One thing confuses me as an engineer on the B744 is why different airlines start using the stab tank at different fuel qtys. If you let the aircraft refuel itself, stab tank starts to be used at around 129000 kgs, but BA starts using it a lot later. What is the reasoning behind this?
I believe it has to do with standard fuel loads for C of G calculation when the stab is used with certain SGs.
Our company avoids standby loads between 125 -132T which I think allows for all SGs, meaning you could end up with fuel in the stab with loads as low as 125T and no fuel with loads as high as 132T
As you said at 129T you should end up with 16.8T in the CWT & 3.2T in the HST based on an SG of 0.785, I believe the problem lies with when you try to top it up to say 131T the system has a problem of adding 1.7T to the CWT and 0.7T to the HST, I think it chucks it all in the CWT which results in a non standard fuel load which requires a loadsheet change for the non standard C of G.
I've never had the chance to see how it loads fuel. With 129T entered and an SG of 0.785 does it fill all tanks at the same time or in a certain order, does it put 16.8T in the CWT and then 3.2T in the HST or does it fill them together?