Yep, had the movement back to front! Sorry about that. Trying to put it together in too big a rush, as was going out (no excuse! lol). Of course, increasing MAC is C of G moving towards the rear. So should read -
So on this chart, the first 7.6 - 7.8 tonne is the main tank fuel, which increases the MAC & moves the C of G rearward as it is loaded & forward as it is burnt. The rest is the centre tank fuel, which decreases the MAC & moves the C of G forward as it is loaded & rearward as it is burnt.
There is no chart that displays what happens when fuel remains in the centre tank while main tank fuel is burnt, as this is not normally what happens. However, from the chart you can deduce that MAC will move forward as main tank fuel is used. Using the example in the initial question, with full mains & 5 tonne in the centre tank, the moment for fuel would be about +1.6 MAC. Then, if the centre tank pumps both fail at that point, MAC would be moving forwards as fuel was burnt from the main tanks. A 2 tonne burn from full tanks would equate to about -5.3 movement in MAC. (from about +9 MAC back to +3.7 MAC). So +1.6 less 5.3 gives a -3.8 decrease in MAC & forward movement in C of G for fuel burn.
As has been said, the effect of this movement would depend on how the aircraft was initially loaded. I don't believe that it is likely to take the aircraft outside C of G limits. Boeing obviously don't think so as well, as there is no mention of C of G problems in the QRH for both centre tank pumps failing. In fact, that checklist is very low key, except for a gentle reminder not to run out of fuel! I guess the only time you would have to be aware is when the centre tanks pumps fail shortly after departure & the aircraft was loaded close to the forward C of G. In that case (both centre tanks failing shortly after departure), you would most likely be considering a return to the departure airport anyway, as you most likely wouldn't have enough fuel to complete the trip & then you would only have to be careful if you were planning to burn off fuel prior to the landing.
Last edited by Oakape; 3rd Oct 2014 at 22:39.