An aircraft hybrid fuel system includes a main tank and a set of flexible bladders, the main tank and the set of flexible bladders defining a fuel containment space. The system further includes a set of pathways coupling the set of flexible bladders to the main tank. The set of pathways is constructed and arranged to vent gas out of the set of flexible bladders into the main tank while fuel from a fuel source is provided into the fuel containment space defined by the main tank and the set of flexible bladders. Along these lines, each flexible bladder can be provisioned with a fuel port to provide fuel, and a separate vent port to vent gas to the main tank.