Under what conditions are flight crewmembers required to keep their safety belts fastened?

Flight crewmembers are required to keep their safety belts fastened during takeoff, landing, and while en route primarily for safety reasons. This requirement is rooted in the need to mitigate the risks associated with unexpected turbulence, sudden changes in speed, and other unforeseen situations that may arise during these critical phases of flight.

During takeoff and landing, the aircraft is in a more vulnerable state, and maintaining a secure position is crucial to ensuring that crewmembers can continue their duties effectively. Meanwhile, keeping safety belts fastened while en route protects against turbulence or potential incidents that could occur mid-flight, contributing to the overall safety and stability of the crew and the operation of the aircraft.

Thus, this comprehensive requirement emphasizes the importance of constant readiness and safety throughout the entire flight, rather than in isolated phases.