For which icing classification is deicing equipment necessary for effective operation during flight?

The classification of icing that necessitates the use of deicing equipment for effective operation during flight is moderate icing. In moderate icing conditions, the accumulation of ice can significantly affect an aircraft's performance, handling, and safety, making it crucial to have deicing systems operational.

Deicing equipment helps remove ice that has already formed on critical surfaces, such as the wings and propellers, ensuring that the aircraft maintains lift and performance close to the designed levels. Moderate icing can lead to a continuous buildup of ice, which can worsen progressively if not managed with effective deicing methods.

Lighter icing conditions, like light and trace icing, may not require immediate action or specialized equipment, as the accumulation is generally manageable and may not significantly degrade aircraft performance. Severe icing, on the other hand, indicates an urgent need for deicing measures, but it often suggests that the pilot should avoid such conditions altogether if possible. Thus, while severe icing does require deicing equipment, the key classification during flight operations where deicing becomes essential to maintain safety and performance is indeed moderate icing.