When must the A/I system be selected ON while taxiing?

The A/I (Anti-Ice) system must be selected ON when taxiing on contaminated surfaces to ensure that ice, snow, or slush does not accumulate on the wings, leading edges, or control surfaces of the aircraft. Contaminated surfaces can significantly affect the aircraft's performance by impacting lift and increasing drag. By activating the A/I system during these conditions, the aircraft can maintain necessary performance and safety standards, as it helps to prevent buildup that could compromise takeoff capability.

In contrast, the other scenarios presented do not warrant the use of the A/I system during taxiing. For instance, it is not necessary when conditions are clear because there is no risk of icing. Additionally, turning on the A/I system whenever the engines are on does not consider the operational need based on environmental conditions. Furthermore, while preparing for takeoff, the focus is typically on ensuring that the systems needed for takeoff are operational, and the specific activation of the A/I system would still depend on the runway and aircraft conditions rather than a blanket rule.