The primary method uses a siphoning action through standpipes that connect the tanks. The siphoning is caused by an absence of air (lower pressure) in the tanks receiving fuel. If reservoir fuel starts to drop below full, air ejector pumps installed at the top of each reservoir tank automatically expel air, thus maintaining the siphon.
ECS pressurization causes the external centerline to transfer to both internal wing tanks. The aft system transfers from the left external by ECS pressurization to the left internal wing, and then to the A-1 tank to the aft reservoir. The forward system transfers fuel from the right external by ECS pressurization to the right internal wing and then to the F-1 tank to the F-2 tank to the forward reservoir.