ATPL flight planning question
Hi guys, I am about to burn AFTs notes on flight planning I have because I cant get my head around something, I dont know if my brain is that fried from studying all day or it is genuinely poorly explained... here lies my problem:
It has to do with calculating the EMZW for cruise fuel flows.
The formula is: EMZW = SZW - (zone distance x ?? kg/nm) /2
#?? kg/nm=SGR#
*The "??" is dependent on 3 things...
1) WEIGHT - AVERAGE weights (70-80 tonnes) = 10kg/nm
- HEAVY weights (ABOVE 80 tonnes) = 11kg/nm
- LIGHT weights (60-70 tonnes) = 9kg/nm
2) WIND - For 50kts headwind + 1kg/nm
- For 50 kts tailwind - 1 kg/nm
3) Altitude - If FL below 280 + 1kg/nm
SIMPLE ENOUGH RIGHT?
The book goes on to give an example:
EG: -The SZW is 72400kg
- distance 488nm
- FL310
- .80mach
- ISA temp
- 50kt HEADWIND
"The SZW figure to use would be 10 (9kg/nm because of the weight. A SZW of 72400kg and 488nm means that the average weight will probably be below 70t, plus 1kg/nm extra because of the headwind which brings the number back to 10kg/nm)"
I understand the wind and the FL bit but what I dont understand is how the "average" weight is accurately determined. There isnt much of an explanation about how it is actually calculated (even if it is just a rough estimate).
So my question is how is this "average" weight determined???
thanks in advance