I'd like to re-open this thread with a question for the 'Tecchies' in Flt Planning departments. It has plagued me for years!
Tankering generally means you cannot achieve the cruise level you would have done non-tankering. Is this taken into account in the 3% or 3.5% (or 5%) rule-of-thumb?
Diving into my 737-400 fuel tables I find:
If I depart on an out-and-back at 54T on a 3hr flight, my optimum cruise is initially at FL360, at 1165kg/hr/engine
If I tanker, say 6T extra, my optimum is now FL340 at 1296kg/hr/engine
That is over 11% extra fuel. Taken, then over a 3 hr cruise, I will burn:
Approx 786kg more for the lower levels at the new weight
Tankering will be decided (by 'established rule of thumb' = 3.5% per hour) on a price diff of around 10.5%
Are we still saving money?
Now I'm on a NG, and can get ALL my return fuel on at departure, so I carry somewhere around 10T extra........................
Should that 3/3.5% be a higher figure?