14 CFR § 137.42
Fastening of safety belts and shoulder harnesses
July 13, 2020
No person may operate an aircraft in operations required to be conducted under part 137 without a safety belt and shoulder harness properly secured about that person except that the shoulder harness need not be fastened if that person would be unable to perform required duties with the shoulder harness fastened.
[Amdt. 137-10, 44 FR 61325, Oct. 25, 1979]