With the aircraft powered via the Battery or External Power, the APU can be started by switching ON the APU MAIN switch, waiting 10 seconds, and then pressing the APU START button.
It is important to wait 10 seconds before pressing the APU START button to allow time for the APU inlet door to fully open.
The GEN3 switch must also be in the OFF/RESET position in order to initiate the APU start sequence.