Icing and freezing rain occurs when the air is saturated with super-cooled water droplets which forms rime ice and/or glaze ice. Rime ice is a white or milky accumulation and is lighter, softer and less transparent than glaze ice. Glaze ice is a generally clear hard coating of ice. Rime forms when super-cooled water drops freeze on exposed surfaces. Glaze ice forms from a film of super-cooled water vapor freezing as a layer.
Method 521.2 can be applied to any material exposed to freezing rain conditions including large items such as vehicles, aircraft, and ships. This method generally does not apply to electronic equipment such as rackmount computer systems.
Some adverse effects of ice and freezing rain include:
a. Binds moving parts together.
b. Adds weight to radar antennas, aerodynamic control surfaces, helicopter rotors, etc.
c. Increases footing hazards for personnel.
d. Interferes with clearances between moving parts.
e. Induces structural failures.
f. Reduces airflow efficiency as in cooling systems or filters.
g. Impedes visibility through windshields and optical devices.
h. Affects transmission of electromagnetic radiation.
i. Provides a source of potential damage to materiel from the employment of mechanical, manual, or chemical ice removal measures.
j. Reduces efficiency of aerodynamic lifting and control surfaces.
k. Reduces (aircraft) stall margins.
The principle goal of the test is to have super-cooled liquid water droplets impinge on the test item and freeze in place. A chamber of suitable size capable of maintaining a temperature of -10 deg C (14 deg F) with the ability to spray cooled water on the test item is required. The test can be performed outdoors if the ambient conditions are prone to icing. Test conditions can be adjusted such as temperature, droplet size, water temperature, etc., to achieve satisfactory icing conditions.