Third officer is a lesser used civil aviation rank. It was primarily used by Pan Am, particularly on its Clippers flying boats during the infancy of extended range airline routes. The third officer would serve as a relief pilot and aircrew member, and could move between pilot, co-pilot, radio officer, and flight engineer positions to provide a rest period for the primary crews. Third officers in modern civil aviation are often not formally titled as such. Rather, these relief pilots take on a junior first officer rank or in some cases a second officer rank.