What condition is necessary for the formation of structural icing in flight?

The formation of structural icing in flight primarily requires supercooled water drops. Supercooled water refers to water droplets that remain in a liquid state even though they are below freezing temperatures. When an aircraft flies through an environment where supercooled droplets are present, they can adhere to the surfaces of the aircraft, leading to ice accumulation.

While visible water can hint at potential icing conditions, it does not account for the critical factor of temperature and the state of water. In the atmosphere, temperature below freezing could suggest conditions that favor icing, but it is the presence of supercooled water that directly leads to the formation of structural icing. Therefore, in understanding aircraft icing phenomena, recognizing supercooled water drops is essential, as they are the actual contributor to ice formation on the aircraft's structure.