Under what conditions does the anti-ice system generally operate?

The anti-ice system is designed to prevent the formation of ice on critical aircraft surfaces, particularly when flying in conditions where temperatures are at or below the freezing point. Ice accumulation can significantly degrade aircraft performance and safety by disrupting airflow over wings and control surfaces, or adding excessive weight. Therefore, it is crucial for the anti-ice system to engage when outside temperatures drop to levels conducive to ice formation, typically at or below freezing.

While the other conditions mentioned might suggest scenarios where ice formation could be a concern, such as during heavy turbulence that could lead to moisture freezing or during taxiing when the aircraft could be exposed to cold air, they do not encapsulate the primary operational trigger for the anti-ice system, which is related directly to temperature thresholds. Thus, the primary condition under which the anti-ice system operates is indeed when temperatures are below freezing.