737 center fuel tank

[cumulusse]

dear all

could anyone explain why the center fuel tank on the 737 is used first? is there something to do with the different pressure of the fuel valve?

many thanks...............

[Mike McGowan]

Has to do with weight and balance ( CG ) while airborne and landing. Center tanks, when installed on the 737, have different pump pressures ( cracking pressure ) with the largest center tank having the highest pumping pressure and the wing tanks having the lowest pumping pressure; therefore, the largest center tanks empty first followed by the next center tank, followed by the wing tanks. The CG of the aircraft will remain within limits if all pumps are turned on initially and turned off as the respective tank is emptied.

Mike

[Notso Fantastic]

Aeroplanes should use fuel from fuselage tanks first to minimise wing bending forces. The less weight held in the fuselage the less the wing bending forces. If you used the wing fuel first, aside from undesigned trim changes, the wing will be undergoing higher forces than by using centre tank fuel first

[Sperm Bank]

Lads the info I have is a little different to that provided. I fly the NG and have no experience on the classic.

The figures for fuel pump output are as follows.
Main tank fuel pumps.....min 10 psi.
Centre tank fuel pumps..min 23 psi.

Perhaps I have missed something but nothing in my notes talks about "crack pressure".

[TR4A]

Perhaps I have missed something but nothing in my notes talks about "crack pressure".

The crack pressure they are referring about is the opening pressure for the check valves.

The Classics have the same fuel pressure (min 10 psi, rate 20,000 pph / 9,071 kgph) on all pumps.

The NG center fuel pumps put out higher pressure than the main pumps - min 23 psi and min flow rate of 20,000 pph /9,071 kgph