What is one in-flight condition necessary for structural icing to form?

The necessary condition for structural icing to form during flight is the presence of visible moisture. This moisture can come from various sources, such as rain, snow, sleet, or clouds containing supercooled water droplets. When an aircraft flies through this moisture at temperatures below freezing, these droplets can freeze upon contact with the aircraft's surface, leading to ice accumulation.

It's important to understand that simply flying at a certain altitude or in specific cloud types does not guarantee icing will occur. While a small temperature/dewpoint spread can indicate potential for moisture, it does not directly lead to icing without visible moisture. Similarly, while stratiform clouds may contain supercooled droplets, it's the moisture itself that is crucial for ice formation, not the cloud type alone. High altitude flight, depending on conditions, may lead to colder temperatures, but again, icing will only occur if there is visible moisture present at those low temperatures. Thus, the presence of visible moisture is the critical factor for structural icing during flight.