One for the fuel systems specialist.
As an ex Test Engineer On Airbus and British Aerospace 125 I post this here because I suspect it is the only place I might gain an answer to this question. I currently fly the 787 and there is a debate in the instructor community as to whether we should have the fuel system schematic up as centre tank contents approach low and the centre tank pumps are selected off.
On the leave the schematic hidden side they say the aircraft will tell you if there is something wrong (fuel low pressure waning) they believe that because the main wing tank pumps are permanently selected on they are running throughout the flight. That the main tank fuel lines are permanently pressurised by the wing tank pumps (but fuel only comes out of the centre tank first because centre tank pump pressure is higher).
Whilst it is clear the centre tank pumps will put out a higher pressure than the wing tank pumps when all are operating the 787 Flight Crew Operations Manual schematic description says
“Fuel Pump(green rectangle) - Fuel pump switch is ON.
Fuel Pump(green rectangle with line) - Fuel pump switch is ON and pressure sensor detects pressure greater than 7 to 8 psig” this implies that when the centre tanks are running and there is no green line coming from the wing tank pumps those lines are unpressurised.
Interested in both general and type specific thoughts.