Bug or not ?
 

I create a simple single aircraft mission in FMB. I set Fuel on board in FMB to 40% and save the mission as a "Single Mission". I then jump in game and check the Loadout page. The Fuel slider is full right showing 100%. The Fuel slider cannot be changed (Niether can the waepons loadout). I then jump in game. The actual Fuel quantity on board is as planned in FMB at 40%.

So is this normal ?
How do I construct a mission that then enables me to change loadouts in game

(This with latest BETA patch)

Check in your options setting that you don't have unlimited fuel on. If you have unlimited fuel checked in your setting then you can not change it in loadout screen.

I have "Limited Fuel" Checked.

Hi. I'm not creating SP missions, but it's a known bug, that the aircraft customization didn't work in SP environment. We met this error, when we wanted to testing the ammunitions. It is only possible if you run the mission as MP host.

It is not possible to change fuel or loadouts in single player so far. I think it was initially done on purpose to prevent using unrealistic loadouts in historical campaigns. Mission designer defines loadout and fuel depending on mission scenario. E.g. to force a player into out of fuel situation or no ammo mission.

Please vote here to make it optional http://www.il2bugtracker.com/issues/247

You can change this for MP missions with SpawnArea only.