (2) the surface temperature of the object, (3) the characteristics of the surface of the structure (configuration, roughness, etc.), and (4) the size of the waterdrops.  

CLEAR ICE (GLAZE). Clear ice, also know as glaze, is the most serious form of structural ice. The large supercooled water droplets that cause glaze strike an aircraft, and spread out on or around the movable and non-movable surfaces of the aircraft before they freeze. Glaze adheres firmly to exposed surfaces, and since it is transparent or translucent, it may go undetected until it is too late for deicing methods to remove it.

When clear ice is transparent, it resembles ordinary ice. When it mixes with snow, sleet, or small hail, the glaze may be rough, irregular, and whitish.

Glaze forms on the leading edge of airfoils and antennas, where it takes on a blunt-nose shape. The glaze tapers toward the trailing edge.

RIME ICE. Rime is a white or milky, opaque, granular deposit of ice created by small supercooled water droplets. It is normally encountered in stratiform clouds where the temperature ranges between 0C and -20C or in cumuliform clouds with temperatures between -10C and -20C.  

Rime ice is not as compact as clear ice, and the small supercooled water droplets do not spread out before they freeze. Rime ice forms and accumulates on the leading edge of exposed parts, producing rough, irregular formations.

FROST. Frost is formed through the process of sublimation; ice crystals form when water vapor contacts a cold surface. Frost may form in the air and on the ground. When aircraft descend through a cold layer of air into a layer of warm, moist air, frost formation is possible. It may also form at flight level when aircraft pass from a sub-freezing air mass into a moist and slightly warmer air mass. On the ground, frost may form during a clear night with subfreezing temperatures. Pilots tend to underestimate the flight hazards of frost formation; therefore, you must make sure they understand its dangers. On a windshield, frost can restrict visibility or cause total loss of visibility. And at low airspeeds, during takeoff and landing, frost is particularly hazardous because it increases drag.

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