Embodiments include a modular fuel pump apparatus for an aircraft refueling chassis and related devices, systems and methods. In one exemplary embodiment, a pump module includes a hydraulic motor driven by a hydraulic pump connected to a chassis motor. A chassis engine is operated at a substantially constant RPM, which causes the hydraulic pump to output hydraulic fluid to the pump module at a substantially constant hydraulic pressure. The speed of the hydraulic motor can be varied by a proportional flow control valve disposed between the hydraulic pump and the hydraulic motor, thereby varying the speed of the fuel pump and the flow rate of the fuel. A controller is configured to operate the pressure control valve such that the flow of the fuel is maintained at a predetermined flow rate.