Before the aircraft disconnects from the ground power, the APU is started first to prevent a complete power outage once the ground power is disconnected. The aircraft then operates on APU power, allowing the ground power to be removed. When taxiing out, the air conditioning must be turned off. After starting the engines, the engine power takes over, at which point the APU can be turned off. Before shutting down on the ground, ensure the APU is started to avoid a power outage upon shutdown. In summary, the aircraft's power supply comes primarily from ground power, the APU, and the engines. The APU requires fuel, so it's typically connected to ground power during long-term parking.
