During what phase of flight are crewmembers specifically required to wear shoulder harnesses?

Shoulder harnesses are specifically required to be worn during takeoff and landing because these phases of flight are critical moments when the chances of an incident occurring are higher. The requirement for shoulder harnesses enhances occupant safety by providing additional restraint in the event of sudden movements or impacts. During takeoff and landing, aircraft experience more dynamic changes in speed, altitude, and potential turbulence than during cruise flight. This is when effective restraint systems play a crucial role in keeping crewmembers secure and reducing the risk of injury.

In contrast, while taxiing or in cruise flight, the requirements for wearing shoulder harnesses are less stringent, as the risks associated with sudden deceleration or impact are typically lower than during takeoff and landing.