What is indicated by the "APU GEN" light on the overhead panel?

The "APU GEN" light on the overhead panel indicates a fault with the auxiliary power unit (APU). When this light illuminates, it is a warning signal that alerts the crew to a malfunction or failure condition related to the APU's generator. Rather than signifying normal operation or successful power generation, the illumination of the "APU GEN" light suggests that the APU cannot provide the necessary electrical power for the aircraft or that it may be experiencing operational difficulties requiring attention.

In contrast, if the APU were running normally and functioning properly, the "APU GEN" light would not be illuminated. Similarly, a green light would indicate that not only is the APU running, but it is also providing electrical power effectively. The APU being serviced typically doesn't trigger this specific light; instead, maintenance procedures would be indicated differently on the panel. Therefore, recognizing the significance of the "APU GEN" light is crucial for the crew to ensure the aircraft's electrical systems remain operational and to take corrective actions when needed.