When must the WING/COWL A/I ON advisory message be displayed?

The WING/COWL A/I ON advisory message is designed to alert the flight crew that the wing and cowl anti-ice systems are activated. This functionality is critical during various phases of flight to ensure that ice does not accumulate on critical surfaces, thus maintaining aircraft performance and safety.

The correct answer pertains to the specific operational requirement that mandates the activation of the anti-ice systems prior to takeoff. If conditions are conducive to icing, which can occur during pre-takeoff roll, the WING/COWL A/I ON message must be displayed to confirm that the systems are operational and that the aircraft is prepared for safe takeoff.

In contrast, during descent, after touchdown, or during engine startup, the activation of the wing and cowling anti-ice systems is not necessarily required. The advisory message's display before takeoff is specifically focused on ensuring that all necessary precautions are in place in a potential icing environment, making it critically important for the safety of the flight.