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A320 APU

Old 22nd Mar 2015, 02:46
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A320 APU

Hi guys,

In the securing aircraft section FCOM dictates that the batteries should not be turned off until the APU flap is closed. If you are on external power, is it necessary to wait for the APU flap to be closed before turning off the batteries?

Also, is APU fire extinguishing available with external power ON and batteries OFF when the APU is running?

Thanks, couldn't find any information on these or maybe I've missed it somewhere!
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Old 22nd Mar 2015, 03:41
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As long as the dc bat bus bar is supplied then it will close and fire protection will work. See fcom electical supply.
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Old 22nd Mar 2015, 04:56
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dream747
If batteries are turned off before flap is closed there will be smoke in the cabin as APU de oil sequence is interrupted. It is true even with external ON. APU does not start with batteries OFF.
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Old 24th Mar 2015, 00:58
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Which APU ??
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Old 25th Mar 2015, 13:50
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As long as the dc bat bus bar is supplied then it will close and fire protection will work. See fcom electical supply.
So with batteries off that'll be a "no", then... ;-)

Confirmation of this on status page or by trying an APU fire test with batteries off.
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Old 25th Mar 2015, 15:55
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In normal configuration TR1 supplies the DC BAT BUS.

i.e. the DC BAT BUS will be powered from the EXT PWR, even if the BATs are selected off.
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Old 29th Mar 2015, 15:07
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I wish to correct my statement. With external power connected and batteries off APU flap will open or close but APU will not start with batteries off.
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Old 29th Mar 2015, 15:12
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Thanks guys for the answers.

So it would be absolutely necessary to keep the batteries on as long as the APU is running regardless of external power status to ensure the availability of the APU fire protection.
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Old 29th Mar 2015, 15:47
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As long as the dc bat bus bar is supplied then it will close and fire protection will work. See fcom electical supply.
The answer is no, battery needed only for starting.
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Old 29th Mar 2015, 20:11
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ECB = Electronic Control Box,
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Old 30th Mar 2015, 09:51
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If batteries are turned off before flap is closed there may be smoke in the cabin as APU de oil sequence is interrupted. It is true even with external ON. APU does not start with batteries OFF.
Fixed it for you.
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Old 4th Apr 2015, 15:58
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A320 fitted with 131-9A APU

If you turn off the battery's the APU will shut down! Due no fire protection. I know because I've done it!

That's not reading from a book, that's experiance!

Fire protection is powered from the hot battery bus, regardless of electrical system configuration. That way you have fire protection during the start sequence on a "dead" aircraft.
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Old 4th Apr 2015, 18:09
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I agree with this:

Originally Posted by whitelabel
As long as the dc bat bus bar is supplied then it will close and fire protection will work. See fcom electical supply.

And


Originally Posted by Checkboard
In normal configuration TR1 supplies the DC BAT BUS.

i.e. the DC BAT BUS will be powered from the EXT PWR, even if the BATs are selected off.

If the APU starter motor is connected to the DC BAT ( by External Power via AC1-TR1-DC1 ) even with the Battery PB off (DSC 24-10-30-20 P2/4 27 NOV 13) why the APU won't start?
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Old 4th Apr 2015, 22:01
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Touge/Whitelable/Checkboard

I think you may all be taking the FCOM diagrams too literally! They are designed to give a basic understanding of system distribution, rather than a full system logic explanation. There are in fact a lot of contractors/relays omitted, just to keep the diagrams simple.

If you attempt to start the APU with the battery's switched off but with ground power ON, the start will initiate but the ECU will sense a lack of fire protection. An AUTOSHUTDOWN will occur.

If you turn off the battery's with the APU running, ECU will sense lack of fire protection. APU will shut down.

Simple..... NO BATTERYS NO APU!

The explanation is in the AMM not the FCOM.
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Old 5th Apr 2015, 01:07
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I tried it last night, turning off the batteries when the APU is running and the APU did not shut down.
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Old 5th Apr 2015, 15:24
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EpsomDog, in general I agree with your comments on the "simplicity" of FCOM ELEC diagrams. They are not wrong, though. Please take care with the terms used, to avoid confusion.
Fire protection is powered from the hot battery bus, regardless of electrical system configuration. That way you have fire protection during the start sequence on a "dead" aircraft.
On a cold aircraft the fire detection loops and bottle #2 only become available after the BAT are selected ON so power is made available to the DC BAT bus.

dream747 I only have the FCOM for guidance, if we look at PRO-NOR-SOP-26-A:

BAT 1 pb-sw and BAT 2 pb-sw ..................................................OFF
Wait until the APU flap is fully closed (about 2 min after the APU AVAIL light goes out), before switching off the batteries. Switching the batteries off before the APU flap is closed may cause smoke in the cabin during the next flight.
If the batteries are off while the APU is running, APU fire extinguishing is not available.
Further up the procedure says EXT PWR as rqrd". The text of BAT1+2 procedure should be good for both cases.

My unerstanding of the italic note:
1) APU flap > this is only important when the EXT PWR had been selected OFF. CAUTION: vilas says otherwise!
2) Fire extingusihing > puzzled here. As per FCOM diagram: if the APU was running DC BAT bus is supplied via DC 1/2 and, together with HOT BAT 2, the fire protection is available... ??

hence my Q:
(i) Is the APU fire protection avilable with batteries OFF but DC BAT bus supplied and BAT 2 normally charged? The DSC and PROC chapters of FCOM seem to contradict. If NOT is the reason electrical (beyond FCOM pilot-digestable diagrams) or logical?

Moreover I wonder to what extent are the failure cases already considered. E.g. EXT PWR trip-off during APU start and shutdown. Or ops with APU GEN u/s.

Last edited by FlightDetent; 5th Apr 2015 at 19:09.
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Old 5th Apr 2015, 18:36
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Even with the APU running and you turn off the batteries, on the ECAM ELEC page the DC BAT bus shows crosses. I would assume from there then the DC BAT bus is completely not powered in this case?

Or anyone has any information on the conditions when the dc tie contractors close?
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Old 5th Apr 2015, 18:51
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Originally Posted by dream747
Even with the APU running and you turn off the batteries, on the ECAM ELEC page the DC BAT bus shows crosses. I would assume from there then the DC BAT bus is completely not powered in this case?

Did you try it in the A/C or in Sim?
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Old 5th Apr 2015, 19:11
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Where is IGh and IFixPlanes when you need them?

On a second thought, probably doing someting reasonable for the Easter ...
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Old 5th Apr 2015, 19:13
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747 ,

Were you doing this as part of the securing the aircraft procedure ? Were the ADIRU's Off ?
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