A318/19/20/21 Aircraft electrical power down.
 

What is the correct way of avoiding the APU flap door staying open while shutting down power from the aircraft?

Ganard, suspect that you are too quick with switching off the batteries.

Extract from A330 should be similar:
On the APU page of the SD:
- a short time after the APU speed decreased to 7%, the FLAP OPEN indication goes off.

- 30 seconds after the APU speed decreased to 7%, the APU speed and EGT indications show XX (amber).

NOTE: Switching the Batteries OFF before the APU air inlet flap is completely closed may cause smoke in the cabin during the next flight, therefore wait as long till SD shows crosses XX in amber to be sure the shut down process is finished.

So waiting 60 secs minimum after seeing flap OPEN indication disappear and then deselect BATT1, BATT2 and APU BATT should improve your lot...

Of course this maybe completely different to the Baby Buses..

Ehrrm, not current anymore, but AFAIK Apu Flap is controlled via the ECB which is powered by the BAT BUS. The BAT BUS is supplied by DC BUS 1and/or 2 and/or the batteries.

So if EXT ELEC ON, the BAT BUS is supplied even with batteries of and the Flap will close.

My 2 cents.

Only if you have no external power you should wait 2 minutes before turning off BATTs OFF to be sure that flap has closed. With external power ON, DC BATT bus is powered, can switch off BATTs without delay. You could confirm flap closed on APU page before deselecting external power off.