as we know, the 744 combi has a ballast fuel alert.
The alert level can be set on the FMC's PERF INIT page in line 3R. When the center wing tank quantity drops below this level, the alert is triggered and the pilot should deactivate the center wing tank pumps.
The center wing tank sits slightly forward of the center of gravity. This tank is the counterforce when the combi has heavy cargo in its tail, so it won't get tail-heavy.
On this subject there's a question about the scavenge pump:
The scavenge pump may autostart when the reserves-to-main-tank transfer begins. Normally, at this time, the center wing tank is nearly empty and the scavenge pump just draws the rest of the soup from the tank floor.
Does anybody know if the scavenge start is inhibited when a fuel ballast value is entered in the FMC? It looks strange when the crew wants to keep fuel in the center tank and the scavenge ignores this desire.
Also unknown is whether the >FUEL OVD CTR L & R messages are inhibited if the crew deselects CTR L & R pump switches when the quantity is below the ballast level.