787 Batteries and Chargers - Part 1

[Chris Scott]

Quote from TURIN, responding to the sugestion that the APU may contribute some power to APU start even when main electrical power is available:

"This concept of the APUC being powered by the APU battery however, regardless of main bus power does seem odd.
I'm not sure if this is the case on other a/c types. Research required methinks."

I can't comment further on the 787 architecture. But I can repeat an ex-pilot's rough understanding of the fix used by Airbus to solve APU start problems on the A320 - in case it helps as background info.

During an early A320 demo flight in 1988, with the president of France on board, an APU start resulted in a TR failure, and a shedding of electrics. At that time, the system used only one of the two main TRs to provide the DC load of >700 amps. The TR was plainly not man enough for the task.

The chosen fix was to introduce the second main TR into the loop, and let it share the load. However, in later years, it became clear to me that the two main batteries (there's no APU battery on non-ETOPS A320s) were also sharing the load. You could see it quite clearly by selecting the ELEC page on ECAM during the APU start. Cannot detail the mechanism involved, but it seems that once the DC voltage falls below the voltage of each battery, that battery's contactor closes. So the two main batteries' contactors are not inhibited from closing during an APU start. (Whether the system has been changed since I retired, I don't know.)

The 787 has radically different architecture, with a dedicated APU battery and (presumably) much more powerful TRs.

Moving on, I wonder if the number of APU starts using the APU battery alone (on the ground before engine start and with no external power available) has been recorded. The report quotes the JAL captain as stating simply that the APU had previously been "turned on about 30 to 40 minutes before the a/c left the gate at NRT".