Fuel is mass, volume, time and distance

How do you convert uplift volume into usable planning mass?

Density is a ratio with units; multiplying or dividing depends on the unknown.

Write the units before the numbers

With density inkg/L, mass kg = volume L ×density kg/L. Litres cancel, leaving kilograms. To find volume from mass, divide by density. To find density, divide mass by volume. The unit cancellation makes the operation clear even when the values are unfamiliar.

US gallons and imperial gallons are different volumes. Use the stated conversion, not a remembered gallon factor without a label. For these exercises,1 USgal = 3.78541 L and 1 imperialgal = 4.54609 L. An apparently small unit ambiguity can become a substantial uplift error.

Separate capacity from endurance

Tank capacity is a volume or mass limitation under stated conditions; usable fuel is the quantity available for engine use under the aircraft definition. Fuel that is physically present but unusable is not available for planned endurance. Start from the aircraft’s documented conventions and the actual fuel state.

Temperature affects fuel volume and density. Use the prescribed measured or corrected density and uplift method. A fixed classroom density is an explicit assumption for the exercise, not a replacement for the operational fuel record.

Reconcile the uplift

Existing fuel plus delivered mass, adjusted for any consumption or other documented change, should reconcile with the final on-board quantity within the applicable procedure and measurement tolerances. Compare like units and like phases. A volume receipt should not be subtracted directly from a mass indication.

The laboratory lets volume and density vary independently so that the causal link is visible. Hold volume fixed and lower density: delivered mass falls. Hold required mass fixed and lower density: required volume rises. Use the inverse calculation as a reasonableness check before accepting an uplift result.

What can the remaining fuel actually achieve?

Protect the stated reserve before calculating endurance or range.

Endurance and range answer different questions

For a constant flow, endurance h = available fuel kg /fuel flow kg/h. Ground range NM = endurance ×GSkt. The fuel available for the planned task is the amount remaining after the quantities that must be protected under the stated plan, not necessarily the entire indicated fuel on board.

Specific air range is TAS divided by hourly fuel flow, giving NAM/kg. Specific ground range uses GS and gives NGM/kg. A wind change affects the latter directly. A speed change can alter both speed and flow, so the highest TAS is not automatically the best range setting.

Solve the inverse problem

To cover a stated ground distance at known GS and flow, required fuel = distance/GS ×flow. To find maximum allowable flow for a fixed task fuel allowance, divide that allowance by required time. Rearranging the same relationship is more reliable than memorising several disconnected formulas.

A reserve expressed in minutes becomes a fuel mass only after selecting the correct reference flow for that reserve condition. Cruise flow at the current level may not be the required holding or reserve-condition flow. Use the applicable aircraft data and operational rule.

Respect changing conditions

Constant-flow endurance is a segment model. In a real diversion, climb, descent, approach, wind changes and a different aircraft configuration affect consumption. A simple range ring is not evidence that every aerodrome inside it can be reached and used safely.

In the synthetic case,800 kg on board minus 200 kg protected leaves 600 kg for the task. At 150 kg/h the task endurance is 4 h; at 100 kt GS that corresponds to 400 NM before the protected quantity is reached. Explain both the protected amount and the assumed flow whenever reporting a range.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.