FAA Ground School Practice Test

One in-flight condition necessary for structural icing to form is:

1. Inadequate engine heat

2. Visible moisture

3. High altitude

4. Clear skies

The correct answer is: Visible moisture

For structural icing to form during flight, one critical condition is the presence of visible moisture. This moisture can originate from various sources such as clouds, rain, or snow. When an aircraft flies through this environment when the temperature is below freezing, the moisture can freeze upon making contact with the aircraft's surfaces. This accumulation of ice can pose significant safety hazards, including increased weight, decreased performance, and control difficulties. The presence of visible moisture is essential because, without it, even if the temperature is low enough, there would be no water to freeze and create icing. Thus, understanding and recognizing the conditions where visible moisture is present is crucial for pilots to avoid or mitigate the risks associated with structural icing.