B737 Pneumatics

[BOAC]

Originally Posted by Allerbeste

But what is the reason why we may not select the bleed to "OFF" before engine start?

- if you are still around? We are struggling with your question. Could you answer post 3?

[Echo_Kilo]

Hi everybody,

dont know if it had been mentioned already; but they ENG Bleed Valve cloeses once the start switch is positioned in to GRD. Afterwards it opens again. So there is no need to close it manually. Probably a good thing to let them in ON. It dimishes on more ocassion, where we could set the AIR COND Panel inproperly.


Cheerio,

[IFixPlanes]

Originally Posted by BOAC

- if you are still around? We are struggling with your question. Could you answer post 3?

Only one posting (16th May 2010) and the last activity was on the 15th Oct 2011 16:47.
Forget it...

[de facto]

Ok i will tell you why you need the BLEED switch ON eventhough you have not replied to my post #3

When the bleeds switches are ON,it energizes relays in the ACAU(air con Accessory unit) to PROTECT the Bleed air system during engine start opearations such as over pressure in the bleed air interstage manifold and over temperature.
You have these protections when the switches are where they are supposed to be,in the ON position.
If you need to do a bleeds OFF take off,configure following the SUPP Procedures.

Safe flight.

[BOAC]

Agreed Ifix - I need to look more closely.. I did not realise Framer had been grave-robbing.

[Yeelep]

de facto,

The bleed/overheat relays in the ACAU are active regardless of engine bleed switch position. They don't provide any protection to the bleed system during starting.

[de facto]

Yes they do,so says my AMM.

[TURIN]

Its been a long time since I worked on the 737-200. Something tells my overused memory that the physical arrangement of the pneumatic components was such that the bleed valve had to be open to allow air to the start valve. When the -3/4/500 was launched, even though the engine pneumatic arrangement is different the SOPS were left the same for starting the engines just to make transition from old a/c to new simpler.
But I could be wrong.
Also, I'm pretty sure the CFM56 has surge bleed valves aft of the booster stage, automatically controlled of course.

[de facto]

Also, I'm pretty sure the CFM56 has surge bleed valves aft of the booster stage, automatically controlled of course.

Correct,variable bleed valvess ,variable stator vanes and transiant bleed valves commanded by the EEC via the HMU.
The valves are mechanically operated via actuators.

[IFixPlanes]

Originally Posted by de facto

...commanded by the EEC via the HMU.

Correct for the CFM56-7 (on B737NG)
...but on the CFM56-3 (B737CL) it is controlled by the MEC (main engine control)

[de facto]

I have heart for the NG only

such that the bleed valve had to be open to allow air to the start valve

Not sure what you mean,,,during start,air from whatever source you are using(APU,ground cart) is directed directly to the starter motor(some small turbine kind of motor) via the start valve.
The motor is then turning the n2 via the gear box,mechanically.
After cut out,the engine is self sufficient,lpt drives lpc etc....

[Denti]

Well, if you have a heart for the NG only you shouldn't quote a post about the Jurassic. Systems change between generations, sometimes quite a lot.

[de facto]

Well, if you have a heart for the NG only you shouldn't quote a post about the Jurassic

I will make sure to get my future questions/quotes via your personnal filter next time.

[TURIN]

Not sure what you mean,,,during start,air from whatever source you are using(APU,ground cart) is directed directly to the starter motor(some small turbine kind of motor) via the start valve.

If you can imagine the air has to get from the source (APU,Ground Cart, Cross bleed) to the engine. I think the JT8 has the bleed valve down stream from the start valve duct junction. So for the air to get to the start valve it has to pass through the bleed valve first. I think. This was why it was important to check the start valve had closed otherwise the engine, when on speed, could continue to drive it's own starter moter even if the bleed valve was closed.

As I say, I'm relying on memory here as I don't have the schematics handy.

[de facto]

The high stage valve is closed during engine start.(737NG).

[Yeelep]

de facto,

There is no extra protection with the bleed switches on. The protection circuit is powered directly through the dc bat. bus. Additionally the overheat/overpressure switches command the PRSOV closed, which is already closed during starting, hence no start protection.

I am curious about where in the AMM it says differently, I don't mind being proved wrong.

Edit: I see where you read it, AMM Chapter 1 36-11-00 pg.27. Its referring to the ACAU closing the PRSOV through the start relay, that's the start protection. But, with the bleed switch off, the PRSOV is already closed.

[sheppey]

c) After Takeoff checklist.

The After Take-off checklist makes no specific mention of checking the pressurisation instruments - only to set or verify that the engine bleeds and air conditioning packs are operating.

It could be argued that is a major flaw in the Boeing published checklist since numerous observations in the simulator often reveal the crew do not specifically check the pressurisation instruments as well as switch positions. - although the actual checklist items are self challenged and responded to by the PM. If the Helios accident crew had bothered to look at the pressurisation instruments during the initial climb after the flaps were up it is likely they would have twigged something was wrong with the pressurisation early in the peace and taken appropriate action. More often than not, simulator observations reveal the PF is too slack or busy on other things to verify that verbal checklist actions of the PM have indeed been done correctly.

Also there is often a tendency for the crew to concentrate only on what the published checklist states and not use their commonsense to keep an eye on allied items appropriate to the systems the check list is written for.

The manufacturer designs a checklist system with the understandable assumption that competent trained crews will use it. As many accidents have proved, not all crews are competent.

[de facto]

Edit: I see where you read it, AMM Chapter 1 36-11-00 pg.27. Its referring to the ACAU closing the PRSOV through the start relay, that's the start protection. But, with the bleed switch off, the PRSOV is already closed.

Understood,thanks

[Denti]

The manufacturer designs a checklist system with the understandable assumption that competent trained crews will use it.

Sadly the main aims with those manufacturer designed checklists from boeing has been shifted to quite different targets in the last 10 to 15 years. Main target is now to produce a fleet commonality where no such thing exists (at least not if compared to airbus), the second main thing is that they have to be checked or written by lawyers to keep free of possible court cases.

For example boeing has already an EASA approval (and i guess FAA as well) for a reduced type rating course for the 787 if coming from a 737, 757 or 767. The next step they work currently on is a CCQ to allow mixed fleet flying between those original types and 787. My company plans to do that and works with boeing to develop those standards, same as we had already EASA approval to fly 737 and 757 as well as 737 and 767 in a mixed fleet. Mixed fleet in this case means that one enjoys the same rules as for example A320/A330 mixed fleet flying, no dual sim-checks, switching between types as easily as between variants and so on.

Apparently (please correct me if i'm wrong) other boeing aircraft have different pressurization setups and it is quite enough to check the bleed valves, which lead to this checklist design.

[TURIN]

The high stage valve is closed during engine start.(737NG).

As I say, my reference is to the original arrangement with the -100/200 PW-JT8 powered 737.

There's a very old sketch here from a website discussing the 727 about 3/4 down the page. The bleed valve is not shown but clearly it would have to be open for engine start to allow air flow to the starter.

Hope this clarifies my point.