One aircraft, several mass limits

Why does each stage have a different mass?

A mass figure is useful only when its contents and flight stage are known.

Build up from a defined baseline

Basic empty mass is an aircraft-specific defined baseline; dry operating mass adds the items required for that operation but excludes usable fuel and traffic load. Use the definitions supplied with the aircraft and the operator: a label alone cannot tell you whether crew, catering, unusable fuel or equipment are included.

Traffic load is passengers, baggage and cargo. Zero-fuel mass is dry operating mass plus traffic load. Ramp mass adds block fuel; take-off mass subtracts taxi fuel; landing mass subtracts the fuel expected to be consumed between take-off and landing.

Why the limits differ

Maximum zero-fuel mass protects structure in a loading condition where wing fuel is not available to relieve wing-root bending. Maximum take-off and maximum landing masses are separate limitations; performance can impose a lower allowable mass on a particular day. Passing one does not establish compliance with the others.

Do not subtract total block fuel from take-off mass to find landing mass: taxi fuel has already gone. Keep a labelled mass chain so that every subtraction corresponds to one flight phase.

Track what each subtraction removes

Draw the fuel quantities as intervals on a timeline: block fuel exists before engine start; taxi fuel is consumed before take-off; trip fuel covers the airborne journey to the destination landing. The same kilogram cannot be consumed twice. A useful reconciliation is ramp mass minus all planned consumption equals landing mass. If this disagrees with take-off mass minus trip fuel, the mass chain contains an inconsistent definition or a duplicated allowance.

Structural maximum landing mass and a runway-limited landing mass describe different constraints on the same planned arrival. A longer runway may improve a performance allowance, but it cannot raise the structural maximum. Conversely, being below structural maximum does not establish sufficient runway or climb performance. Use a reason for every limit rather than treating the smallest printed number as a universal aircraft maximum.

Which limit sets today’s traffic load?

Solve each mass constraint for payload, then select the lowest allowance.

Turn limits into allowances

For a given DOM, the ZFM allowance is MZFM − DOM. The take-off allowance is allowable TOM − DOM − take-off fuel. The landing allowance is allowable LM − DOM − landing fuel. A landing restriction can therefore limit departure payload even when take-off mass is acceptable.

Use the lower of structural and performance allowances at the applicable phase. Fuel-volume, compartment and centre-of-gravity restrictions remain additional checks; the minimum mass allowance does not prove the load arrangement is acceptable.

Interpret the result

If the three allowances are 15, 12 and 13 t, only 12 t can be loaded under these assumptions. Adding 1 t of fuel usually reduces the take-off payload allowance by 1 t, but its effect on the landing allowance depends on how much additional fuel will remain at landing.

A negative allowance means the proposed operating baseline and fuel already exceed a constraint. It is not permission to load a negative payload: change the plan.

Explain a change before recalculating

The minimum of the allowances is the active constraint, while the difference between that allowance and each other allowance is a margin. If an extra tonne of take-off fuel remains entirely on board until landing, it reduces both take-off and landing payload allowances by a tonne. If it is consumed before landing, only the take-off allowance changes by that full amount. A route change can therefore alter the limiting constraint even with identical block fuel.

This calculation is an inequality problem: DOM plus traffic plus the phase fuel must be less than or equal to the phase maximum. Write that inequality first when a question uses unfamiliar terminology. Then isolate the unknown traffic mass. Check the result by rebuilding the original masses; a correct rearrangement must satisfy every original inequality, including any stated ramp limitation.

Can you explain and apply the mechanism?

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