A320 APU Bleed Use
 

In the A320 if the Engine Fire pushbutton is pressed APU bleed should not be used in case of Eng. 1 Fire pb pressed and APU Bleed can be used with the cross bleed rotary switch in shut position in case of Eng 2 Fire pb pushed. Can someone please explain the reason why APU bleed should not be used. Is it to prevent a reverse flow?

Hi
When there engine fire or damage fire push button of that engine is pushed. If you use cross bleed the smoke fumes from that side will enter in the aircraft. As the APU bleed is connected to the left of cross bleed, if no.2 fire push button is pushed you can open APU bleed with cross bleed closed to supply no.1 pack and not the other way around. If no1 button is pushed APU bleed cannot be used as the cross bleed should not be opened.

Thanks Vilas. I made a mistake while typing. Omitted to mention Eng 2 Pb. Thanks for the response. But I fail to understand how smoke will enter if there is only damage and no fire.

You'll have discharged an agent into the engine if damaged.

Damage is unpredictable. It can create Fuel vapours or oil burning smell. You can end up doing smoke procedure, not very welcome. So no chances are taken.

Like wise. I fail to understand how any smoke, fuel, fire extinguisher, burning smells etc. could find their way against the differential pressure of a working bleed system v a system which is shut down and at ambient pressure.

It was explained to me that an engine with damage (i.e. no N1 / N2 or puncture in the casing giving a fire warning) will probably have metal debris from turbine / compressor blade rub / failure. If the metal particles get into the bleed system then the Non Return Valve function may have been damaged and cross bleed air could leak past the valve and into the damaged engine unnoticed (no bleed leak message). That could cause an excessive bleed demand on the working engine.

Same ways as the suction pump works. Bleed air will flow towards operating engine bleed sucking whatever is trapped in the damaged side.

Hi vilas,
http://www.eaton.com/ecm/groups/publ.../ct_207260.pdf
"Due to the action of the motive flow, fuel in the tank is drawn into the jet pump through the induced inlet and mixed with the motive flow and then discharged through the pump discharge outlet port, to be deposited close to the boost pump inlet."

I understand how the venturi effect can be used to move fuel from adjacent tanks - but I fail to see why Airbus would design such a venturi in their bleed air system.

Guys,

I think everyone's missing a much simpler point here, while poring over venturis and smoke procedures. Vilas, in this case, I think you're wrong.

The FCOM (DSC 36-10-20) doesn't really help because the diagrams are not to scale and are only schematics.

The Bleed Valve is bolted just inches away from the engine casing. Downstream of the Bleed Valve, the ducts then go to the pre-cooler (between the engine and the pylon), and then onward to the packs, routed via the pylon and wing.

If you have engine damage, it's almost certain that the engine would have shaken itself to bits and the attached ancillaries (like the thin-walled bleed ducts) would be damaged too.

Remember, these ducts are all downstream of the bleed valve. So if the APU is trying to supply air, the air will now leak thru these broken ducts on the side of the damaged engine, pylon and wing.

Two obvious issues with that:

1. You have a leak!!! Why force air into a broken duct causing overheating, structural weakening (and if the duct is damaged in the engine, you'll be blowing away the agent that you just discharged).

2. The APU is wasting it's breath blowing into a broken duct, and your pack will almost certainly not have enough pressure to operate.

In short:
Engine Damage -> Duct Damage -> Don't put air in a broken bleed duct!

Whatever the reason the cross bleed is not to be opened when engine is damaged.

Quite so.

In the event of engine damage, you don't know exactly what has happened, or what substances, {smoke, fumes, fuel, engine oil, hydraulic fluid, extinguishing agent, CSD oil}, might have got into the bleed duct.

The last thing you want to do is incapacitate yourselves or your crew and passengers, with smoke or fumes, which would compound what started as a bad day into a disastrous one !

Don't use the affected bleed duct(s) in such a situation, If the left engine is affected, don't use the APU bleed either, because it shares part of the engine #1 pipework.