B747-400: BAT switch with APU running

[Bluescan]

Good morning folks.

We had a discussion in the cockpit last night regarding what would happen if the BAT switch was turned OFF during ground operations with the APU running, APU generators connected and STBY POWER switch in AUTO. We all imagined that "nothing" would happen, considering that the relevant busses would be fed by the AC system.

Allegedly, someone turned BAT OFF by an accident in mentioned condition and everything went black and the APU subsequently shut down. Could anyone please explain why?

Best regards,
Bluescan

[JammedStab]

Seems to happen in many Boeing jet types.

[Stan Woolley]

I seem to remember that the APU will shut down on the ground, but not in the air. In a 737.

Would I bet my mortgage on that? No! It's been over five years since I've flown anything.

[Icelanta]

On ground, if the battery is put to OFF when the APU is running, the aircraft will lose all power, and APU will shut down.
The whole eicas will go "nuts" and your engineers will be very pleased 😏...

[Crispy Pig]

If I recall correctly, the battery bus supplies power to the APU fire protection system. In the scenario you describe the APU goes into an auto shutdown without a cooling cycle as a safety precaution - this is the case on the 757 anyway!

[NSEU]

Definitely shuts down the APU. Ask me how I know :P

From my 747-400 manuals:

"Turning the Battery Switch off de-energizes the BCU Standby Power Relay (R7301), which provides a false duct overheat signal to the APUC. The APUC then shuts down the APU. Logic in the EIUs prevents false messages in this situation."

This autoshutdown has no cooldown period.

[ACMS]

That's my understanding.

Maybe do it in the Sim......

[Bluescan]

Thank you, it's a bit clearer now