Formation and Accretion of Supercooled Water Droplets and Their Effect on Aircraft

MIL-STD-81D currently has no procedure to test equipment that will be exposed to supercooled fog and the resultant ice accretion. Supercooled fog icing is one of the most difficult conditions to simulate. Thus, when a piece of equipment must be subjected to this type of icing, the Program Manager or Test Engineer must design a custom icing test with no formal Military Standard guidance. These conditions can be especially hazardous to helicopters and fixed-wing aircraft that must fly through icing conditions. This paper discusses several technical aspects of the icing process, including such topics as the supercooling process, nucleation and freezing of water droplets, ice type definition, types of problems to be expected during flight through icing conditions, and what icing parameters are important for an icing simulation. This paper can be used as an introduction to the icing phenomenon and will equip the reader with enough basic information to begin a discussion on icing test requirements.