Originally posted by BuffyTVS65
I can't believe that lightening a fully laden 747 by around 9kilo per Y passenger (about 2,700 kilo) will significantly affect the fuel consumption on an aircraft that weighs about 350,000 kilo fully loaded.
You're making the aircraft either:
a) ~3 tonnes lighter
or
b) releasing ~3 tonnes worth of cargo capacity
To put this into perspective, Air New Zealand operate a mixed fleet of Rolls Royce RB211 and GE CF6 engined 747-400s.
Their initial order was for 4 RR engined aircraft, because the 747 Classics they replaced (which went to VS, incidentally) were RR powered.
When they added extra 747-400s to the fleet, they leased two GE engined examples, and bought another two.
Why the swap? They found that each GE engine is a tonne lighter than the corresponding RR engine, so the BOW of each GE aircraft is 4 tonnes lighter than an RR powered counterpart.
If I remember correctly and got my maths right (if G-VOPS or someone else can correct me if I'm wrong), on an AKL-LAX flight (12 hour block time) the lighter engine saves something in the region of 4 tonnes of fuel... and for every extra tonne of fuel you carry, you have to carry more fuel to carry the fuel, IYSWIM

.
Cheers!
Mike