What happens when the APU reaches self-sustaining speed during the starting process?

When the APU reaches self-sustaining speed during the starting process, the combustion process begins. At this stage, the APU has accelerated to a point where it can maintain its operation without relying solely on the starting system. The ignition system activates and fuel is injected into the combustion chamber, where it mixes with the air entering from the compressor. This mixture is ignited, leading to the generation of hot gases that expand and drive the turbine, creating the necessary power to support various aircraft systems. This transition is crucial for the APU to function effectively and to provide electrical power and pneumatic pressure for the aircraft's needs, especially during ground operations and before main engine start-up.