Aircrew, also called flight crew, are personnel who operate an aircraft while in flight. The composition of a flight's crew depends on the type of aircraft, plus the flight's duration and purpose.
In commercial aviation, the aircrew are called flight crew. Some flight crew position names are derived from nautical terms and indicate a rank or command structure similar to that on ocean-going vessels, allowing for quick executive decision making during normal operations or emergency situations. Historical flightdeck positions include:
Captain, the pilot highest-ranking member or members of a flight crew.
First officer (FO, also called a co-pilot), another pilot who is normally seated to the right of the captain. (On helicopters, an FO is normally seated to the left of the captain, who occupies the right-hand seat).
Second officer (SO), a person lower in rank to the first officer, and who typically performs selected duties and also acts as a relief pilot. The rank of second officer was traditionally a flight engineer, who was often the person who handled the engine controls. In the 21st century, second officers on some airlines are pilots who act as "cruise relief" on long haul flights.
Third officer (TO), a person lower in rank to a second officer, and who typically performs selected duties and can also act as a relief pilot. Largely redundant in the present day.
'Relief Crew' members in the present day are fully licensed and trained captains and first officers who accompany long-haul airline flights, and who relieve the primary pilots during designated times from the commercial operator or consented portions between the two crews to provide them with the opportunities for rest or sleep breaks to avoid the risk of pilot fatigue (some large wide-body airliners are equipped with special pilot sleeper berths, but more typically reserved seats in the section closest to the flight deck, or cockpit, are used for the relief crew). A relief crew will take over most predominantly during the middle portions of a flight when an aircraft is usually on autopilot and at cruising altitude.