Structural Limitations: Why the heaviest loads are usually placed in the over-wing section?

Cargo movement would result in an uncontrolled Center of Gravity shift during flight. That is why every Unit Load Device (pallet or container) must be secured to stay in place at all times.

There is a special system mounted on the floor which allows locking ULD to prevent its unexpected motion in 3 planes: forward-aft, left-right, and up-down, which assures that the CG position is only affected by the burning of fuel, which is predictable, and up to some point, can be manually controlled.

Each position available in the aircraft (spot at which the ULD can be locked & secured) can accommodate specific ULD types. The weight of that ULD is limited by the maximum capacity of the position it is placed on. This capacity is determined so as to prevent damage to the floor or other structural components.

Capacity for a given ULD type can differ depending if it is located in the forward, middle, or aft part of the fuselage. It is usually the highest in the overwing section, as the airplane structure is the most rigid and strengthen in this section due to the placement of extra frames used for attachment of the wings. The capacity is lowest, on the other hand, at the aft part and it is usually connected with preventing tail tipping.

Maximum load capacity of the choisen position

In the Evionica Weight and Balance solution, whenever the weight of the ULD exceeds the maximum capacity of the position, the error is indicated on the list, deck map, and the loadsheet preview is blocked:

Weight exceeds Maximum Capacity of DR position
Weight exceeds Maximum Capacity of DR position

Moreover, in the case of some aircraft, like a variant of B747-BDSF, the maximum weight allowed on the given position might vary depending on the Actual Zero Fuel Weight. This dependency can be reflected in the solution. The capacity limit is set up in such a way that it is automatically adjusted if an airplane reaches the critical AZFW value.

For example:

When AZFW <= 477 500 lb Max weight allowed on position A1 = 4773 lb

Low capacity of A1 position
Low capacity of A1, A2, B2 position

But if AZFW > 477 500 lb maximum weight allowed on the same position A1 is 5770 lb.