What is the primary function of an aircraft's FMS (Flight Management System)?

The primary function of an aircraft's Flight Management System (FMS) is to assist in real-time navigation and flight planning. The FMS integrates data from various aircraft systems, allowing pilots to input flight routes and waypoints. It automatically calculates the optimum flight path, taking into consideration factors such as wind, air traffic, and aircraft performance. This system enhances the efficiency of the flight by providing continuous updates and guidance, which helps in managing the aircraft's navigation throughout the different phases of flight. Thus, the FMS plays a crucial role in ensuring safe and accurate flight management, allowing pilots to focus on other operational aspects of flying.

In contrast, other options refer to functions that are not within the primary scope of the FMS. Managing cabin pressure and air conditioning is the responsibility of environmental control systems, controlling the aircraft's engines during cruise is typically handled by the throttle and engine control systems, and communication with air traffic control involves separate radios and systems specifically designed for that purpose.