Why are the center tank pumps deactivated when slats are extended "Take Off - Configuration" on the A320 ? - The Aviation

 

Why are the center tank pumps deactivated when slats are extended "Take Off - Configuration" on the A320 ? 

Remember that Center tank pumps overpower the wing pumps, because of the sequence valves.

The sequence valve acts as a relief valve and keeps the output pressure of a wing tank pump 5 psi lower than that of the center tank pump.

Slats extended means a takeoff / climb configuration, or the aircraft is on approach with a TOGA mode.

These configurations require high fuel pressure, If a center tank pump fails, it will be 1 pump supplying 2 engines, compared to 2 pumps for each engine.

If a center tank pump fails, there is no automatic command that can be sent to the sequence valves to change the sequence, or to engage the crossfeed valves.

If one center pump fails, the pressure may remain high enough to keep the valves closed, but not high enough to supply two engines during high demand.

Add to that, there is no suction (gravity) feeding from the center tank like the bypass suction valve that installed on the engine feed line, downstream of the main pumps

BYPASS SUCTION VALVE

If both wing tank pumps fail with no center tank pump running and the crossfeed valve closed, the bypass suction valve lets fuel be sucked from the wing tank by the engine fuel pump system and thus engine supply is done by "gravity".