BAT ONLY STARTING OF THE APU (A320)
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BAT ONLY STARTING OF THE APU (A320)
Hi all,
A little question about the start of the APU in the Airbus A320 (msn446) when flying on BAT ONLY.
In the Fcom Pro-Abn-24-Q elec Emer config:
"CAUTION In case of simultaneous engine generator failure, the probability of a successful APU GEN coupling is low. Therefore APU start attempts should be avoided, as this will consequently reduce the flight time on batteries only (by about 3.5 min for one start attempt)."
But how is it possible to start the APU when the ECB-starter is INOP on BAT ONLY like said in QRH?
QRH Abn-24.01A
Thanks a lot for your answers.
A little question about the start of the APU in the Airbus A320 (msn446) when flying on BAT ONLY.
In the Fcom Pro-Abn-24-Q elec Emer config:
"CAUTION In case of simultaneous engine generator failure, the probability of a successful APU GEN coupling is low. Therefore APU start attempts should be avoided, as this will consequently reduce the flight time on batteries only (by about 3.5 min for one start attempt)."
But how is it possible to start the APU when the ECB-starter is INOP on BAT ONLY like said in QRH?
QRH Abn-24.01A
Thanks a lot for your answers.
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Because generally in ELEC EMER CONFIG the only time you're running on batteries is during the emergency generator coupling or on the ground below 80kts*.
Airbus say this because the possibility of both generators failing independently is miniscule. The problem is far more likely to lie with one or both AC buses, hence the ECAM actions of isolating them by opening the bus tie. If this doesn't restore normal power there's little point in starting the APU because it's just as unlikely to couple to the network.
On top of this (even if the APU ECB is powered on batteries only) attempting an APU start would almost certainly fail - next time you start the APU watch the ELEC page and notice the enormous load placed on the DC network. Would you really want to do that while these same batteries are keeping you airborne?
* Of course, depending on how antique your A320 is you could also be battery only during flight with the gear down. Same principles apply.
Airbus say this because the possibility of both generators failing independently is miniscule. The problem is far more likely to lie with one or both AC buses, hence the ECAM actions of isolating them by opening the bus tie. If this doesn't restore normal power there's little point in starting the APU because it's just as unlikely to couple to the network.
On top of this (even if the APU ECB is powered on batteries only) attempting an APU start would almost certainly fail - next time you start the APU watch the ELEC page and notice the enormous load placed on the DC network. Would you really want to do that while these same batteries are keeping you airborne?
* Of course, depending on how antique your A320 is you could also be battery only during flight with the gear down. Same principles apply.
Last edited by Fursty Ferret; 23rd Aug 2012 at 07:05.
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I managed to get the answer today,
- BAT ONLY in flight, it is NOT possible to Start the APU because the ECB-starter is INOP
- BAT ONLY on the ground it is POSSIBLE to start the APU (*if speed below 100kts)
The procedure Fcom Pro-Abn-24-Q elec Emer config takes into account the fact that the RAT is out, so the EMER GEN is working and the BAT ONLY procedure is non applicable because it is irrelevant. In that case starting the APU will not be an issue.
- BAT ONLY in flight, it is NOT possible to Start the APU because the ECB-starter is INOP
- BAT ONLY on the ground it is POSSIBLE to start the APU (*if speed below 100kts)
The procedure Fcom Pro-Abn-24-Q elec Emer config takes into account the fact that the RAT is out, so the EMER GEN is working and the BAT ONLY procedure is non applicable because it is irrelevant. In that case starting the APU will not be an issue.
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it will start
Recently a student wanted to know if the APU could be started on batteries in flight. Now, forgetting any discussion as to the wisdom of attempting this and looking at the question in purely technical terms....the answer is "yes, it can be done".
FCOM 1 tells us that the BATT BUS powers the APU starter motor, also we know this is lost in EMER ELEC CONFIG + FLIGHT WITH BATTERIES ONLY. But it also states, and you have to search for this with a magnifying glass, that positioning the APU MASTER SW to ON, closes the BATT BUS contactor for 3 minutes, sufficient time for a start attempt. When I performed this in the sim, the current drain started at 260 amps and over two minutes reduced linearly to 60 amps. That's about one quarter of the battery's total capacity assuming a perfect, fully-charged 23AH battery.
That's why I wouldn't try it for real.
FCOM 1 tells us that the BATT BUS powers the APU starter motor, also we know this is lost in EMER ELEC CONFIG + FLIGHT WITH BATTERIES ONLY. But it also states, and you have to search for this with a magnifying glass, that positioning the APU MASTER SW to ON, closes the BATT BUS contactor for 3 minutes, sufficient time for a start attempt. When I performed this in the sim, the current drain started at 260 amps and over two minutes reduced linearly to 60 amps. That's about one quarter of the battery's total capacity assuming a perfect, fully-charged 23AH battery.
That's why I wouldn't try it for real.
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That's about one quarter of the battery's total capacity assuming a perfect, fully-charged 23AH battery.
