Originally Posted by
FlightDetent
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
-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.