Apu starting sequence in aircraft :
In figure the APU control unit receives its power from the aircraft battery.
By moving the APU switch to ‘ON', power is provided to the intake door actuator and an LP fuel valve which starts to open. When they are both fully open, switches energise the starter system, igniters are energised and the APU accelerates with assistance from the starter motor to idle speed. The starter motor cuts out between 50 & 60%.
The fuel system controls the fuel flow during start. Once the engine is at idle (100%, lower if the APU is fitted with an idle power setting), the APU will be ready to load either electrically or pneumatically. This is indicated by a ‘Ready to Load' light or ‘APU Power Available ‘light coming 'ON'.