Page:CAB Accident Report, United Airlines Flight 21.pdf/52

 conditions at temperatures hovering around the freezing point or below, or in any cloud the temperature of which is at or below freezing, and in precipitation below clouds even though no precipitation is evident on the surface of the ground. The rate of accumulation of ice is also dependent upon such factors as the size of the aircraft, its speed, the length of time in the icing condition, the rate of accumulation, the size, temperature and rate of descent of the water droplet and the angle at which it strikes the aircraft surfaces. This combination of variables makes it almost impossible for meteorologists or pilots to predict either the kind of ice or the rate of its accumulation under reported weather conditions. Also, it is often quite difficult during conditions in which ice is forming for the pilot to see, from the cockpit, the type or the rate of accumulation of ice. In addition, an examination of the aircraft after landing seldom gives any reliable indication of the type or amount of ice which the aircraft had accumulated at flight altitudes because of the effects of evaporation, temperature changes, and vibration.

Generally speaking, pilots classify ice as either "rime" or "glaze" ice. Rime ice is usually milk in color, of a rough uneven globular form, described as tending to build up very quickly but brittle in nature and easy to break off with de-icing equipment. Glaze ice is described as clear and smooth, tending to build up very rapidly but clinging to and preserving the form of the object upon which it accumulates. Glaze ice is harder to remove and when broken off by the action of the de-icer boot, tends to stick in patches and build up over the cap strip behind the boot. Glaze ice will not blow off as easily as does the more brittle and rough rime ice.