In-flight crew relief (commonly referred in noun form as the relief aircrew, relief flight crew, or just relief crew), is a term used in commercial aviation when referring to the members of an aircrew intended to temporarily relieve active crew members of their duties during the course of a flight.[1] The term and its role are almost exclusively applied to the secondary pilots of an aircrew, commonly referred to as relief pilots, that relieve the primary and active captain and/or first officer (co-pilot) in command of an aircraft to provide prolonged breaks for rest or sleep opportunities.[2][3]