Don't know if this has been posted but it is the report on the BA flight from LA and explains the occurrence quite well.
http://www.aaib.gov.uk/cms_resources...LG%2006-06.pdf.
Just a guess but gravity feeding about 3000kg an hour would be impossible with the size of the fuel lines on any acft.
The following paragraph is taken from this document.
http://www.united-virtual.com/upload...Training_2.pdf
So the use of APU in the 747 apart from gate power is limited to an APU-PACK departure. The reason is that
the engines on the 747 are deemed to have a very strong windmill, so even if "dead" they are able to turn freely
to generate electricity and of course there are 4 of them, so losing them all is very unlikely. So it was decided on
the 747 to allow the engines to windmill for electricity, or to have one of the running engines support power.