Build trip fuel from a consistent profile

Why can a cruise-only estimate understate trip fuel?

Each phase has its own time, distance and fuel relationship.

Start with the same endpoints

Define what the supplied climb and descent tables include. A climb table may begin at a stated take-off condition and include acceleration; a descent table may stop before the approach. If one table includes a terminal segment and another includes it again, the sum double-counts fuel and distance.

Subtract climb and descent ground distances from the total planned ground route to obtain cruise distance, provided all distances use the same endpoints and wind treatment. Do not subtract an air distance from a ground route without converting it under the stated assumptions.

Integrate the cruise segment

For a constant cruise GS, cruise time is cruise ground distance divided by GS. Multiply by the appropriate average or tabulated cruise flow. At a changing mass, a more detailed method uses segment or integrated performance data rather than treating initial flow as constant for a long flight.

Add the climb, cruise, descent, approach and landing quantities according to the supplied data definitions. Taxi remains a separate pre-take-off component in the illustrated mass/fuel chain. A change in route distance or level may require recalculating more than the cruise distance.

Check that the geometry is possible

If the supplied climb and descent distances exceed the route distance, there is no positive cruise segment in that assumed profile. Do not hide the inconsistency by setting a negative cruise time. A lower level, a different profile or appropriate short-sector data is needed.

The laboratory uses fixed synthetic phase quantities so that the remaining cruise segment is clear. The aircraft moves through the climb, cruise and descent while fuel used accumulates from the same profile. This synchronisation helps detect a common mistake: reading time from one profile and fuel from a different level or mass row.

How do you read between performance table rows?

Interpolate one controlled variable at a time and keep the chart domain explicit.

Read the supplied variables

A performance table can depend on mass, pressure altitude, temperature, speed and wind. Identify which variables are fixed and which need interpolation. Use the table’s units and correction order. A fuel value at one mass cannot be paired with a time from a different mass unless the published method specifically supports that combination.

For linear interpolation between two points, calculate the fraction of the input interval and apply it to the output interval. If mass 60 t gives flow 1,200 kg/h and 70 t gives 1,400 kg/h, mass 65 t gives 1,300 kg/h under the stated linear assumption.

Perform a second interpolation deliberately

For a rectangular two-variable table with permitted bilinear interpolation, first interpolate across mass at each altitude, then interpolate those results across altitude. Reversing the order gives the same result for a bilinear surface; disagreement can reveal an arithmetic or input error.

Do not extrapolate beyond the supplied domain. A smoothly extended line may look convincing while predicting an unsupported operating condition. Use an appropriate table or revise the plan when the requested state lies outside the data.

Interpret trends before accepting numbers

For the synthetic data, fuel flow rises with mass. An interpolated value between 60 and 70 t must lie between the two endpoint flows. If it does not, inspect the fraction and units. A trend check is useful but does not prove that a real aircraft always follows that same relationship.

The laboratory exposes one table with a fixed speed and altitude assumption. It teaches interpolation and inverse reading: find the mass that corresponds to a selected flow threshold. Actual flight planning requires the applicable approved performance material, including its corrections, ranges and stated limitations. A classroom straight line is not dispatch data.

Can you explain and apply the mechanism?

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