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A fuel system for an auxiliary power unit includes a mechanical fuel pump, an electric fuel pump, and a controller. The mechanical fuel pump provides fuel flow to the auxiliary power unit and has an output dependent upon an operational speed of the auxiliary power unit. The electric fuel pump provid

A fuel system for an auxiliary power unit includes a mechanical fuel pump, an electric fuel pump, and a controller. The mechanical fuel pump provides fuel flow to the auxiliary power unit and has an output dependent upon an operational speed of the auxiliary power unit. The electric fuel pump provides fuel flow to the auxiliary power unit, and is located in flow series with the mechanical fuel pump. The controller causes the electric fuel pump to provide fuel flow during starting of the auxiliary power unit.

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1. A fuel system for an auxiliary power unit, the fuel system comprising: a mechanical fuel pump for providing fuel flow to the auxiliary power unit, the mechanical fuel pump having an output dependent upon an operational speed of the auxiliary power unit;an electric fuel pump for providing fuel flo

1. A fuel system for an auxiliary power unit, the fuel system comprising: a mechanical fuel pump for providing fuel flow to the auxiliary power unit, the mechanical fuel pump having an output dependent upon an operational speed of the auxiliary power unit;an electric fuel pump for providing fuel flow to the auxiliary power unit, the electric fuel pump located upstream of the mechanical fuel pump on a common fuel supply conduit; anda controller for causing the electric fuel pump to provide fuel flow during starting of the auxiliary power unit;a bypass conduit having a first end and a second end, the first end of the bypass conduit fluidly connected to the fuel supply conduit upstream of the electric fuel pump and the second end of the bypass conduit fluidly connected to the fuel supply conduit downstream of the electric fuel pump, wherein the first and second ends of the bypass conduit are upstream from the mechanical fuel pump;a bypass check valve located on the bypass conduit, the check valve having an open configuration that allows fuel to flow from the first end of the bypass conduit to the second end of the bypass conduit and a closed configuration that prevents fuel from flowing from the second end of the bypass conduit to the first end of the bypass conduit;a relief conduit having a first end and a second end, the first end of the relief conduit fluidly connected to the fuel supply conduit downstream of the electric fuel pump and the second end of the relief conduit fluidly connected to the fuel supply conduit upstream of the electric fuel pump; anda relief check valve located on the relief conduit, the check valve having an open configuration that allows fuel to flow from the first end of the relief conduit to the second end of the relief conduit and a closed configuration that prevents fuel from flowing from the second end of the relief conduit to the first end of the relief conduit. 2. The fuel system of claim 1, further comprising: a fuel metering unit for managing the fuel flow to the auxiliary power unit, the fuel metering unit located downstream of both the electric fuel pump and the mechanical fuel pump on the fuel supply conduit. 3. The fuel system of claim 2, further comprising: a recirculation conduit having a first end and a second end, the first end connected to the fuel metering unit and the second end connected to the fuel supply conduit at a location downstream of the electric fuel pump. 4. The fuel system of claim 1, further comprising: a boost fuel pump for increasing fuel flow to the auxiliary power unit, the boost fuel pump located on the fuel supply conduit upstream of the mechanical fuel pump. 5. The fuel system of claim 4, wherein the boost fuel pump is connected to the mechanical fuel pump by a rotational shaft. 6. The fuel system of claim 5, wherein the electric fuel pump is located upstream of the boost fuel pump. 7. The fuel system of claim 5, wherein electric fuel pump is located downstream of the boost fuel pump. 8. A fuel system for an auxiliary power unit, the fuel system comprising: a fuel supply conduit fluidly connecting a fuel supply to a combustor of the auxiliary power unit;a fuel metering unit located on the fuel supply conduit upstream of the combustor;a mechanical gear driven pump located on the fuel supply conduit upstream of the fuel metering unit;a mechanical boost pump located on the fuel supply conduit upstream of the mechanical gear driven pump;an electric pump located on the fuel supply conduit upstream of the mechanical gear driven pump;a controller for commanding the electric pump to flow fuel through the mechanical gear driven