The pilot and passenger seat belt and shoulder harness requirements are referenced in FAR 91.107 and FAR 91.105.
During Takeoff and Landing:
Crew members: Must wear both seat belts and shoulder harnesses (if equipped) during takeoff and landing, unless the shoulder harness would interfere with their duties.
Passengers: Must wear seat belts and shoulder harnesses (if available) during taxi, takeoff, and landing.
In-flight:
Crew members: Must keep their seat belts fastened while at their station (pilot seat). The use of a shoulder harnesses in flight is not required, but it is encouraged when feasible.
Passengers: Seat belts are required during flight whenever the seat belt sign is on, during turbulence or when instructed by the crew.
It’s also a requirement for the PIC to ensure that each passenger is briefed on how to use (fasten and unfasten) their seat belt and shoulder harness.
