In some MP servers, FC3 planes (BRRRT version A too) start with a lower amount of fuel (e.g. 59%). This is to make their startup last as long as the fully clickable modules.
When you rearm the plane, your fuel doesnt go down, it already is down. Set the planned amount of fuel to 100%, set the ammunition/flares/chaffs/liveries, wait for fuel and armament and then BRRRRRRRRRRRttttt KA-BOOOOOOOOOOM