@@_K3PLR that's because I have 2 different fuel type and engine on the craft(hydrolox and liquid hydrogen), the fuel percentage on the UI only show the fuel of the fuel type of the engines that are active, if all hydrolox engines disabled and only LH engines active the UI only show the percentage of the LH fuel type, if both hydrolox and LH engine are active then the UI will calculate both of the fuel type the reason why the fuel percentage suddenly jump to 80% is because I turn off all my hydrolox engine, 80% is how much my liquid hydrogen is left