Thanks fargoo
Any idea why though?
Dc BAT BUS will be on in both cases??
Thanks again for the info.
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
)
APU START CONDITIONS
An electric DC motor starts the APU.
On ground operation conditions,the APU is started by the aircraft batteries or aircraft batteries in combination with Transfomer Rectifier Units (TRUs). A start in flight is possible up to an altitude of 11900 m (39000 ft.) with the use of TRUs.
A start in flight is possible up to an altitude of 7630 m (25000
ft.) with the use of the aircraft batteries only.
For successful APU starting on ground or in flight, both batteries PB must be switched to the AUTO position according to Airbus procedures.