Structural Icing in VMC Structural icing in Visual Meteorological Conditions may sound like an oxymoron to many pilots. These conditions constitute the physical phenomenon known as supercooled water. Supercooled Water One of the qualities of water is that it can be found in its three forms at or below 0 degrees Celsius. That is water, vapor and ice. The state of water at or below freezing is defined as supercooled water.
Supercooled water is in an unstable liquid state.
Even a small impact on a supercooled droplet will violate the fine balance, causing it to freeze. As an aircraft moves through supercooled water the formation of structural ice is imminent. The ice forms on all the surfaces that are exposed to the initial impact. That includes the leading edge of the wings, the empennage and the propellers, as well as the struts, the windshield, etc. The types of structural ice are clear, rime and a mixture of the two.