What must be ON for the autothrust system to arm when thrust levers are advanced for takeoff?

For the autothrust system to arm during takeoff, it is essential for the Flight Directors (FDs) to be turned ON. When the flight directors are active, they provide the necessary guidance information to the autothrust system, enabling it to function correctly and support the flight crew in managing thrust during critical phases of flight like takeoff.

The Flight Directors play a crucial role in indicating the proper power settings and flight path. If the FDs are OFF, the system may not engage properly or follow the desired configuration, which could lead to issues in thrust management. Thus, having the FDs ON is a prerequisite for the autothrust system to arm when the thrust levers are advanced for takeoff.