B737 Pneumatics
 

For some reason the Engine Bleed switches of the B737 should be in the "ON" position before engine start. I see no specific reason for this. I know that the valves are only open if there is engine bleed pressure, and anyway the valves are closing when the starter switch is in "GROUND", going to the selected position after starter cut-out. But what is the reason why we may not select the bleed to "OFF" before engine start? :confused:

Could be as simple as having the bleeds setup for flight...nice to have pressurization .

Allerbeste,

May i reverse your question...why wouldn't you want the bleeds selected ON?

Having the Bleeds ON is their STANDARD position, switching them OFF would require you to go through the SUPPLEMENTARY procedure.

Good to know your systems,but make your life easier and follow standard ops:ok:

Whatever any technical reason, one word - Helios. Something as simple as a bleed switch, pressurisation mode selector etc can be the first card in a deck to cause a disaster. To this day there are NO warnings of an incorrectly configured bleed/press panel on the 737.
Our airline requires a safety report to be filed and subsequent investigation if we ever arrive at an airplane and find bleeds off or PMS set wrongly.

ALLERBESTE,

This is to prolong the life of the solenoid. It is actually de-energized on the ON position, not the other way around.

If this is the main thing your airline learned after the Helios accident, they did not understand what happened there.

Quote out of the final Report:
Direct Causes
1. Non-recognition that the cabin pressurization mode selector was in the MAN (manual) position during the performance of the:
a) Preflight procedure;
b) Before Start checklist; and
c) After Takeoff checklist.
...

Contributing Factors to the Accident
1. Omission of returning the pressurization mode selector to AUTO after unscheduled maintenance on the aircraft.
...

/Quote
At the time the accident happened the AMM did not demand to put the pressure mode selector to the position AUTO after the the leakage test.

capt solopsist correct
 

The captain S is correct. Very simple reason.
The fail safe for bleed air is open hence the solenoid powers the engine bleed off.
Same for many hyd systems.Fail safe to hyd on, powered to off position.

Having the bleed valve open removes some compressed air from the 5th and 9th stage compressor, before that air enters the combustion chamber. This enrichens the mixture, acts as a surge control and helps the start.

Do the engine bleed air switch positions control the bleed valves associated with surge control, starting, low/high thrust conditions? Or, are they purely connected to the pneumatic system in terms of supply on/off?

Maybe no aural warning like configuration or other bells and whistles. But there are two qualified Being 737 pilots on the flight deck. Plus two gauges that show cabin rate of climb and cabin differential and cabin altitude. Plus a checklist read as challenge and response. Plus airmanship. How many other warnings do you want if the problem cannot be seen by a competent crew. The operative word being "Competent".

Ifixplanes - We had this policy even before Helios. There are of course many other points to notice and rectify the problem, but I think there is no harm in having good processes in place to ensure things like this don't get left in those positions to start with i.e. the best method of detecting the problem is to remove it in the first place! Obviously other things have changed since thing including the takeoff config/cabin alt warning lights as well as a mentioning it in the brief on first flight of the day. Its amazing that the AMM didn't require the return to the auto position - didn't know that until now (not that any of it excuses the flight crew from checking it)

There is no surge control valve in the CFM56. I remember one fitted to the TFE731 (found on many business jets), I think. The bleed air valve on the CFM simply provides air to the packs for air conditioning, and for cross-bleed starting.

Now the AMM stated at the end of the check:

"Move the pressurization mode selector on the forward overhead panel to AUTO."

Lesson learned ...

checkboard
 

What you say is probabaly right from an operational point of view (not a driver here) but the bleed s/w's are on 99% of the time so they dont overheat/ wear or burn out while being powered off all the time.
When you climb into the front seat almost without fail you'd find the bleeds on whether your procedure requires them on or off for starting.

An engine start sense relay located in the M324 air conditioning accessory unit for the left
engine and in the M1455 air conditioning accessory unit for the right engine is energized when
the respective engine start switch is positioned to GRD. This relay sends a closing signal to the
latching solenoid on the bleed air regulator. This eliminates the control pressure to the PRSOV
pneumatic actuator and ensures the valve remains closed during starting.

Quote: From IFix Planes
Originally Posted by ballyboley
... Its amazing that the AMM didn't require the return to the auto position - didn't know that until now (not that any of it excuses the flight crew ...

Now the AMM stated at the end of the check:

"Move the pressurization mode selector on the forward overhead panel to AUTO."

Lesson learned ...

The AMM wasn't SPECIFIC. It did however have the statement "return the airplane to the usual condition" or words to that effect.

No, the wording was “Put the airplane back to its initial condition” and this was only the headline.
Then the AMM list 3 steps which have to be done. The selector was not mentioned.

Regarding Original Question
 

The start relay energizes when the engine start valve opens.
The energized relay contacts send a signal to close the bleed
air regulator solenoid valve.

i.e. Bleed is closed automatically during start, restores to selected position after start.

The open and close coils on the bleed air regulator solenoid
valve are constant duty coils. The solenoid valve also has a
mechanical latch. When there is a loss of electric power, the
solenoid valve will stay in the last electrically commanded
position.

The Engine Hyd Pumps are enegized OFF. The bleed system is not the same it's a twin coil latched type.

That last post has thrown me. To clarify,
When the bleed switch is in the OFF position, is it be energised to OFF or is it being held by a mechanical latch?

When the switch is selected OFF, the close coil of the solenoid is continuously powered. If power is lost the mechanical latch will keep it closed. The same applies when the switch is selected ON. As Terraplaneblues said, its a twin coil latch solenoid.

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

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,

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

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

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.

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

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.

Yes they do,so says my AMM.

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.

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.

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

I have heart for the NG only:E
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....

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.

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

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. :ok:

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

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.

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.

Understood,thanks:ok:

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.

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. :ok: