When must the cowl anti-ice system be on?

The correct answer indicates that the cowl anti-ice system must be activated when the outside air temperature (OAT) is 10°C or below. This requirement is critical for ensuring that ice does not form on the engine inlets during flight operations and is particularly important during phases of flight where icing conditions are possible, such as taxiing or takeoff.

When operating in temperatures at or below 10°C, there is a risk of ice accumulation, especially if the aircraft is flying through higher humidity or visible moisture. Activating the cowl anti-ice system helps maintain airflow into the engines and prevents engine performance degradation caused by ice buildup.

This limitation underscores the importance of closely monitoring temperature conditions during flight operations to enhance safety. It's also worth noting that the other options provided do not align with the specific operational guidelines necessary for safe operation regarding ice accumulation on the engine inlets.