In flight, when does the DC fuel pump come on automatically?

The DC fuel pump is designed to automatically activate when motive flow pressure is lost from the primary ejector. This is crucial for ensuring an adequate fuel supply to the engine, especially in scenarios where the primary method of fuel delivery—utilizing the ejector—fails or is insufficient. The primary ejector relies on the pressure generated by the engine to draw fuel from the tank. If that pressure drops, the DC fuel pump provides the necessary fuel flow to keep the engine operating safely.

Other options do not accurately represent the conditions under which the DC fuel pump switches on automatically. For instance, the activation of the fuel pump is not contingent upon all electrical systems being active, a specific altitude being reached, or fuel levels dropping below a particular threshold. These factors may affect overall operations but do not trigger the automatic activation of the DC fuel pump. Thus, the correct choice highlights the essential role that pressure dynamics play in fuel management during flight.