When will you see the APU GEN ON advisory?

• A. When AC power is available from the main generator
• B. When it is the sole source of AC power
• C. When the APU is off and on battery power
• D. During post-start normal operation

Answer: (B)

APU GEN ON appears when the APU’s generator is online and supplying AC power to the aircraft. In the UH-60 electrical system, the main generators normally feed the AC buses, but the APU can take over as the power source if the main generators are unavailable. Seeing this advisory means the APU is the sole source of AC power, which is exactly the situation when the APU is powering the electrical system because the main generators aren’t providing AC. If AC power is already coming from the main generator, the APU’s generator isn’t the active sole source, so the advisory wouldn’t be shown. If the APU is off or only on battery power, there’s no APU generator online to trigger the advisory, and during post-start normal operation the status isn’t defined as the APU being the sole AC source.