In icing conditions, when must the cowl anti-ice system be turned ON?

The cowl anti-ice system must be turned ON when the ice detection system annunciates ICE. This is crucial because the cowl anti-ice system is designed to prevent ice accumulation on the engine inlets and helps ensure proper engine performance during flight. In icing conditions, allowing ice to build up can lead to airflow disruptions and potential engine performance issues, including the risk of ice ingestion.

When the ice detection system indicates the presence of ice, it acts as a direct prompt for the crew to activate the anti-ice systems to maintain safe operational conditions. This proactive measure is central to maintaining the aircraft's safety and performance in known icing environments.

The other options are either too limited in scope or do not account for the system's operational requirements in all phases of flight. For example, while cowl anti-ice may be required before takeoff or during ground operations, the critical factor is when the ice detection system signals the risk of icing; that is the moment when action is necessary for safety.