14 CFR § 91.105

During takeoff and landing, and while en route, each required flight crewmember shall—

Be at the crewmember station unless the absence is necessary to perform duties in connection with the operation of the aircraft or in connection with physiological needs; and

Keep the safety belt fastened while at the crewmember station.

Each required flight crewmember of a U.S.-registered civil aircraft shall, during takeoff and landing, keep his or her shoulder harness fastened while at his or her assigned duty station. This paragraph does not apply if—

The seat at the crewmember's station is not equipped with a shoulder harness; or

The crewmember would be unable to perform required duties with the shoulder harness fastened.