When does structural icing typically occur during flight?

Structural icing typically occurs during flight when moisture in the atmosphere comes into contact with the aircraft at temperatures at or below freezing, specifically at 32°F (0°C) or colder. This is because when the air temperature is at or below this critical freezing point, any water droplets in the atmosphere will freeze upon contact with the aircraft surfaces, forming ice. This phenomenon is especially prevalent in clouds where supercooled liquid water exists, allowing for convection and the accumulation of ice on various aircraft surfaces, including wings and control surfaces.

While heavy precipitation can contribute to icing conditions, it is not the sole determining factor; the key element is the temperature at which the moisture encounters the aircraft. Therefore, other choices that suggest icing can occur in warmer temperatures or under different conditions are not accurate as they do not conform to the temperature criteria that are essential for structural icing formation.