What is the lowest ambient temperature at which engine ice is likely to form in air clear of visible moisture?

Ice can form on aircraft engines at temperatures near or below freezing, even in clear air without visible moisture. The phenomenon is often associated with supercooled water droplets, which can exist in the atmosphere above freezing temperatures.

The lowest ambient temperature at which engine ice formation is likely, in clear air conditions, generally aligns with understanding that ice can start to form when the temperature is around +5 degrees Fahrenheit. At this temperature, any residual moisture present in the air can still lead to ice accumulation on engine surfaces, particularly in environments where the engine is operating under certain airflow and temperature conditions.

Therefore, the correct answer reflects the temperature threshold based on aviation safety practices, which emphasize vigilance in cold weather conditions to prevent ice buildup, even when visible moisture is absent.