During what phase of flight are flight crewmembers prohibited from performing duties except those required for the safe operation of the aircraft?

The correct answer is the critical phase of flight. This term describes key moments during the flight operation when maintaining safety and reliability is of utmost importance. The critical phases typically include taxiing, takeoff, and landing, as well as any portion of the flight during which the aircraft is operating close to the ground.

During these phases, the potential for operational challenges and unexpected events is higher, making it essential for flight crewmembers to be focused exclusively on the aircraft’s operation. Distractions must be minimized to ensure that all crew members can respond swiftly and effectively to any changes in the environment or the aircraft's status. By limiting duties to those required for safe operation, the crew can maintain a heightened level of situational awareness, which is critical for ensuring safety.

In contrast, while there are important duties to perform during the cruising phase, taxi phase, and landing phase, these do not encapsulate the full breadth of critical interactions and workload that define the critical phase of flight. Hence, the emphasis on restricting non-essential duties specifically during the critical phases helps to bolster the overall safety of flight operations.