An illuminated APU MASTER switch FAULT light indicates that the APU has what condition?

An illuminated APU MASTER switch FAULT light indicates that the APU has shut down automatically. This scenario usually occurs due to a critical fault detected by the APU system, which must take immediate action to protect the unit and the aircraft. When the APU experiences issues such as over-temperature, low oil pressure, or other failures that compromise its safe operation, the automatic shutdown feature is triggered. This is an essential safety mechanism, as it prevents potential damage to the APU and ensures the safety of the aircraft and its systems.

The other conditions mentioned, while related to the APU's operation, do not directly result in the FAULT light activation. For instance, if the APU generator is not supplying the rated voltage, it may indicate a performance issue, but it does not necessarily trigger an automatic shutdown. Similarly, a sheared generator drive shaft or an overheating galley would not directly cause the APU to shut down in the same immediate manner as a detected fault in the APU itself. Thus, the FAULT light specifically signifies that the APU has taken protective action by shutting down due to an identified risk to its operation.