What components are included in the ignition portion of the APU electrical system?

The ignition portion of the Auxiliary Power Unit (APU) electrical system primarily focuses on the components that initiate and sustain combustion within the engine. The correct choice includes the exciter and ignition plug.

The exciter serves as a high-voltage power source that converts low voltage from the APU's electrical system into a high-voltage spark needed to ignite the fuel-air mixture in the combustion chamber. The ignition plug, also known as a spark plug, is responsible for creating the actual spark that ignites this mixture, enabling the APU to start and run efficiently.

Understanding the role of these components is crucial for proper maintenance and troubleshooting of the APU, as they are integral to its operation. The exciter must be functioning correctly to generate the necessary voltage, and the ignition plug must be clean and operational to ensure reliable ignition for the combustion process.

In contrast, while fuel pumps and pressure switches are critical to the fuel delivery system, monitoring sensors and relays play a role in system diagnostics and control, and transformers and batteries contribute to power supply management, they do not directly function in the ignition sequence necessary for APU operation.