Fuel is calculated by multiplying fuel flow by the time. Using current time and fuel quantity, ETE and fuel remaining can be calculated. These calculations can be done for each leg of the flight plan or for the whole flight plan. The FMS calculates time and fuel for each leg, including the active leg, which are added up for the complete flight plan. This article concentrates on what ground speed and fuel flow are used in the calculations.
Distance is a product of the flight plan and is the same under all options.
Just as a note, the distance shown in the active flight plan is the actual distance to be flown by the aircraft. Because the aircraft flies to the inside of the turn, the distance shown in the flight plan may be shorter than the actual distance between the waypoints. The distance is a function of the course change at both ends of the leg. Ground Speed - Planned ground speed for each leg is calculated from planned TAS for each leg and wind. The first step in calculating planned TAS is to divide the flight plan into climb, cruise, and descent segments.