Read a load sheet as a chain of evidence

Why does an index not have units of metres?

An index is a scaled moment representation with a declared reference and offset.

Decode the convention

A synthetic index system uses I = 40 + Σ[m(x − 12)]/1,000. The reference arm is 12 m and the scaling is 1,000 kg·m per index unit. Adding 500 kg at 20 m increases the index by 500(20 − 12)/1,000 = 4 units.

For total mass W, recover CG with X = 12 + (I − 40)1,000/W. This formula belongs only to the stated convention. Another aircraft may use a different offset, reference arm or scale; do not copy coefficients between systems.

Read each phase

A load/trim sheet builds a dry operating index, traffic contributions and fuel contributions for the required phases. Check aircraft registration, configuration, flight, units, baseline revision, passenger zones, cargo, fuel and limiting masses before accepting the result.

A printed trim setting is an aircraft-specific output from an approved system. A classroom percent-MAC calculation does not supply a transferable stabiliser-trim setting.

Check the scale with one known change

Before trusting a spreadsheet index conversion, test it with a simple known load change. In this convention, 100 kg added exactly at the 12 m reference changes mass but leaves the index unchanged. The recovered CG still depends on the total mass and the previous nonzero moment about that reference. An index value is therefore neither a direct CG coordinate nor a percentage.

A graphic trim sheet may use sloping guide lines, incremental load marks and fuel corrections to perform the same moment arithmetic. Read each scale in the direction specified by the chart, carrying forward the cumulative state. Jumping to an unrelated axis after one contribution can lose the baseline. Use an arithmetic check where appropriate, but retain the manufacturer’s scale, offset and phase-specific fuel treatment rather than imposing this synthetic convention.

What makes a last-minute change valid?

Apply a permitted amendment and repeat the affected checks.

A change has mass and location

Removing 200 kg of aft baggage changes ZFM, take-off mass, landing mass, index and possibly trim. Adding a passenger changes the appropriate zone contribution as well as total mass. The permitted last-minute-change limits and procedure are operator/aircraft specific.

A change outside the authorised amendment conditions requires a revised load/trim sheet according to the procedure. A small total mass change can still produce a large balance change if it occurs far from CG.

Close the control loop

The loading instruction must match what was physically loaded; the final load sheet must reflect that actual state. Cross-check loading confirmation, amendments and crew acceptance through the applicable system. A mathematically correct sheet describing a different load is not adequate.

Retain the distinction between planned and actual fuel. If the uplift differs, recalculate using the correct tank distribution and density/mass record rather than changing one displayed total alone.

Reconcile the physical and documentary states

An amendment is meaningful only if the loading team and crew refer to the same final state. A removed bag left on the sheet and a bag removed from the sheet but left on the aircraft create opposite errors. The required communication and acceptance process closes this gap; simply editing a calculator does not confirm what happened on the ramp.

Trace a change through the chain: baseline remains the same for a baggage removal; traffic decreases; zero-fuel, take-off and landing masses decrease; the baggage moment or index is removed at its station; CG is recomputed with the new mass; any required trim output is revised. This sequence explains why subtracting a kilogram value from every field is wrong. Some outputs are sums, others are ratios or aircraft-specific chart results.

Can you explain and apply the mechanism?

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