How is an APU typically started?

An auxiliary power unit (APU) is typically started with its own electrical starter, which is designed to provide the necessary power to initiate the combustion process within the APU. This electrical starter is usually a turbine starter or an electric starter that engages the APU’s turbine to spin it up to operational speeds.

Once the APU reaches the required speed, fuel is introduced, and ignition occurs, allowing the APU to generate power and bleed air for auxiliary systems on the aircraft. The self-contained nature of this starting mechanism is crucial since it allows the APU to operate independently without needing assistance from external power sources or manual intervention.

Other methods mentioned, such as using external power sources or pneumatic assistance, can be utilized in specific circumstances or other types of engines, but they are not the standard method for starting an APU. Manually cranking is not a practical method for starting an APU due to the design and operational requirements of these units.