Final answer:
The wing anti-ice system should be activated on the ground when the OAT is at or below freezing to prevent ice formation on the wings, crucial for aircraft safety.
Step-by-step explanation:
The wing anti-ice system should be turned on the ground when the Outside Air Temperature (OAT) is at or below freezing, which is 0°C (32°F). This is necessary to prevent the formation of ice on the wings, which could disrupt the airflow and negatively impact the aircraft's lift and safety during takeoff.
The wing anti-ice system works by using hot air from the engines or electric heating to keep the wing surfaces above the freezing point, preventing ice from building up.