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

The APU can ingest the exhaust gasses from the main engines, the "clean" air from the apu bleed is only as clean as the air coming from the inlet.

There was a long and detailled thread on this about 6 weeks ago.

Great, if you have the link please post it here, cause I couldn't find it.

Thanks

Just as an aside...
 

Switching off the APU Bleed (by the captain), as well as returning the Start Selector back to Normal (also by the captain) is a signal to the First Officer to begin his/her After Start flow. I have problems with this, if the aircraft is being pushed back.

Part of the First Officer's flow includes setting the Flaps/Slats to the takeoff setting. My company's SOP prohibits making any configuration changes, performing any flows or checklists, etc., until AFTER the ALL CLEAR (or the AIRCRAFT IS CLEAR TO TAXI) signal is given by the ground man (person).

Now, assuming a long push-back...and engine start during push-back...both engines will have been started (all start sequences complete) prior to the completion of the push-back. Turning off the APU Bleed immediately after the starting is complete (thereby initiating the after start sequence) prior to receiving the ALL CLEAR signal will violate my company's SOP.

I solve this dilemma by holding off on the engine start until well into the push back. This way, by the time the engines are started, the tug and tow bar are disconnected, all the ground people are clear or clearing, we're getting the ALL CLEAR signal from the lead ground man, etc......then, we start moving flaps/slats, etc.

Additionally, our SOP requires that we not call for taxi clearance until we receive the ALL CLEAR. All this ensures that we don't get in a hurry and hurt any of the ground people.

Just an aside...just for info...



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