What are the indications of a malfunctioning APU during operation?

The indications of a malfunctioning APU (Auxiliary Power Unit) primarily include abnormal lights on the APU control panel, unusual noises, or fluctuations in output parameters. These symptoms are critical because they provide immediate visual and auditory feedback about the APU's operational status.

The control panel is designed to alert crew members to any deviations from normal operational parameters, and any abnormal lights suggest that the system is not functioning correctly. Unusual noises can indicate mechanical issues or failure within the APU, while fluctuations in output parameters, such as power output and temperature, can signify underlying problems that could affect the APU's ability to provide the necessary support.

Monitoring these indications is essential for ensuring the reliability of the APU, as it serves crucial functions, including providing power and pneumatic energy for starting the main engines and supporting onboard systems. Quick identification and remediation of these symptoms can prevent further damage and ensure the safety and operability of the aircraft.