What is the primary source of power for the airplane when the engines are running?

The primary source of power for the airplane when the engines are running comes from engine-driven generators. These generators are connected to the aircraft's engines and convert mechanical energy produced by the engines into electrical energy. As the engines operate, they drive the generators, providing the necessary power to run various aircraft systems, including avionics, cabin systems, and flight controls.

These engine-driven generators are designed to be the main source of electrical power during flight and are capable of supplying power in sufficient quantities to meet the demands of all the aircraft's systems. This makes them essential for ensuring that the aircraft can operate safely and efficiently during its flight operation.

In contrast, the auxiliary power unit (APU) generator provides power when the engines are not running, such as during ground operations. The static inverter converts DC power to AC but is not the primary source during engine operations. External power is used mainly when the aircraft is on the ground and not operating under its own power. Hence, the engine-driven generators are the correct choice as they actively generate power when the engines are in operation.