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DEICE/ ANTI-ICING SYSTEMS On days when there is visible moisture in the air, ice can form on aircraft leading edge surfaces at altitudes where freezing temperatures start. Water droplets in the air can be supercooled to below freezing without actually turning into ice unless they are disturbed in some manner. This unusual occurrence is partly due to the surface tension of the water droplet not allowing the droplet to expand and freeze. However, when air-craft surfaces disturb these droplets, they immediately turn to ice on the aircraft surfaces. The ice may have a glazed or rime appearance. Glazed ice is smooth and hard to detect visually. Rime ice is rough and easily noticed. Frost is formed as a result of water vapor being turned directly into a solid. Frost can form on aircraft surfaces in two ways. First, it can accumulate on aircraft parked in the open over-night when the temperature drops below freezing and proper humidity conditions exist. Second, it can form on aircraft surfaces, caused by flying at very cold altitudes and descending rapidly into warm, moist air. In this case, frost deposits will result before the structure warms up because of the marked cooling of air adjacent to the cold skin. Ice or frost forming on aircraft create two basic hazards: (1) the resulting malformation of the airfoil, which could decrease the amount of lift; and (2) the additional weight and unequal formation of the ice, which could cause un-balancing of the aircraft, making it hard to control. Enough ice to cause an unsafe flight condition can form in a very short period of time, thus some method of ice prevention or removal is necessary. Presently there are two methods for removing or preventing ice. One method, deicing, employs a mechanical system to break up and remove the ice after it has formed. The second method, anti-icing, uses heated bleed air to prevent the formation of ice. Deicing systems are common to older aircraft, and are now generally being replaced by anti-icing systems.
Deice Boot Systems The deice system for the wing, horizontal, and vertical stabilizer leading edges of E-2 aircraft is an example of a typical deice boot system. The system removes accumulated ice from the wing surfaces through the use of rubber deice boots, which are bonded to the leading edges. The cells or tubes of the deice boots (fig. 1-7) are inflated and deflated alternately by applying pressure and suction, causing a wavelike motion that cracks the formed ice and allows it to be carried away by the airstream. The E-2A deice boot system shown in figure 1-8 is pneumatically operated and electrically controlled. Engine bleed air provides the necessary air pressure. The bleed air is regulated by the pressure regulator and relief valve. Suction is provided by the ejector and regulated by the suction relief valve. Suction and pressure gages provide a means of monitoring system readings that indicate proper or improper system operation. The deice system consists of the follow-ing main components: electronic timer, three distributor valves, pressure regulator and relief valve, ejector, suction relief valve, the de-ice boot sections, and pressure and suction gages.
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