Time for a short update!
I'm working on the system side and have almost done with the GUI and the weight and loading. So, let me explain the overall design philosophy, which extends to the whole project.
Your role in the DC-9 is the Captain in command. You will have the role and the responsibilities due to that position. That means, for example, that you will adjust the fuel you need for your flight, but not in which tank the fuel will go. This will be done automatically by a background "load manager". The same with passengers and cargo. Of course, will be your responsibility to make sure that the loading limits are not exceeded!
Image 1: Pax loading. The bottom graph displays the limits (red markers) and the blue and pink markers the current ZFW index and TOW Index respectively. The limits will also move depending the current gross weight, to project the 2D chart (see image 4) into a 1D line. I believe the rest are self-explanatory...how they work!
Image 2: Cargo loading. Note: Each passenger is computed to carry 1 item of 13 kg baggage, which will take some cargo space. I will also add at the bottom the same graph as in the passengers tab.
Image 3: Fuel Loading. You will just adjust the total fuel. Also I will add the same graph here too.
Image 4: Aircraft Loading. This is the final tab you will have to look at, before you start your flight, because beyond having the full chart available, you will use the GW marker to check in which slope line you are close, to get the stab trim setting (STAB TO ANU) for the take off!.