The interesting thing is according to PMDG those values from Simbrief that GSX is using are not a "balanced" CG which is why they take the simbrief ZFW and then adjust the payload to get their own CG.
I guess PMDG is referring to the distribution of cargo between FWD, AFT and Bulk zones, which is the only thing I can think of. GSX doesn't take anything from Simbrief performance calculator, it only takes the dispatched plan, and there, Cargo is expressed as a single figure, there's no distribution done by Simbrief.
What GSX does next, depends (as explained in the manual where the PMDG integration is covered) if your airplane profile on Simbrief assumes a standard passenger weight or not.
- If the passenger weight in the flight plan is the standard one, GSX will enter the Passenger number distributed over three classes (the CDU has 1st, business and economy, while the modeled cabin has only business and economy, this it's a confirmed bug of the CDU but it won't affect this too much) and will distribute the (single) cargo weight reported by Simbrief in the 3 cargo holds.
- If the passenger weight is not standard, GSX will JUST enter ZFW, and the airplane will do the rest, just AS IF you typed the ZFW yourself. GSX is not doing any weight distribution here.
So, I guess the only possible issue PMDG is referring to here, is how GSX distributes the (single) cargo weight it got from Simbrief across the 3 holds (when it's not using the ZFW method), and the answer it very simple: it distributes cargo EXACTLY like the PMDG CDU does, when you set the "Cargo Level" to a certain value, and of course we derived it by making a chart of the various values, to understand the CDU own strategy, which is as follows:
Cargo Level FWD AFT Bulk CG
10% 1817.59 1547.89 354.50 25.5%
33% 5999.03 5108.87 1170.09 25.3%
50% 9087.96 7739.46 1772.58 25.1%
66% 11997.08 10216.92 2339.90 24.9%
100% 18177.87 15480.58 3545.54 24.6%
The above Table is what the PMDG CDU sets if you set a certain Cargo Level, and it's basically perfectly linear, the Cargo is always distributed according to these values:
FWD 48.86%
AFT 41.61%
Bulk 9.53%
Again, this is without GSX involved, it's just what you'll get when you set a certain Cargo "Level" in the CDU, from 0% to 100%, the distribution is always the same, the CG% changes slightly.
Now let's put GSX into the picture, and try a real world example using Simbrief. I made a flight plan from LSZH to KLAX in Simbrief, which called for:
290 Passengers
6577 kg of Cargo
ZFW 200161 kgs
Since the passenger weight is standard, GSX explicitly set the Passengers number and distributed Cargo, with the *Airplane* calculating the ZFW, and this is what we have in the CDU after GSX set the values:
7 Pax in 1st, 42 in Business, 241 in Economy
Total Pax = 290 CORRECT
3213 kg in FWD hold
2736 kg in AFT hold
626 kg in Bulk hold
Total Cargo = 6575 (2kg difference is rounding), and if we calculate the distribution:
3213 is 48.86% of 6577
2736 is 41.61% of 6577
626 is 9.53% of 6577
So, again, not only GSX took the correct amount of Cargo from Simbrief, but it also distributed it exactly AS IF you typed a Cargo Level without even using GSX. And of course, the ZFW calculated by the airplane is CORRECT too, at 200.2 Tons, which matches our Simbrief plan of 200161 kgs.
So, I really don't know what else is to say or why there's a need to change these values after GSX already set them correctly from Simbrief.