If the intent was to get the CG forward, what would be the point in transferring fuel to anything other than the forward trim tank? In particular why to the engine feed tanks which are arranged to be CG neutral?
However, the BEA report says:
It has been established that the aircraft began taxiing with tanks completely full. Before line-up, the crew carried out fuel transfer so as to bring the CG to 54% for takeoff. During this operation, the fuel burnt from the feeders during taxiing was replaced by the fuel contained in tank 11.
As a result of the transfer, feeder tanks 1 to 4 were full before line-up. In addition, main tanks 5 and 7, which had not been called on during taxiing, had remained full.
Between 14 h 41 min 55 s and 14 h 43 min 10 s, the time when the tank ruptured, the quantity of fuel burnt by each engine is estimated at 219 kg (15 kg between 14 h 41 min 55 s and engine power-up, 204 kg between power-up and the rupture). This was therefore the quantity of fuel taken from each feeder tank.
The transfer of fuel from tank 5 to feeder tank 1 deliberately only starts when the level in the feeder reaches 4,000 kg, that is to say 198 kg less than full. This leads to estimate that 219 kg – 198 kg = 21 kg was the quantity of fuel taken from tank 5.
I can't make that compatible with tank 5 being continually topped up no matter how I try