In which mode is it NOT possible to engage the autopilot?

TOGA mode, which stands for Takeoff/Go-Around mode, is designed to provide maximum thrust during critical phases of flight, specifically during takeoff and when executing a go-around. During this phase, the focus is on achieving optimal performance and safety, rather than on maintaining specific flight parameters that the autopilot would typically control. Therefore, autopilot engagement is not allowed in TOGA mode to ensure that the pilot has full control of the aircraft's thrust and flight path.

In contrast, the other modes—Climb, Cruise, and Descent—are phases of flight where autopilot can be engaged. These modes allow the autopilot to manage the aircraft's altitude and speed to maintain desired flight profiles, which is essential for efficient and safe flight operations. In summary, engaging the autopilot in TOGA mode would contradict the necessity for manual control during critical flight segments where precise management of thrust is required.