Fuel Endurance Calculator

How to use this calculator

1. Enter Usable fuel on board and choose its unit.
2. Enter Fuel flow per hour. It is taken in the same unit as the fuel, per hour.
3. Optionally enter Fuel to keep unused, again in that unit. Left blank, it counts as 0.

Usable fuel is less than tank capacity, and the quantity actually on board is something only your own check of the aircraft can establish. The usable figure and a fuel flow for the power setting and altitude come from that aircraft's flight manual. Fuel to keep unused is entirely your number: it follows from the applicable regulations, the operator and the flight manual, and the page offers no default. It must be smaller than the usable fuel.

Formula

What the variables mean

usable fuel

fuel on board that can actually be burned, from the aircraft's own data

fuel kept unused

any quantity you choose to exclude; this calculator does not suggest one

fuel flow

fuel burned per hour, in the same unit as the fuel

endurance

time in hours

Worked example

Reading the result

When some fuel is kept unused, the first row reads Endurance on the fuel counted; otherwise it is plain Endurance. Fuel counted shows the quantity that was divided by the flow and the amount set aside. Time to empty tanks uses all the usable fuel at the same flow. It marks the point where this constant-flow model runs dry and is there so the gap between the two times is visible; it is not flying time to plan on. The decimal hours beside each time are not minutes: 4.21 hours is 4 h 13 min.

Assumptions and limitations

• Fuel flow is constant from the first minute to the last, although real flow is higher in the climb and changes with altitude, mixture and power.
• Every unit of fuel entered as usable can reach the engine; unusable fuel has to be excluded before you type the number.
• Fuel for start, taxi and run-up is not deducted unless you deduct it yourself.
• No reserve is calculated. The only fuel left out of the count is the amount you type.
• Fuel and fuel flow share one unit, and no conversion between volume and mass takes place.
• The quantity on board is taken as exact, with no allowance for gauge error or uneven tanks.

Common mistakes

• Entering total tank capacity where the usable quantity belongs.
• Assuming the tanks hold what the gauges show without confirming the quantity another way.
• Applying a cruise fuel flow to a whole flight that includes a long climb.
• Pairing gallons on board with a fuel flow read in pounds per hour.
• Reading the time to empty tanks as time available to fly.
• Turning endurance into distance with true airspeed when it is ground speed that covers the ground.