Togue, we tried it in the aircraft. With the APU running, we turned off the batteries and had a look at the ECAM ELEC page, where it shows the DC BAT BUS in crosses. STATUS page shows APU BAT START NOT AVAIL. APU FIRE DET was not part of the inop system as I would have expected similar to the DC BAT BUS FAULT.

Stone, The ADIRS were on when we tried it out. Aircraft was in transit.

Can you check this section of FCOM for the A/C you were sat in? What does it say about the crosses?


http://i58.tinypic.com/5nq6ub.png
because mine says nothing:


(3) DC BAT indication
It is normally in green. It becomes amber, if DC BAT voltage ≤ 25 V.

Mine describes it exactly the way yours do. Which is also why the indications caught us by surprise when we turned off the batteries.

Have you tried it on your aircraft?

I myself wrote this to earlier query on the same subject on 12/12/2014 now I remember.

Different APUs are fitted depending upon MSN of the aircraft. I am giving below from FCOM. However my experience is APU auto shuts down with batteries off,


Applicable to: MSN 0185-0249, 0264, 0384-0400, 0724
The APU may obtain power for starting from the aircraft's batteries or normal electrical system, or from ground service.

Applicable to: MSN 0138-0148, 0138-0148,0148- 0255, 0148- 0255, 0497-0557, 0932-0943
The APU may obtain power for starting from the aircraft's batteries alone or in combination with the external power, or from ground service.

Applicable to: MSN 0250, 0301-0335, 0440, 0758, 0976-1818
The APU may obtain power for starting from the aircraft's batteries or normal electrical system, or from ground service

Had the opportunity to secure our aircraft today and we tried once again putting the batteries off while the APU is running, and it doesn't shut down.

External power was connected and we put the batteries back on and shut down the APU, and turned off the batteries while the APU flap was still indicated open. We monitored it and it closed eventually.

MSN 5xxxx aircraft.

As I said on the other thread the flap is controlled by the APU master and doesn't need the battery but it is required for start.

As an SLF I might easily be missing something, but it seems like an APU shutting down because batteries were unavailable would be an unnecessary (and presumably undesirable) decrease in redundancy.