“…When the aircraft performs a time constrained descent in idle thrust, the descent is conceived assuming a hypothetical 4D trajectory, where the Air Traffic Controller imposes the aircraft to fly through a specific waypoint at a specific time. In case of a Closed Path trajectory with constant Flight Path Angle, for example, whether it is a Constant-Descent Final Approach (CDFA), a Three-Degree Decelerating Approach (TDDA) 22,23 or an Advanced Continuous Descent Approach (ACDA) 24 , the FMS function can be seen as a tool that computes a reference airspeed, for an assigned flight path angle, to select the appropriate autopilots and feed them with the reference signals to match. The calculated trajectory is thus tailored to the specific airplane and requires advanced flight management on board, as well as strong interaction with the ground air traffic management, which consider local traffic, flight scheduling, weather, and capabilities of every the single aircraft.…”