At what temperature range can clear structural ice form on an aircraft?

Clear structural ice can form on an aircraft primarily under conditions where the temperature is conducive to supercooled liquid water existing in the atmosphere. This typically occurs when the temperatures are between 2 degrees Celsius and -10 degrees Celsius. Within this range, the water droplets can remain in a liquid state even below freezing temperatures, which is essential for the formation of clear ice upon contact with the aircraft surfaces.

When temperatures are at or above 0 degrees Celsius, the liquid water is not supercooled, meaning it will not freeze upon contact, thus clear ice will not form. Conversely, temperatures below -10 degrees Celsius generally lead to a different type of ice formation, such as rime ice, which is characterized by a different structure and freezing process. Therefore, the conditions between 2 degrees Celsius and -10 degrees Celsius allow for the most critical formation of clear structural ice on an aircraft, making this the correct range for the question.