A320 APU Bleed after engine start
FCOM 3.03.09 states that the APU Bleed should be turned off just after engine start to avoid ingesting engine exhaust gases.
What I don't understand is that APU Bleed as well as engine bleed is clean air coming from their respective compressors and going to the packs. So exhaust gases from where?
Besides, after a while we can turn the APU Bleed back on and takeoff with the packs on the APU.
Any Ideas?
Cheers