pump to the fuel metering unit during starting of the auxiliary power unit;a bypass conduit having a first end and a second end, the first end of the bypass conduit fluidly connected to the fuel supply conduit upstream of the electric fuel pump and the second end of the bypass conduit fluidly connected to the fuel supply conduit downstream of the electric fuel pump, wherein the first and second ends of the bypass conduit are upstream from the mechanical fuel pump;a bypass check valve located on the bypass conduit, the check valve having an open configuration that allows fuel to flow from the first end of the bypass conduit to the second end of the bypass conduit and a closed configuration that prevents fuel from flowing from the second end of the bypass conduit to the first end of the bypass conduit;a relief conduit connecting an outlet of the electric pump to an inlet of the electric pump; anda relief check valve located on the relief conduit, the check valve having an open configuration that allows fuel to flow from the outlet of the electric pump to the inlet of the electric pump and a closed configuration that prevents fuel from flowing from the inlet of the electric pump to the outlet of the electric pump. 9. The fuel system of claim 8, further comprising: a fuel shut-off valve located on the fuel supply conduit between the fuel metering unit and the combustor, the fuel shut-off valve having an open configuration and a closed configuration,wherein fuel flows along the relief conduit when the fuel shut-off valve is in the closed configuration. 10. The fuel system of claim 9, further comprising: a starter motor, wherein the controller is configured to command the starter motor to start rotation of the auxiliary power unit after fuel is heated from flowing along the relief conduit. 11. The fuel system of claim 8, further comprising: a starter relay connected to the electric pump;a starter motor connected to the starter relay; anda power source connected to the starter relay, wherein the starter relay simultaneously energizes the starter motor and the electric pump. 12. The fuel system of claim 8, wherein the electric pump is located between the mechanical boost pump and the mechanical gear driven pump. 13. The fuel system of claim 8, wherein the electric pump is located upstream of both the mechanical boost pump and the mechanical gear driven pump. 14. A method of starting an auxiliary power unit, the method comprising: using a controller to provide power to a starter motor for starting rotation of the auxiliary power unit; andproviding power to an electric pump for flowing fuel through a fuel supply conduit to the auxiliary power unit, wherein providing power to the starter motor and providing power to the electric pump occur simultaneously;flowing fuel through a relief conduit having a first end and a second end, the first end of the relief conduit fluidly connected to the fuel supply conduit downstream of the electric fuel pump and the second end of the relief conduit fluidly connected to the fuel supply conduit upstream of the electric fuel pump, wherein a relief check valve located on the relief conduit has an open configuration that allows fuel to flow from the first end of the relief conduit to the second end of the relief conduit and a closed configuration that prevents fuel from flowing from the second end of the relief conduit to the first end of the relief conduit;driving a mechanical pump, connected in series with the electric pump and downstream of the electric pump along the fuel supply conduit, wherein the fuel supply conduit is fluidly connected to the auxiliary power unit, and wherein the mechanical pump has an output dependent on operational speed of the auxiliary power unit;flowing fuel through the fuel supply conduit for starting the auxiliary power unit;terminating power to the electric pump when the output of the mechanical pump provides sufficient fuel flow to sustain operation of the auxiliary power unit; andflowing fuel through a bypass conduit having a first and a second end, the first end of the bypass conduit fluidly connected to the fuel supply conduit upstream of the electric fuel pump and the second end of the bypass conduit fluidly connected to the fuel supply conduit downstream of the electric fuel pump, wherein the first and second ends of the bypass conduit are upstream from the mechanical fuel pump, and wherein a bypass check valve located on the bypass conduit has an open configuration that allows fuel to flow from the first end to the second end of the bypass conduit and a closed configuration that prevents fuel from flowing from the second end of the bypass conduit to the first end of the bypass conduit. 15. The method of claim 14, wherein providing power to the starter motor and providing power to the electric pump occur through a starter relay.