14 CFR § 91.105 - Flight crewmembers at stations.
(a) During takeoff and landing, and while en route, each required flight crewmember shall -
(2) Keep the safety belt fastened while at the crewmember station.
(b) 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 -
(1) The seat at the crewmember's station is not equipped with a shoulder harness; or
(2) The crewmember would be unable to perform required duties with the shoulder harness fastened.