What is indicated by the FAULT light on the APU MASTER switch?

The FAULT light on the APU MASTER switch signifies a potential system failure within the Auxiliary Power Unit (APU). This light illuminates when the APU detects that it is experiencing a condition that could compromise its performance or safety. Such conditions might include issues related to the APU's oil pressure, fuel supply, or overheating.

When the FAULT light is illuminated, it is crucial for the flight crew to perform appropriate checks and potentially take corrective actions, as this indicates that the APU may not function correctly, which is vital for providing electrical power and air conditioning while on the ground or as backup power systems during flight.

Normal operational status would not cause the FAULT light to illuminate, as it only indicates anomalies. The life of the APU pertains to its operational lifespan and maintenance scheduling, while low battery voltage is related to the aircraft's electrical systems rather than specifically to APU operational issues. Thus, the presence of the FAULT light serves as a critical warning for ensuring the safety and reliability of APU operations.