Many thanks guys for the info. It now makes much morew sense to me!
However, the other point I was curious about is that BA are considering routes such as London-Pheonix for the 777. This is a very long route (I think) for a twin jet; is it a technological 'breakthrough' that a twin jet can achive these sorts of ranges? Are the GE90's (in this case) so much more efficient than RB211 on the 747? Why don't all new aircraft now have these engines if they can produce so much power at high these high levels of efficiency? Furthermore, is there more to it than a powerplant issue?
3g.