Which of the following is NOT a type of structural ice?

The correct answer is "Trace." Trace refers to a very light accumulation of ice that is often not significant enough to affect aircraft performance critically. In aviation, structural ice typically refers to ice formations that can accumulate on an aircraft and impact its aerodynamics and safety.

Rime ice forms when supercooled water droplets freeze upon contact with the aircraft surface, creating a rough, opaque coating. Clear ice, on the other hand, is a smooth, glossy layer that forms when larger water droplets freeze more slowly, allowing them to spread out and create a denser ice formation. Mixed ice describes a combination of both rime and clear ice characteristics and can present unique challenges in terms of removal and danger to flight operations.

In contrast, trace ice does not typically represent a hazardous level of accumulation and can often be disregarded in the context of aircraft operations. Therefore, it is not categorized as a type of structural ice, making it the exception among the options provided.