Eletrical emergency configuration

[calden320]

hi guys, during ECAM action why is it the APU if avail will be started since AC1 and AC2 are already lost meaning even if the APU will start it still cannot supply any AC busses.

Also, on ground why is it that APU can only be started below 100knots but then again the AC busses are inop.

Thanks.

[Sidestick_n_Rudder]

The AC busses are lost, because there are no generators to power them. Once you have started the APU, you should have the AC busses back.

[halfofrho]

If the actual AC1 and AC2 busses fail, then yes, firing up the APU is pointless. However if you have entered Emer Elec because the two engine generators are bust then firing up the APU makes sense as it will be able to power AC1 and AC2.

[calden320]

During elec emer the engine generators are not busted but its just the failure of 2 ac busses right? However I do not see ( and it doesnt say) any diagram in the electrical where the APU can supply the aircraft electrical network when the AC busses are inop during elec emer. Thanks for further info.

Guys, Have you seen elec emer diagram where the APU will connect to the aircraft electrical network while AC busses are inop?

Likewise can anyone clarify why the APU can only be started below 100kts on ground while emergency generator is running. Thanks for your help.

[Uplinker]

Highly unlikely to have a dead short on both main AC busses simultaneously.

Airbus boffins won’t have written the ECAM to perform useless tasks !

[Sidestick_n_Rudder]

@calden

Elec Emergency Config essentially means loss of all generators. It assumes all AC Buses to be operational, just not powered. Therefore APU start will restore the normal AC power. AC bus loss is a different procedure and I don’t think Airbus even envisaged losing both AC buses at the same time

As for APU start on ground. The system logic is that on ground, you’re not getting enough power from the RAT and DC BAT bus is only connected to the batteries below 100kt. I don’t know the actual reasons for that, but basically if you’re on GND and above 100kt:
- RAT is not available (because min RAT speed is 140kt)
- BAT bus is not yet powered by the batteries

Hence - you can’t start the APU