A320 exhaust smell in cabin area

[alawax]

Hi guys ,


I have question & I hope to found answer

during engine start , there are exhaust smell coming in aft of the cabin area
with engine start only ,
what is this ? and this is normal ?


Thanks

[Meikleour]

Sounds like re-ingestion of engine exhaust fumes via the APU inlet if starting in a strong tailwind. Can happen if Packs are supplied by APU for a long period after engine start.

[sonicbum]

Hi guys ,


I have question & I hope to found answer

during engine start , there are exhaust smell coming in aft of the cabin area
with engine start only ,
what is this ? and this is normal ?


Thanks

APU exhaust ingestion with tail or crosswinds. A few years back there was an OEB instructing to start the upwind engine first to avoid excessive engine exhaust gas ingestion from the APU with old ECB standard.

[SW1]

When pushing back and having a direct tailwind from behind the engines can cause that smell as previously mentioned.

Ive found setting the extract push button to override- closing the avionics inlet and extract valves- once the smell is first perceived eliminates this problem. In the cockpit at least. Not sure if it elimintes the smell from the cabin.

After second engine start. PF does normal flow and sets the override push button back to normal. Inform the other guy as well before you do this so neither of you forget!

[dream747]

If I understand the replies correctly, due to the tailwind the APU ingests its own exhaust gas and hence the smell. But why does it only happen during engine start? Surely sitting at the gate with the tailwind and the APU bleed being the source of air for the packs, we should be smelling it consistently?

What am I missing here?

[EEngr]

What am I missing here?Possibly the exhaust velocity. Once the engines (or APU) are up and running, the exhaust velocity is sufficient to push the vapors far beyond the aircraft in all but severe tail/crosswinds. :8

[Uplinker]

The smell is caused by the APU ingesting the engine's exhaust gases, not its own.

Hence why this only happens during main engine start and with certain winds from behind - if you'll pardon the expression !!

After the engines are started, the aircon packs are fed from the engine bleeds instead - which are "clean" - and the APU is shut down, so the problem goes away.

[alawax]

Thanks guys ,


as your replay , it's APU ingesting the engine's exhaust gases .
but , it's coming ONLY with Engine start within few seconds everything back to normal .
no strong tailwind & crosswinds


this happens it's come always with Engine start only

[Goldenrivett]

Hi alawax,

I agree with SW1's diagnosis in post #4.

During the first automatic engine start attempt, only 1 igniter is supplied, but fuel is being supplied to every combustor. It takes some time for the flame to propagate around to all the combustors. During that time, the fuel that has not been ignited, is exhausted into the atmosphere as vaporised fuel and is very smelly.

As you push back into the smelly air, the Avionics Ventilation system Blower fan sucks the air in and blows it around the avionics equipment bay. Hence you get to smell it first.

see FCOM DSC-21-30-20 Avionics Ventilation, Normal Operation, Open-Circuit Configuration.

[lomapaseo]

It takes some time for the flame to propagate around to all the combustors

about the time that a whoom sound makes under spin-spark and spray

[alawax]

I will try next flight :) thankx

[Goldenrivett]

Hi lomapaseo
whoom sound
Ok. More accurately, the unburned fuel due to poor air flow at very low engine RPM.

see Page 4: http://www.air.flyingway.com/books/engineering/CFM56-3/ctc-142_Line_Maintenance.pdf

"When an engine is started.....gases in the exhaust that smell bad and can cause injury or irritation to your eyes and lungs."

[safelife]

I think it's not ingested via the APU, the packs are off during engine start anyway. I think it's the avionic venting which is dragging in the fumes.

[ShotOne]

Sorry guys, need to back up a little. Why would THE APU ingesting exhaust gases, whether it's own or the engine's, cause odour/contamination of cabin air??

[IFLY_INDIGO]

It doesn't happen all the time.

Here is the what FCOM, NORMAL PROCEDURE, SOP - SECURING THE AIRCRAFT says:

"wait until the APU flap is fully closed (about 2 min after the APU AVAIL light goes out), before switching off the batteries. Switching the batteries off before the APU flap is closed may cause smoke in the cabin during the next flight"