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Boeing 737NG crossbleed engine start

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Boeing 737NG crossbleed engine start

Old 5th Dec 2011, 19:50
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Boeing 737NG crossbleed engine start

Hey folks,
After starting engine #1 from external power (GPU and ASU) there is no enough bleed air at the isolation system for starting engine #2
(something around 15 PSI).
So, according the pocedures I need to raise the engine #1 thrust for rasing the duct air for allowing to start the engine #2.
and my question is: Why there is no enough bleed air at the isolation system after engine #1 starts, why the pressure reduced only because the engine start ?
a schematic chart will be very helpful too.
Thank you very much,
Avi.
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Old 5th Dec 2011, 21:01
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What? I am not sure if I understand your question, but if you are asking why the pressure is too low: The engines deliver less bleed pressure than the APU, so after engine one is started using a huffer the external air source is disconnected and the pressure in the duct will drop to the 15-20 psi that the engine delivers. That is insufficient for a normal engine start, thats why you need a bit of thrust on engine one to start number two... Is that what you meant?
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Old 5th Dec 2011, 23:51
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Sounds like a flight simulator issue.
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Old 6th Dec 2011, 04:56
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STBYRUD,
First of all thank you very much for your reply.
I think that you didn't understand me because my English and I'm sorry about that.
Well, I know that the pressure will drop to 15-20 PSI duct air in the isolation system, the question is why ?
Why after starting engine #1 the PSI drops and why do I need to disconnect the ASU external source ?
So, if I understand you, the ASU disconnected automatically when engine #1 starts and because the engine is on idle thrust, the engine needs more bleed air and that's why I need to raise the thrust - for providing more bleed air.
More thrust - more bleed air.
am I right ?
But why the ASU automatically disconnected ?
Again,
Thank you very much,
I'm really appreciate your help !
P.S: it is a flight simulator issue, but the aircraft's behavior is the same like the reality.
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Old 6th Dec 2011, 05:27
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But why the ASU automatically disconnected ?
Because that's probably the way PMDG(?) programmed it. In reality, the ground staff/engineers would disconnect the external air supply with their gloved hands. The aircraft would then be pushed back from the gate and the second engine started (with the running engine at higher-than-idle rpms).

it is a flight simulator issue, but the aircraft's behavior is the same like the reality.
So you're saying a $100 simulator program is 100% like the real aircraft? I don't think even PMDG would claim that
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Old 6th Dec 2011, 05:29
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but the aircraft's behavior is the same like the reality.
No it's as close to reality as the person who programmed it can get it. I bet it doesn't simulate generator frequency "wobble" either. If you want to get close to reality in a simulator try a Level D sim manufactured after 1995 or so, and even that's quite a bit different than the plane.
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Old 6th Dec 2011, 05:45
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So having dived into the books to maybe learn something new and offer my perspective about why systems do certain things I find it pretty frustrating to find out behaviour has actually been observed on a $100 desktop sim. It's pretty disingenuous if you ask me.
There are plenty of forums for sim pilots and many are frequented by real word guys too.
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Old 6th Dec 2011, 06:23
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The Boeing 737 NGX PMDG is very realistic, specially his behavior, and now you'll ask me how can I say it if I even don't fly a real aircraft, that's why I'm reading the menuals and trying actions according it.
For sure it's not 100% same but I would say it is something like 90% (without considering the G effects :P)
I tought that the ASU automatically disconnected when engine #1 starts and I was wrong.
So, why the ASU doesn't provide more bleed air after engine starts ?
I feel that I don't belongs here, but I'm asking you because I want a professional answer which I can find here for sure.
I can tell you that the most of the heavy aircraft's simulator pilot (FSX, FS2004) don't know how to fly a full IFR flight with Cessna without using the GPS and that's why I'm asking my questions here.
If you think that I don't belong here because I'm a SIMULATOR pilot and maybe only 15, I'll accept it.
I'll be glad for help anyway,
Thank you very much,
Avi.
P.S. aviatorhi, if you mean to the CPS frenquancy of the AC source - it simulated.
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Old 6th Dec 2011, 06:44
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Look kid, it's a computer game, call/email the people who made it. We can't help you here because your game doesn't behave like a real aircraft, if it did you wouldn't be having these issues.

