To the technos out there - how is the fuel transfered from the Main to the Centre tank on the 777?
Same as on all airliners. You open the refuel panel. Select Defuel. Open the refuel valve for the Centre tank. Then go to the flight deck and turn on the booster pumps in the wing tanks, and open the crossfeed valve. Some aircraft have a manual defuel valve like B737. (On the B737-200 you have to open the right engine LP fuel cock).
But can only be done on the ground with the refuel panel open.
Trouble with a B777 is that the refuel panel is a long way up, so you need a big set of steps.
I do it regularly on a A319. It arrives with about 3000kg of fuel in the inners at M20degC. I transfer it into the centre tank before refuelling. Then the wing tanks are filled with fuel at about P5degC, and the frost melts. Saves deicing.