What is required to start the APU on DC power?

To start the Auxiliary Power Unit (APU) on DC power, the requirement is to have the DC fuel pump operational. The APU needs a reliable source of fuel to initiate the start sequence, and in the case of using DC power, the DC fuel pump ensures that fuel is being delivered to the APU during this critical phase.

The DC fuel pump operates to raise the fuel pressure and provide the necessary flow to start the APU under DC power conditions. In contrast, the AC fuel pump is not necessary for this particular scenario, as the question focuses specifically on the requirements when starting the APU using DC power.

While both BATT 1 and BATT 2 must be operational to provide sufficient electrical power for starting processes, the key aspect of starting the APU specifically on DC power hinges on the functionality of the DC fuel pump. A ground power unit is not required when the APU is being started on internal DC power; it typically serves to provide external electrical power when the aircraft's batteries are insufficient or when the APU is not running.