What is the most significant factor contributing to structural icing?

The most significant factor contributing to structural icing during flight is the presence of supercooled liquid water in the atmosphere, particularly when flying in clouds or precipitation at temperatures below freezing. While excessive altitude, atmospheric pressure changes, and de-icing procedures can impact flight safety and performance in various ways, they do not directly contribute to the formation of structural icing.

Icing occurs when supercooled droplets make contact with aircraft surfaces and freeze, leading to the accumulation of ice. In this context, the listed correct choice about increased drag and fuel consumption can be connected to icing as a secondary effect. If icing accumulates on an aircraft, it alters the aircraft's shape, increasing drag and resulting in greater fuel consumption and potential weight gain. All of these effects can exacerbate the risks associated with flight in icing conditions.

This understanding emphasizes the importance of recognizing flight conditions and understanding the hazard posed by supercooled liquid water rather than just focusing on the physical state changes or maintenance processes like de-icing.