When should the cowl anti-ice system be activated for takeoff?

The cowl anti-ice system should be activated for takeoff when visible moisture is present below 400 feet AGL with an outside air temperature (OAT) at 10°C or below. This is important because the combination of low temperatures and visible moisture increases the risk of ice formation on critical engine components and airframe surfaces, which can adversely affect performance and safety during takeoff.

At temperatures above 10°C, the risk of icing is significantly reduced, even in the presence of visible moisture. This means that activating the system in those conditions (when OAT is above 10°C) would not be necessary and could lead to unnecessary weight and potential loss of engine efficiency. Similarly, activating the system only when it is raining does not account for the risk of ice in cool conditions where other forms of visible moisture, such as fog or mist, could still lead to icing. Lastly, relying solely on low clouds ignores the broader context of visible moisture and temperature, which are critical factors in ice formation.

Thus, the correct timing for activating the cowl anti-ice system is linked to both temperature and the presence of moisture below 400 feet AGL, ensuring the aircraft operates safely during takeoff.