Very interested to read more about the 380 weight restrictions on the DFW-SYD sector. Note the distance in 13,815k and the range of the generic 380 is 15,700k. Wondering is the Qantas range the same as this? Wondering how much fuel needs to be carried for alternates? Wondering how much the range reduces for each pax or tonne of freight?
You have not accounted for the headwinds. Air Nautical Mile VS Ground Nautical Miles (or KMs in your case).
13800km route at 490kt TAS into a 50kt headwind equates to 15350 air KMs.
13800km x TAS490/GS440 = 15368km
PS I just pulled the 50Kts average Headwind out of the air but I am guessing it could get that high on a bad day.