Shoulder harnesses for flight crewmembers are required to be fastened during:

The requirement for flight crewmembers to have their shoulder harnesses fastened during takeoff and landing unless they are unable to perform their duties is rooted in safety protocols established to minimize the risk of injury during critical phases of flight. Takeoff and landing are considered the most hazardous times during a flight, with a higher likelihood of emergency situations arising. By ensuring that crew members are securely fastened in their seats during these stages, they are better prepared to respond quickly and effectively to any issues that may occur.

The provision allowing crew members to unfasten their harnesses when necessary to perform their duties acknowledges that there are times while conducting essential operational tasks that they may need to be unrestrained. However, this is a calculated exception to the safety rule, as the primary focus remains on protecting the crew and maintaining readiness throughout the most vulnerable moments of the flight.