Moving the head on the rails is possible to balance in each flight the center of gravity of the aircraft. In this way, the aircraft can fly in a sure way with or without container. Another additional possibility is to load containers of smaller longitude, to reduce weight in function of the load type that the container transport in each flight. The head-tail assembly can occupy multiple positions along the rails to get the correct balance of the center of gravity.

AMFM. Center of Gravity with container and without container.