Flight Planning and Navigation Computations: Fuel, Time, Leg Calculations and Replanning
Leg, time and fuel calculations
Flight planning converts distance, ground speed, fuel flow and time into a navigation log. For each leg: time = distance ÷ GS; fuel = fuel flow × time. Keep units consistent: hours for time, kg or lb for fuel, NM for distance, kt for speed. Endurance is time; specific range is distance per unit fuel. A safe plan includes trip fuel, contingency, alternate fuel, final reserve and any additional fuel. ETA should be computed in UTC and then converted to local/standard time only for presentation.
In-flight replanning
After a fix, compare actual time, fuel and wind with the plan. If GS has changed, recompute the remaining leg time and fuel: remaining distance ÷ revised GS; remaining fuel = flow × revised time. For a diversion, use the diversion distance, expected GS and fuel flow, then check that landing fuel will not breach reserves. Replanning is a continuous loop: fix, update wind/GS, recalculate ETA/fuel, decide. Use the latest fix as the new datum; do not keep recalculating from the original departure point unless the whole plan is being reworked.
Worked example
A leg is 210 NM, forecast GS 140 kt, fuel flow 45 kg/h. Time = 210/140 = 1.5 h. Trip fuel = 45 × 1.5 = 67.5 kg. Add a 30-minute reserve: 45 × 0.5 = 22.5 kg, so planned fuel for the leg plus reserve = 90 kg. If an actual fix shows GS 120 kt, remaining time becomes 210/120 = 1.75 h and trip fuel 78.75 kg, an increase of 11.25 kg.
Traps
Using TAS instead of GS inflates or deflates time and fuel. Mixing minutes with hours is common: 45 min = 0.75 h. Do not omit reserve or alternate fuel when assessing a replan. Do not compare fuel in kg with flow in lb/h. ETA time-zone conversion is a presentation step, not a change to the UTC calculation.