Which in-flight condition is necessary for aircraft structural icing to form?

• A. Visible moisture
• B. Dry air
• C. High altitude with clear skies
• D. Strong surface winds

Answer: (A)

Ice forms on an aircraft when liquid water droplets are present in the air and those droplets freeze on contact with the airframe. This requires visible moisture such as clouds, fog, rain, or spray. If the air is dry or the skies are clear, there are no liquid droplets to freeze, so structural icing cannot develop. Being at high altitude with clear skies still lacks those droplets, and strong surface winds don’t create icing by themselves—they don’t provide the necessary liquid water in the right conditions. In short, the key factor is the presence of visible moisture in which droplets can be supercooled and freeze upon impact with the wing or other surfaces.