Which icing intensity is defined as creating a problem over prolonged flight?

The correct answer is light ice. In aviation, icing intensity classifications are important for understanding the potential impacts of icing conditions on aircraft performance and safety.

Light icing is recognized as causing an accumulation of ice that can create challenges over an extended duration of flight. Although it might not immediately disrupt normal operations, the effects can become significant if the flight is prolonged in these conditions. This could lead to difficulties in controlling the aircraft, degrading lift, and increasing weight, thereby necessitating careful management by the flight crew to ensure safety.

In contrast, trace ice typically doesn't accumulate in significant amounts, while moderate ice poses a more urgent concern as it can significantly affect aircraft performance, if encountered for longer periods. Severe icing is the most dangerous level and can lead to critical failures in aircraft systems quickly, rather than over a prolonged period.