PROCESS OF ICE FORMATION ON AIRCRAFT

The first step in ice formation is when the supercooled droplets strike the surface of the aircraft. As the droplet, or portion of it, freezes, it liberates the heat of fusion. Some of this liberated heat is taken on by the unfrozen portion of the drop; its temperature is thereby increased, while another portion of the heat is conducted away through the surface in which it lies. The unfrozen drop now begins to evaporate due to its increase in temperature, and in the process, it uses up some of the heat, which, in turn, cools the drop. Due to this cooling process by evaporation, the remainder of the drop is frozen. Icing at 0C will occur only if the air is not saturated because the nonsaturated condition is favorable for evaporation of part of the drop.

Evaporation cools the drop below freezing, and then ice formation can take place.

There are three intensities of aircraft icinglight, moderate, and severe.

The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the deicing/anti-icing equipment is used.

The rate of accumulation is such that even short encounters become potentially hazardous, and the use of deicing/anti-icing equipment or diversion is necessary.

The rate of accumulation is such that deicing/anti-icing fails to reduce or control the hazard. Immediate diversion is necessary.

Certain icing hazards exist on or near the surface. One hazard results when wet snow is falling during takeoff. This situation can exist when the air temperature at the surface is at or below 0C. Wet snow sticks tenaciously to aircraft components, and it freezes if the aircraft encounters markedly colder temperatures during takeoff.

If not removed before takeoff, frost, sleet, freezing rain, and snow accumulation on parked aircraft become operational hazards. Another hazard arises from the presence of puddles of water, slush, and/or mud on airfields. When the temperature of the airframe is colder than 0C, water blown by the propellers or splashed by wheels can form ice on control surfaces and windows. Freezing mud is particularly dangerous because the dirt may clog controls and cloud the windshield.

OPERATIONAL ASPECTS OF AIRCRAFT ICING

Due to the large number of types and different configurations of aircraft, this discussion is limited to general aircraft types, rather than specific models.

These high speed aircraft generally cruise at altitudes well above levels where severe icing exists. The greatest problem will be on takeoff, climb, and approach because of the greater probability of encountering supercooled water droplets at low altitudes. Also, the reduced speeds result in a decrease of aerodynamic heating.

Turbojet engines experience icing both externally and internally. All exposed surfaces are subject to external airframe icing.

Internal icing may pose special problems to turbojet aircraft engines. In flights through clouds that contain supercooled water droplets, air intake duct icing is similar towing icing. However, the ducts may ice when skies are clear and temperatures are above freezing. While taxiing and during takeoff and climb, reduced pressure exists in the intake system, which lowers temperatures to a point that condensation and/or sublimation takes place, resulting in ice formation. This temperature change varies considerably with different types of engines. Therefore, if the free air temperature is 10C or less (especially near the freezing point) and the relative humidity is high, the possibility of induction icing definitely exists.

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