In which scenario is the DUAL mode of the FMGC preferred?

The DUAL mode of the Flight Management and Guidance Computer (FMGC) is preferred during takeoff because it provides enhanced redundancy and safety. In DUAL mode, both FMGC units are active and continuously monitor each other's performance. This means that in the event of a failure in one computer, the other can immediately take over without any loss of functionality or guidance, which is critical during the high-demand phase of takeoff.

During takeoff, the aircraft is transitioning from a stationary position to a high-speed environment, and any potential issues must be addressed as quickly and safely as possible. The DUAL mode ensures that the pilots have the utmost reliability and integrity of the systems managing flight performance and navigation.

In contrast, the other phases—descent, cruise, and approach—while still important, do not carry the same immediate risk as the takeoff phase. The aircraft is generally stabilized in the latter phases, and in the event of an issue, there is typically more time to react. Thus, while the DUAL mode is beneficial throughout the flight, its use during takeoff maximizes safety and operational effectiveness.