Does A320 APU start only by battery?
 

In case of engine failure ,Most of instructors were saying that we should start the apu for electric backup source.
meanwhile there was one instructor who says that we might want to reserve the battery power by not starting the apu,
because he said the Starting APU uses battery power..

He was insisting that if the apu didnt start for some reason then it would end up using battery power.
Which means, if we get to lose remaining generator for some reason (I dont think so,. but...)
it would lead to electric emergency configuration,
if we had have used battery power with starting apu, saying that it would shorten the time with electricity .


I looked up the FCOM. but I couldn't the answer.
Anyone knows the answer?

He was kind of disrupting person anyway.
But I kinda wanted to know the real answer.

I thought the remaining generator could start the apu even with the engine failure.

Are we talking OEI or ALL ENGINE FAIL?
One engine out: start the APU, different TRI\TREs have different opinions about when, though.

Seems like your instructor likes to test the A320 redundancy to the design limit. Somebody spent a lot of time and money fitting an APU. Ask him why.....

Seems to me a good time to use an Auxilliary Power Unit is when you need auxilliary power..? Some people overthink things.
I’d say don’t wait for the checklist if you lose a generator, just fire it up if you know it’s a checklist step in a few minutes anyway.

Join the A320 Discussion Group on Facebook, it will give you answers to questions such as this, and others you haven't thought of asking yet. There is lots of input from A320 pilots of all levels from students to examiner.

EFATO when do you guys start the APU?

Hm, too little strong knowledge usually complicates things. It's a long road coming to understand why 330 STS says consider APU start and 320 does not.

2 suggestions:

A) FCTM purist would start before STS, as part of the reset/recovery procedure. This is the most logical moment in the task chain of handling the abnormals.

B) When reviewing the ELEC page of the affected systems.

Personally I think A) is correct while B) achieves the same quicker and with less effort. As long as we're on the same page, it makes no difference.

With the present and previous operator alike, choice B) is the agreed practice.

So why does the 330 Status say consider APU while the 320 does not?

..The correct procedure is to follow the Engine Failure - checklist..What does it say concerning the APU..??

Fly safe,
B-757

To answer OP's original question - no, the APU does not only use battery power to start. The general APU info in FCOM says it may obtain power from the batteries, electrical system or GPU for starting.

Regarding the rest of your post, have a read of the ELECTRICAL CONFIGURATION section in FCTM, Engines, All Engines Failure. APU start can reduce flight time on batteries but as the manual says, if fuel remains crew should attempt an APU start to recover pressurisation, additional electrics and have the bleed available for an assisted relight if required.

In agreement, practical experience shows (on ELEC pg)
- APU does not start with BATT pbs in OFF
- the charging from BAT BUS stops during APU start and the reversed, discharging current is quite significant

If the APU doesn't start, whatever battery power you lost would be replenished by the remaining generator. To take the instructor's logic to the absurd conclusion, why ever start the APU at all?

We don't have a defined point at which we start the APU after engine failure. We just do it when we get to it. As somebody said, this is usually done by the time we're reviewing the ELEC page.

A330 has an APU battery. A320 shares the battery with aircraft and APU.

I think the OPs TRE was getting confused.

If you are down to just one main Gen, of course you can start the APU. That's what the APU Gen is for; to power part or all of the aircraft electrical system, (apart from load shedded galleys etc), at any time. We did exactly this when we lost a main Gen coming home from Faro one day, (A321); started the APU and carried on, (after securing the fault and DODAR obviously).

I think what your TRE might have been confusing this with is flight on batteries only? With no Gens, you have (from memory) ~ 30 mins battery life, depending on exact config, to get on the ground. In this situation you could attempt an APU start from batteries only, but if the start fails, you will have lost ~ 8 mins time (I think) from your batteries. Or, if the reason for your electrical problem is an AC 1+2 busbar failure, then even if you successfully start the APU, its Gen will be unavailable, so not much point wasting battery time starting it.

NOTE an APU start normally uses TRUs and batteries. The in-rush current when the APU starter is first energised is more than 900 Amps initially, which is more than the TRUs can supply, so the batteries are used to assist with the load.
As someone said though; if you have at least one main Gen available, it will then recharge the batteries, so in that situation there is no harm in trying, and the batteries will be replenished in about 20 mins.

Are we in agreement already, the OEM procedure is not to start the APU? (while nothing prevents the crew from doing so).

It's not like the PPRuNers of 2021 discovered something the engineers and designers in 1986 were not aware of.

320busboy 😉

APU uses either battery only or combination of battery plus AC to start. Start is much quicker than on battery only.

To answer the original question - if you don't start the APU then you're in a world of pain if the remaining engine generator goes offline, and you'll still need to start the APU. Then if it doesn't work you're back in the exact situation you wanted to avoid in the first place.

Starting the APU means that you're in a much happier place from an electrical perspective. I'm convinced some people come up with these crazy theories just to get a name for themselves.

On top of that the APU is much more likely to start successfully if you have both AC and battery power available, as opposed to trying to start it with the battery simultaneously supplying the entire DC ESS network.

Boils down to two simple questions:

1) If this happened in real life, would the TRE practice what he preaches or actually start it?

2) Which moment? :}

Not starting APU because of battery discharge is silly because in normal electrics batteries get charged. However it's true that ECAM doesn't ask you to start APU. But according to Airbus for any failure after ECAM actions are complete time permitting you should refere to FCOM for additional information. If you refer to FCOM abnormal procedures ENG 1(2) SHUTDOWN there are three notes at the bottom. The first one says:
If available, the APU may be started and the APU GEN used.
So this is the place Airbus wants yo to start APU. However because one may forget to do so, airlines have their favourite spots to start APU. Most common is at secondary failures ELEC page. However in case of APU FIRE automatic shutdown is not available in the air. So some airlines at ELEC page just push APU MASTER but not the start swich. So when you remove the status page APU page appears on SD which serves as a reminder to start APU and they start it here.

-Apu does not start with BATT pbs OFF because the ECB Supply, the starter motor, the APU Loop A and B, the BTL SQUIB B and the APU AUTO EXT are powered by the DC BAT(only the BTL SQUIB A is powered by HOT 1).

FCOM APU general :
In flight
‐It backs up the electrical system
‐It backs up the air conditioning
‐It can be used to start the engines.

So the instructor does not recognize the need to use the APU as an electrical back up, or to relight the engine if applicable.


The answer to the instructor would be :

We can start the APU now with one GEN available(the batteries will charge right after the start up), or we can wait to to be in elec emer if the second generator/engine fails(the reason to preserve the batteries according to his opinion).


ELEC emer configuration procedure :

APU (IF AVAIL) ..........START

APU start is not available for 45 s after the loss of both engine generators. This 45 s delay prevents any interference with emergency generator coupling.

IIRC there are a couple of MSN’s where APU start is not recommended after loss of AC power (inc emer elec config). They are few and far between, I think we have one of them. That might be part of his confusion, or it might not.

All from memory, please be gentle.

LD