What is defined as structural icing?

Structural icing refers to the formation of ice on the external surfaces of an aircraft, which can impact its performance and safety. When an aircraft flies through supercooled liquid water droplets in clouds or freezing rain, these droplets can freeze upon contact with the aircraft's surfaces, such as wings, tail, and fuselage. This accumulation of ice changes the aircraft's aerodynamic characteristics, potentially leading to reduced lift and increased drag, which can compromise the aircraft's stability and control.

In contrast, the other options address different aspects of icing issues related to aircraft but do not accurately define structural icing. Ice formation within fuel tanks or on internal engine components, while concerning for operational safety, refers to issues that are not part of the aircraft's external structure. Additionally, icing is not limited to high altitudes; it can occur at various altitude levels based on temperature and atmospheric conditions. Thus, the understanding of structural icing is critical for pilots and engineers working to ensure safe aircraft operation in icy conditions.