A320 APU
 

Hi guys!

I have some questions regarding the A320 APU (131-9A in particular)

If the aircraft is currently powered by the external Ground Power Unit (aircraft batteries not switched on), can I start the APU using only the GPU (without switching on the 2 aircraft batteries)?

If yes, will the APU take over the supply of electricity to the aircraft from the GPU? Or will the GPU remain the primary supplier since the GPU has priority over the APU in supplying the 2 networks (AC BUS1 and AC BUS2)?

Is there any situation where the GPU supplies one AC BUS and the APU supplies the other AC BUS?

Thanks!

<<If the aircraft is currently powered by the external Ground Power Unit (aircraft batteries not switched on), can I start the APU using only the GPU (without switching on the 2 aircraft batteries)?>>

No, in this case you would be able to switch on the master switch, but a short time after pressing start you will get a start fail on ecam.

<<will the APU take over the supply of electricity to the aircraft from the GPU? >>
No, external power button will be blue on with the GPU supplying. Once APU is available you will need to press the external power button to txfr over to the APU genny.

I would note I am a pilot not an engineer.

Thanks for taking the time to reply!

May I also ask what are the requirements (eg. flight level, number of starts, length of operation, etc) for an APU in-flight start that the A320 needs to perform for ETOPS operation?

**If the aircraft is currently powered by the external Ground Power Unit (aircraft batteries not switched on), can I start the APU using only the GPU (without switching on the 2 aircraft batteries)?**
Yes you can as the APU needs only the DC BAT BUS for starting which is energized with the aircraft power network on GPU.
And here is the starting procedure reference with the external power
49-00-00-860-008-A.
I recommend reading it if you have access to the AMM.

**If yes, will the APU take over the supply of electricity to the aircraft from the GPU? Or will the GPU remain the primary supplier since the GPU has priority over the APU in supplying the 2 networks (AC BUS1 and AC BUS2)?**
GPU has a priority over the APU.
The priority logic is: Onside IDG> GPU> APU> Offside IDG.

**Is there any situation where the GPU supplies one AC BUS and the APU supplies the other AC BUS?**

No as both require both Busties to be energized hence paralelling occurs which is not allowed on the A320.
I hope I answered your questions.

Thanks!

I got the part about the paralleling when I looked at the schematic again. It's definitely not allowed.

However as I was reading through the AMM chapter that you mentioned, I found that one of the steps requires the Batteries to be in Auto (On) and a Note saying "If the APU is to be started on EXT POWER, make sure that the batteries are switched ON. If not, the APU may not start and shut down."

But you're right about the DC BAT BUS supplying power to start the APU. Just can't figure out why it still needs both batteries to start.

Edit: Re-format

By the way what tom775257 said regarding APU failure to start on GPU only is also correct as there is a likelihood of APU shut down if it's started on GPU only.

Ok that's a Good question however I assure you that it starts with GPU only as I have done it on one of our stored aircraft personally.
I'll do some digging and find an answer for you.:ok:

Here is what I came up with:
It could be the same reason why 2 batteries are required to start the APU, APU starter motors draw significant current and with a GPU only thats supplying the full network it would probably overload it so batteries are required to be paralleled with the GPU to assist in the start operation.
Thats as far as I understand, I couldn't find a specific answer though.
Lets wait and see if anyone out here has an answer.

Maybe down to the APU manufacturer you have fitted but our definitely don't start without batteries when the GPU is on.

You get a fault light and no accel as the fault recorded on the CFDS.

p.s. From the Airbus training notes

Thanks fargoo
Any idea why though?
Dc BAT BUS will be on in both cases??
Thanks again for the info.

Guys, BAT PB off only as I understand. The BATs are still installed and available to connect, via the PB you just prevent their charging via the BCL.
Guess the reason is that you always want them to be fully charged in order to power the hot bus in case you need it with no other sources available.

In the apu start with external power task it says apu may shut down if push buttons are not in Auto positions.

A320 APU
 

Its exactly as Fargoo says. No start unless Battery pushbuttons in AUTO. If you look at the status page with the Batteries off, you'll see a green "APU BATT START NOT AVAIL" as a wee clue to turn them on.
Cheers

apu
 

Batteries need to be in auto, otherwise the fire protection will not be active and when thats the case then tha apu will spool up but then autoshutdown before it gets anywhere close to self sustaining speed

APU FDU power supply comes from the dc bat bus which is energized by the TRUs, squib power supply comes from hot bus however that has nothing to with bat pushbutton switchs in auto.
Fire protection hence is not lost with the normal elelctrical network on gpu and battery push buttons are released.

It's all down to the high load from the APU DC starter motor. In theory the TRUs can supply enough current to start the APU but if it's cold and it could be too much for them. The APU ECB via the BCLs ensures the batteries are connected to the bus and the assist in starting if the external power is on.

I found another helpful section Chap 24 of the notes. Again it may be down to the APU make you have fitted but I've never seen one of ours start with the batteries off. It's a mistake people often make and they quickly remember to try again with the bats on. (The other one is to try and start the APU too soon after applying external power - the FDU does a power up test and the ECB sees this as a fire and does an Autoshutdown :E)

Thanks thats some good information.
I assumed the same thing aswell initially but could not find any notes or refernce from the AMM to back up my assumption.

I just tried to start the APU of one of our 320s with the external power connected and BAT pushbuttons off, APU shut down at 20%, shut downs report shows no accel acft batteries not selected.

thanks for all the info guys!

I did a little investigation of my own.

The TSM notes that "If the BAT 1 and 2 pushbutton switches are selected OFF, the start contactors are disconnected from the power supply"

After digging out the ASM 49-42, it does look as if the Main start contactor 5KA is connected to BCL 1 & 2. Can anyone confirm this?

No, the start contractors are disconnected due to the aforementioned logic not met after 20%. a no accel auto shutdown then occurs.

Looking at the ASM The two start contractors 5KA being one are connected on to the batt bus. So as also mentioned before will allow current flow to 8KA Starter allowing the start if the TRUs are supplying DC from external AC power but only up to 20% where the ECB then aborts.