Aircraft Fuel Planning: Fuel Burn, Reserve & Endurance
Calculate each flight segment using its expected time and fuel flow. Add taxi, climb, alternate, reserve and other allowances required for the flight. A cruise-burn calculation alone is not a complete fuel plan.
Trip fuel
Use a realistic burn rate for the phase of flight and aircraft configuration. Taxi, climb, cruise, descent, approach and expected delays may use different amounts of fuel.
Reserve fuel
The reserve entered in a calculator is only an arithmetic input. Applicable regulations, operating rules and company procedures determine the actual reserve requirement.
Fuel remaining
A positive margin does not automatically make a flight legal or prudent. Weather, alternates, reroutes, ATC delays, unusable fuel, fuel-system limitations and aircraft-specific procedures all matter.
Endurance versus range
Endurance is time; range is distance. Wind changes range because it changes groundspeed, even if fuel flow stays the same.
Trip fuel and a 45-minute reserve
Assume 50 gal usable fuel available for planning, a 10 gal/hr estimated burn, 2.5 hours enroute, and a deliberately selected 45-minute planning reserve. Trip fuel is 10 × 2.5 = 25 gal. The entered reserve is 10 × (45 ÷ 60) = 7.5 gal. This simple calculation therefore gives 32.5 gal before additional allowances and an arithmetic margin of 17.5 gal against the entered 50 gal.
That 45-minute input is an example, not a declaration that the flight meets a regulation. The applicable reserve depends on flight rules and operating category: for example, Part 91 airplane VFR fuel rules are in 14 CFR 91.151, while IFR airplane fuel rules are in 14 CFR 91.167. Other operating rules can differ. Calculate trip phases, alternate needs, forecast winds, delays, usable fuel, and aircraft-specific requirements separately.
Do not mix time and fuel units
Convert minutes to hours before multiplying by gallons per hour. Thirty minutes is 0.5 hour, not 0.30 hour. Also make sure the burn rate matches the phase of flight you are calculating; climb fuel flow can differ significantly from cruise.
Build margin from real aircraft data
Use current POH or AFM performance information, actual usable fuel, forecast winds, routing, expected delays, and the fuel requirements that apply to the flight. A calculator can total the numbers, but the quality of the plan still depends on the inputs.
Estimate endurance and range
Endurance is time aloft; range is distance over the ground. For a first estimate, divide usable planning fuel by fuel flow, then multiply the remaining time by groundspeed. Treat each step as an estimate: climb, descent, changing power, wind and required reserves affect the result.
Example: With 36 gallons available for the calculation and a steady 9 gallons per hour, the arithmetic gives 4 hours before allowances. If 1 hour is reserved and groundspeed is 110 knots, the remaining 3 hours imply 330 nautical miles. This example does not establish a legal reserve or an aircraft's actual range. Use the current POH/AFM and the rules for the operation.
For a deeper explanation, see the endurance calculator and the time, speed and distance guide.