If you want to learn the real aircraft systems I'll suggest you buy yourself an FCTM and FCOM or some systems manuals, they're easily obtainable on ebay and go for $20. Reality is that you won't find any aircraft simulator (even $100,000+ FTDs) which behave exactly like a real aircraft.

PS. I mean variance in frequency output based on engine speed, even a CSD has a delay in response.
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Old 6th Dec 2011, 07:03
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If I was not telling you that it is a simulator issue you maybe didn't reply like that.
I came here to ask one question about a REAL procedure without considering it's a simulator.
Maybe you don't know the abilities of the simulator today and that's why you call it "game", you can absolutely do a classic IFR flight with a GA aircraft all over the world.
You can do a lot of the real procedures and I'll give you some details:
SID, STAR, Holdings, prescion and non-prescion approach, DME arc and much more.
P.S. The NG's engines have an IDG and not a CSD (I think at the classic have).
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Old 6th Dec 2011, 13:34
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Ignoring all of the above so that we can move on...

Well, the huffer/ASU/Apu on wheels is connected to the right side of the airplane very near engine 2 - that is the reason why it has to be removed before starting engine 2 and why engine 1 is started first. Clear?
1) ASU delivers relatively high pressure air to the right side of the bleed system
2) ASU is disconncted once engine one is running, pressure drops to the normal ground idle duct pressure
3) Engine one is throttled up to give sufficient pressure to start engine two without the help of the ASU.
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Old 6th Dec 2011, 13:46
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STBYRUD,
Thank you very much for understanding me - really !
I got the answer !
Am I welcome to ask more questions when I need to ?
You are the best !
Thanks again,
Avi.
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Old 6th Dec 2011, 14:00
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Might it better if people playing on sims put their techie questions in the spotters area, im sure there are plenty of pilots and engineers who read those forums who would help out
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Old 6th Dec 2011, 14:16
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1) Thrust has to be increased because engine bleed output pressure at IDLE power is small(as you noted below 30 PSI).

2) ASU(or ground pneumatic cart) has to be disconnected(by ground crew) before engine no.2 start for safety of the GROUND CREW. In 737 ground crew's interphone jack is located on the right side of the airplane(in front of the engine no.2).

Hope this helps
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Old 6th Dec 2011, 14:21
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VLADKO,
Thanks for your help too !
According to you answer: More thrust - more bleed air at the isolation system, am I right ?
Thanks again !
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Old 6th Dec 2011, 14:53
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Not isolation system but {bleed air duct} to be exact!

Isolation VALVE closes/opens, that way it separates(when closed) and connects(when open) left and right sides of bleed duct. In case of crossbleed start, it is open to allow bleed air flow from left side to right side.
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Old 6th Dec 2011, 15:12
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VLADKO,
roger that mate !
it is open to allow bleed air flow from left side to right side
also from right side to left side in case of starting right engine, right ?
Thank you !
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Old 6th Dec 2011, 15:33
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Well in Normal start, you'd begin with engine no.2 start, and you'd use APU bleed air for that. You'd also use APU bleed air to start no.1 engine after no.2 is started.

But yes Isolation valve would be open.However it has its own logic for opening/closing while switch selector is in AUTO.
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Old 6th Dec 2011, 21:49
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Avis1,
I think that many pilots will be happy to answer your questions if they know from the start that it is a gamer asking the question. Others will simply ignore it.
So, in order to tap into the pilots that are willing to help you, make the title of your post "Flight Simulator Question" or something similar. That way pilots who don't want to be involved in that conversation don't spend their time reading your post and then feel peeved when they realise that it's not aircraft related.
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Old 7th Dec 2011, 05:14
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Avis !
The Boeing FCOM's that come with the PMDG NGX have all the info you need. This forum can be helpfull if you have something to ask that is not really explained in the FCOM's.

Just ignore the ignorant guys on here.
I will mention no names but you know who I am referring to.

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