In which conditions must anti-ice systems be used?

The use of anti-ice systems is critical for maintaining the safety and performance of aircraft during flight. The correct answer identifies the specific conditions under which these systems should be activated, which involves temperatures of 10°C or below combined with the presence of visible moisture.

This situation is key because icing can occur when the aircraft is flying through clouds or precipitation where the temperature is near or below freezing. The moisture in the air can freeze upon contact with the aircraft’s surfaces, leading to ice accumulation that negatively affects aerodynamics, increases weight, and can obstruct critical sensors.

Activating anti-ice systems in these conditions helps prevent icing, ensuring that the aircraft maintains its performance and safety standards. Other options do not adequately capture the operational parameters required for safe anti-ice system use. For instance, using the systems only during takeoff or landing ignores scenarios where the aircraft might encounter icing at other flight phases, while relying solely on a temperature criterion or using the systems in any weather condition is neither safe nor effective in preventing ice accumulation.