Move a load, preserve total mass

Why is redistribution simpler than adding ballast?

Moving an existing item changes moment while total mass stays fixed.

Moment change

Move mass w from station a to station b. Moment change is w(b − a), so CG change is ΔX = w(b − a)/W. W is the total aircraft mass including the moved item. Positive aft movement increases the CG arm under the usual aft-positive convention.

This equation is exact for the stated fixed mass and fixed station arms. It does not require the item to start at the aircraft CG. The original CG is needed only to find the final CG.

Separate movement from addition

If cargo is moved out of the aircraft, total mass changes and the removal formula applies. If passengers move during flight, mass stays constant but the balance and operating restrictions still matter.

Do not use the moved mass alone in the denominator. A 500 kg item moving 6 m does not move a 10,000 kg aircraft’s CG by 6 m.

Separate total moment from changed moment

The redistribution equation works because all unchanged moments cancel when final and initial conditions are subtracted. Only the moved item contributes a difference, w times its change of arm. This lets you calculate a correction without re-entering every unchanged component, but the total aircraft mass in the denominator still includes all those components.

To verify a result, compute the moved item at its old and new stations independently. If 300 kg moves from 16 m to 8 m, its moment changes from 4,800 to 2,400 kg·m, a reduction of 2,400 kg·m. Divide that change by the unchanged total mass. This reveals the sign physically: the aircraft has lost aft moment, so its CG must move forward under an aft-positive convention.

Which load movement corrects a CG error?

Calculate the moment correction required before selecting a practical movement.

Solve for distance or mass

Required movement distance d = WΔX/w. Alternatively moved mass w = WΔX/d. Use signed quantities to preserve direction. A forward correction needs a negative moment change; it cannot be achieved by moving the same item farther aft.

The aircraft may have several practical station choices. Calculate the required moment, then determine whether a real compartment or seating move supplies it without violating local mass, floor, restraint or occupancy constraints.

Allow for discrete loads

A computed 428.6 kg movement is a mathematical result; a set of 80 kg containers moves in discrete increments. Test the resulting actual CG after rounding to a feasible arrangement. Never round a correction towards a limit without checking the final state.

Redistribution can correct CG without consuming structural payload allowance, but it can alter local structural loads and loading sequence. Confirm the aircraft remains stable on its landing gear during loading.

Choose an achievable correction

Several combinations of mass and distance can supply the same required moment. Moving 200 kg through 5 m gives the same 1,000 kg·m change as moving 500 kg through 2 m. The practical choice depends on approved stations, compartment limits, loading time and the ability to secure the item. An algebraic solution is only the start of that choice.

If no available station pair supplies enough movement, redistribution alone cannot solve the problem with the selected item. You may need a different cargo arrangement, an approved ballast provision or a payload change. After selecting a feasible movement, calculate the final CG using actual station arms and actual masses. Treat a target exactly on a boundary with the precision and margins required by the approved method.

Can you explain and apply the mechanism?

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