777 fuel question

[McAero]

Hi folks,

I've been having a discussion with some people about the 777 fuel and hope someone here can help.

My question is this: at what center tank quantity should the pumps switch control from the override center tanks to the wing tanks? My understanding is that the fuel from the center tanks is depleted until the center tank is "empty" which could be between 250kg - 750kg, however a colleague has suggested that the value is in fact much higher than that, approximately 1.1T. I'm not convinced....

Thanks in advance

[777AV8R]

Fuel Low Center

Message comes on when either center fuel pump is on and fuel quantity less than 2400 lbs.

[you'll be right son]

Wing Tanks

Anyone know the reason why the B777 has only one fuel tank in each wing?
Is it purely for simplicity, less things to go wrong?

[Old Aero Guy]

Yes, the parts that aren't there can't fail.

[McAero]

Message comes on when either center fuel pump is on and fuel quantity less than 2400 lbs.

I believe the FCOM says "fuel quantity less than 2400 lbs and pump pressure low". Should the pump pressure go low at this magic value of 2400 lbs (approx 1.1 T) or should it go low at a value less than 2400 lbs? Also, would the altitude of the aircraft have a bearing on what volume of fuel the pumps would switch over?

I read something today about the center tanks having a "cheek tank" which determines how much total fuel is used by the center pumps...........

[aa777phil]

Large 777 tank

Also the large tank takes 8-9 hours to reach the total air temp. Very cold OAT's are not much of an issue.

[gas path]

The CWT on the 'A' market aircraft is just the cheek tanks on the inbd section of the wings and joined across ship with 2 large diameter tubes. On the 'B' market aircraft the whole of the centre wing section (and the cheek tanks) make up the CWT.
The CWT pumps will transfer out below 1 tonne (especially) in the air due to the natural flying angle. The remainder of the fuel will be aspirated out by the motive flow jet pumps from the wing tank boost pumps.