Which form of structural icing is considered the most hazardous and difficult to see?

Clear icing is often considered the most hazardous type of structural icing due to its ability to accumulate on aircraft surfaces and its characteristics that make it difficult to detect. This type of icing forms when supercooled liquid water droplets freeze upon contact with an aircraft's surface, creating a transparent layer of ice that can be extremely hard to notice visually.

The main hazard associated with clear icing is that it can create significant weight and alter the aerodynamic properties of an aircraft, leading to performance issues. Since it is not easily visible, pilots and air traffic controllers may not realize that icing is occurring until it has reached a dangerous level, making it particularly risky during flight operations.

Other types of icing, like rime icing, is more opaque and easier to identify visually on surfaces. While mixed icing combines elements of both rime and clear icing, it does not carry the same level of stealth and danger as clear icing does. Structural icing itself is a general term that encompasses all forms of icing that build up on aircraft surfaces, but it does not specify the particular hazards associated with each type. Therefore, clear icing is recognized as the most hazardous due to its invisibility and the serious impact it may have on aircraft performance.