When two pilots fly a fixed-wing aeroplane which requires a two-person flight crew, the aircraft commander, who must be appropriately qualified and hold the rank of Captain, will normally occupy the left hand seat and the First Officer or Co-Pilot will normally occupys the right hand seat.
Before the commencement of each flight sector, the aircraft commander decides which pilot will take direct responsibility for flying the aircraft for the complete flight or for particular parts of it such as the Descent/Approach and Landing and they become 'Pilot Flying' (PF) for that sector or the specified part of it. The other pilot is then designated for that sector or relevant parts of it as 'Pilot Monitoring' (PM) or alternatively as 'Pilot Not Flying' (PNF) and in that role must monitor the flight management and aircraft control actions of the PF and carry out support duties such as communications and check-list reading. The Operations Manual will specify fully the roles for the PF and PM/PNF, but one of the most important aspects of the duties of any PM/PNF is the cross-check of the actions of PF. Indeed, this part of the role represents one of the most important single reasons why a two-pilot flight crew is specified.
Subject absolutely to the determination of the aircraft commander, the two roles during any particular crew duty will normally be shared so that both undertake PF and the PM/PNF duties. Whatever their role, the designated aircraft commander is responsible for all aspects of the safe operation of the aircraft. Appropriately qualified Check/Training Captains may occupy either seat as aircraft commander for crew training purposes.
When the terms PF and PM /PNF were first introduced, it was normal for the roles to be designated, subject only to limitations on the ability to directionally control an aircraft on the ground imposed by the absence of a steering tiller for the pilot in the right hand seat, to be designated for a complete flight sector. Nowadays, some operators use the Monitored Approach concept in which the pilot who is PM during the descent and approach will become PF for the landing in order to improve the operational safety aspects of the transition from instrument flight to the visual reference required for touch down after almost all approaches.