Selecting ON at 3L causes ETE and fuel remaining to be calculated based on planned numbers from PERF INIT entries. Selecting OFF at 3R causes ETE and fuel remaining to be calculated based on current conditions. The selection is independent on each FMS and can be different, even when operating in dual. Both of the options for calculation of time and fuel are discussed below. The basic elements of time and fuel calculations are distance, ground speed, and fuel flow.
Time is calculated by dividing the distance by ground speed.
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.