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I've seen some procedures where one engine is started with APU bleed, then APU Bleed is turned off to avoid ingestion of non-desired gases and then the second engine is started with ENG 1 Bleed on.

However, my question would be: Would it be possible to start BOTH engines with the XBLEED Valve in the AUTO position, APU BLEED ON and assuming there are no critical issues (fire, ...)?

To sum up: XBLEED AUTO, APU BLEED ON. Could you start both A320 engines with that configuration?

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  • $\begingroup$ Dunno about the A320, but with very similar engines the 737 absolutely can start both motors like that. $\endgroup$ – Ralph J Nov 8 '20 at 13:59
  • $\begingroup$ On a footnote here, the "ingestion of non-desided gases" is a minor issue. Even though the APU intake would ingest some fumes from the engines, no bleed air is fed to the AC PACKs while the engine start selector is set to start. So the fumes would not enter the cabin. $\endgroup$ – Sami Dec 13 '20 at 11:17
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I have been flying A320s for over two years now and this is the first time I am hearing this. Normally the engines are always started using the APU with its bleed providing air for the air starter motor. So, in normal operations, cross bleed is set at AUTO, APU bleed is turned on and both the engines are started with the help of the APU. Once the engines are started APU bleed is turned off and the APU is shut down. Below is the after start flow of the A320. The green shows the flow of the pilot who starts the engines. As you can see, once the engine mode selector is set to norm, APU is shut down.

enter image description here

The procedure you are talking of is called a cross bleed start which is used when the APU is inoperative for the flight. It is also used by airlines that enforces one engine taxi to save fuel. When APU is not operational, we start one of the engines using an external air source. Once the engine is started the external unit is removed and then to start the next engine we use the bleed from the already started engine. To do that we open the cross bleed and increase the thrust on the running engine until the bleed pressure is sufficient enough for the second engine start up. As soon as both the engines are running we set the cross bleed to AUTO and revert back to normal operation.

As I have told before, I have never heard of any concerns regarding harmful gases coming of the APU bleed. Back when I used to fly Dash 8s we had issues in some of our airplanes smelling of burning oil once engine bleeds are turned on after start up. The airline consulted with Bombardier and they sent a bulletin which said that sometimes oil drippings from engine bearings can fall onto the bleed ducts. Because the duct temperature tends to be quite high, the oil burns off and the smell goes into the cabin. According to them, the lubrication oil used in airplanes is not hazardous or poisonous to human beings. I would say, it applies to all transport category airplanes with gas turbine engines because nearly all of them uses the same type of oil.

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  • $\begingroup$ It's not a surprise that manufacturers claim their products to be safe, but the toxicity of oil fumes should be regarded as disputed. See en.wikipedia.org/wiki/Fume_event . $\endgroup$ – bogl Nov 30 '20 at 15:34
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    $\begingroup$ @bogl The wiki article you referred here has an event that happened in the 50s as an evidence that turbo oil can be harmful. How long it has been since then? I am pretty sure a lot of things have changed. $\endgroup$ – Anas Maaz Nov 30 '20 at 15:43
  • $\begingroup$ I didn't claim there is evidence in the article. I only referred to it to link the issue to its name, that is 'fume event'. It has gathered significant media attention in the past couple of years. Further to that, I am not saying it is toxic or not toxic - because I sincerely don't know. The point is, that the issue is disputed! google.com/… $\endgroup$ – bogl Nov 30 '20 at 17:07
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    $\begingroup$ @AnasMaaz, but this article is recent and quite well researched and clearly indicates that oil heat decomposition products and certain oil additives are toxic (so is hydraulic fluid, which also sometimes causes fume events, but this accident is blamed on oil). $\endgroup$ – Jan Hudec Nov 30 '20 at 22:50
  • $\begingroup$ @Jan Hudec The thing is we have been flying for quite a long time now and the amount of evidence we have for such events is too less to come to a conclusion. At the moment all I can say is that it is completely safe. $\endgroup$ – Anas Maaz Dec 1 '20 at 2:27
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Yes you can, and at least some five years ago it was the A320 standard procedure.

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    $\begingroup$ I don't disagree with your answer, but could you maybe add some more details (have you personally done this?) or a source (FCOM, etc.)? $\endgroup$ – Bianfable Nov 8 '20 at 16:32
  • $\begingroup$ @Bianfable you're right. I should have. Though I don't have A320 FCOM anymore. I flew the A320 for a year, and we always started both engines with APU. $\endgroup$ – Sami Dec 13 '20 at 11:13

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