Engine Bleed Air Switches

[Xiropillo 13]

Good evening,

 

One question please.

 

During start up of engines in 737, the FCOM v1, page 216 says that Engine Bleed Air Switches must be selected to On but if I select On position my duct press is near 0 and no pressure for starting engine.

 

Could you please tell if that is wrong, manual is wrong or if I am doing something wrong.

 

I set APU bleed on, packs Off, recirc fans auto and isolation valve open.

 

Getting crazy about this issue.

 

Thanks a lot and nice flights.

 

Xiropillo

[downscc 1,327]

APU running?  I usually start with L Pack on and R off but both off is alright.

[Xiropillo 13]

Yes, I tried with APU or Ground Air. With both packs off and still psi under 20

[jmp308 1]

I've noticed that I have to have L off, R on to get to start pressure;  w APU bleed on; crossbleed auto or open.

 

Joe

[Xiropillo 13]

Me too. And that is wrong because tutorial and FCOM says both to be Off. Nobody reported this before?

[Doug Pelton 1]

A real-time 737 pilot, who instructs now (as I understand it)...has this video on how his company does start up from C&D. Thought it useful.

 

 

Doug Pelton

[Budbud 227]

Me too. And that is wrong because tutorial and FCOM says both to be Off. Nobody reported this before?

 

I always started the engines with both packs off with no issues for years. But now it's been a couple of month since I have flown the NGX, so I'm not sure anymore of the reading of the pressure.

[Xiropillo 13]

Thanks for your answers. I was blind looking at pressure gauge and didn't realized that engines do start normal.

 

But my question now is. Is that normal that pressure is so low and engine starts without problems?

 

I mean, I have been testing cross bleed engine start and always reading about minimum pressure so as it was too low I found something going wrong.

 

Now I have mixed things and would like to know how real PMDG 737 is about this systems on both procedures. Normal and without APU starting.

 

Thanks again

 

Xiropillo

[Xiropillo 13]

A real-time 737 pilot, who instructs now (as I understand it)...has this video on how his company does start up from C&D. Thought it useful.

 

 

Doug Pelton

 

Great videos! Thanks for that. It helps a lot.

 

Xiropillo

[JoeDiamond 67]

It's a demand system.  With the packs off there is no demand for bleed air so the pressure regulating bleed shutoff valve for the APU isn't open, so no duct pressure.  When you select ground on the engine start switch there is a demand for air and the valve opens.  That is why there is no reference to the duct pressure for a normal start using the APU.

 

During a cross bleed start you need to check the duct pressure prior to start to verify that your source is providing enough air for the start.

[wisefox7524 9]

When the engine start switch is selected to GND, the start valve opens and the bleed air valve for the associated engine closes. The physical engine bleed air switch on the overhead doesn't have any bearing on this.

[Xiropillo 13]

Thanks very much.

 

I thought that due to bleed air needed, the duct pressure should indicate high pressure.

 

Maybe is a difficult technical explanation...

 

I said, if for second engine during crossbleed is necessary to increase power, I will also need a minimun pressure indication for the first one with APU starting.

 

I don't know if I explain myself. Sorry ; )

[JoeDiamond 67]

 

 

I will also need a minimun pressure indication for the first one with APU starting.

 

No you don't.  Please see my earlier post.

[Xiropillo 13]

No you don't.  Please see my earlier post.

 

Yes, understood, I was telling you what I was thinking before your explanation. Thanks again!

[SirBismuth 9]

Yeah, was also wondering that although my duct pressure was low (<30psi), my engines still started.  Since then, I had actually forgotten about it (until reading this thread), and merely glance at the guage during engine start, and focus on N2 etc.  

 

The posts in this thread have explained why the engines still start, although duct pressure is low.  In my FS9 days I flew the 135 from Feelthere, which I presume has a different system that isn't automated, as the engines would NOT start if the engine bleeds were set to open in the cockpit.