Bleed air, FCOM, questions off course

[jns278 0]

Hi, I need help/answer for these questions.

 

1) FCOM says(concerning APU vlave&dual bleed) (2.20.02):'there is a possibility of APU bleed air backpressuring the 9th stage modulating and shutoff valve. This would cause the 9th stage valve to close.' ok and? whats the problem when it's closed?

 

2)is it possible to damage something(valve, ducts etc.) on the ngx by pilot missbehaviour? e.g.: when I'm in a dual bleed configuration and set thrust to more than idle will it damage the APU as it is mentioned in the respective FCOM section? I tried it with 100% N1 on ground an nothing happened.. APU + valve were fine...

 

3)PMDG general simulation depth: did PMDG simulate everything?, e.g. in the maintenance manual of the 737 the bleed air schematics are far more complex as those of the FCOM.. there are more bleed trip temp sensors and other components (like precooler) shown that work independently of pilot input. so are all of these valves and sensors physically simulated? is the bleed air temperature really simulated because I don't think there's a need for that since you can't read the actual bleed air temp by any gauge and there are only two failures in the FMC programmable(+ may pilot errors) that cause an overtemperature in the bleed air system. so it could be programmed like this:"If left bleed air overtemp failure is activated, shutoff left eng bleed valve and indicate 'BLEED TRIP OFF'" or is it like: when temp failure is activated via FMC each air atom gets more thermodynamic energy which causes the bleed trip sensors to interpretate the temp as too high and send an electrical signal with 0,1 AMPS in 0,12 sec to the respective computer(or controller as on the MD-11) whose CPU then needs more electrical power to give the comment to shutoff the DC bleed air valve with 0,53 AMPS?

 

the latter seems very visionary to me since if it was like that there should be like 10^10^10 possible failures available in the FMC. (BTW i already found out that bleed air valves can't adopt continuous positions.. when you turn on eng bleed air and let the respective valve fail simultaneously it will be either closed or fully opened, not half or 3/4...=> they are not simualted as real modulating valves(?)!)..

 

would be happy if anyone knows answers to these let's say specific questions!!

thx in advance!! 

[Spin737 102]

1) FCOM says(concerning APU vlave&dual bleed) (2.20.02):'there is a possibility of APU bleed air backpressuring the 9th stage modulating and shutoff valve. This would cause the 9th stage valve to close.' ok and? whats the problem when it's closed?

 

9th stage bleed air is from the high pressure section. If the engine needs to bleed that section for whatever reason - say during takeoff acceleration - and you've blocked that valve, you could cause compressor stalls. Sad airplane. :( Aborted take off :( FOQA Data :( Chat with Chief Pilot :(

 

Don't expect Level D from a PC for a price that's reasonable. This isn't a maintenance trainer.

[jns278 0]

9th stage bleed air is from the high pressure section. If the engine needs to bleed that section for whatever reason - say during takeoff acceleration - and you've blocked that valve, you could cause compressor stalls. Sad airplane. Aborted take off FOQA Data Chat with Chief Pilot

 

Don't expect Level D from a PC for a price that's reasonable. This isn't a maintenance trainer.

OK!, thank you for answering! 

do you know why it could stall? on takeoff the highstage should be closed anyways since 5th stage should be sufficient on take off...

 

PS: I did not expect a level-d, i just wanted to ask how deep they simulated and what is "behind the panel"...that would have been my next question then: why would someone simulate systems even a advanced simmer would never come across... My PC and I are happy that it's not a maintenance trainer (for pilots?!), too.  :wink:

[downscc 1,327]

PMDG used much more information than just "user manuals," thanks to the license they have with Boeing to create a "study simulator."  Your term modulating valve sounds like something from the automotive field. I am pretty certain that the high pressure bleed valve is a solenoid operated shut off valve. A pressure/flow controller in this application just doesn't make sense. Solenoids use a specific amount of current regardless of the flow/pressures in the stream, which is why we use them in refineries to shut down fuel gas to direct fired process heaters.

[Spin737 102]

do you know why it could stall? on takeoff the highstage should be closed anyways since 5th stage should be sufficient on take off...

 

During engine acceleration, there may be some opening of the bleeds to match the flows. Not sure; it's been a while since my Principles of Turbine Engines class.

 

If that's not the case, then at idle, the 9th stage might have too much pressure and need to bleed. If that outlet is blocked, then the LP section might get backed up.

 

Either way, the engine wants to get rid of the extra air sometimes. And that's a job of the bleed.

[jns278 0]

PMDG used much more information than just "user manuals," thanks to the license they have with Boeing to create a "study simulator."  Your term modulating valve sounds like something from the automotive field. I am pretty certain that the high pressure bleed valve is a solenoid operated shut off valve. A pressure/flow controller in this application just doesn't make sense. Solenoids use a specific amount of current regardless of the flow/pressures in the stream, which is why we use them in refineries to shut down fuel gas to direct fired process heaters.

TY for you reply!!! "modulating valve" is  not my term found in in several manuals from boeing... and this valve is referring to beoing "pressure operated" maybe same principle like on a governor. 

During engine acceleration, there may be some opening of the bleeds to match the flows. Not sure; it's been a while since my Principles of Turbine Engines class.

 

If that's not the case, then at idle, the 9th stage might have too much pressure and need to bleed. If that outlet is blocked, then the LP section might get backed up.

 

Either way, the engine wants to get rid of the extra air sometimes. And that's a job of the bleed.

yep thx again... that what I assumed, too.^^ But it's hard to find definite answer on the inet for that

[downscc 1,327]

Okay, I should not have opened mouth before engaging brain.  The FCOM although only a schematic representation does indicate the 9th stage valve is a pressure valve, or a modulating valve, which adds bleed air to the system as required. It is DC operated and the controller also cuts back high temperatures. Believe me, a high temperature high pressure DC operated pressure control valve is a very expensive item. Of course, not much compared to engine costs but considerable engineering goes into an item like this.  I was wrong, not solenoids.

[kevinh 409]

During engine acceleration, there may be some opening of the bleeds to match the flows. Not sure; it's been a while since my Principles of Turbine Engines class.

 

If that's not the case, then at idle, the 9th stage might have too much pressure and need to bleed. If that outlet is blocked, then the LP section might get backed up.

 

Either way, the engine wants to get rid of the extra air sometimes. And that's a job of the bleed.

The CFM56 has variable bleed valves and variable stator vanes to control excess flow and pressure and so prevent surge or stall. The service bleeds to the manifold are separate (though of course they can also help by offloading the compressor). If the engine relied on HP or LP bleed for surge protection you wouldn't be able to do an APU bleed takeoff, which leaves one engine with no service bleed.

The VBV is like a surge bleed valve but it can modulate to provide better control.

[jns278 0]

hi, one more question about the ngx: do you know if there are PACK failures avaiable for the 737? The fms only shows bleed air related programmable failures...? 

[kevinh 409]

Any pack failures available should be visible in the CDU menu. Pack doesn't need to be capitalised btw, it isn't an acronym.

[Spin737 102]

The CFM56 has variable bleed valves and variable stator vanes to control excess flow and pressure and so prevent surge or stall. The service bleeds to the manifold are separate (though of course they can also help by offloading the compressor). If the engine relied on HP or LP bleed for surge protection you wouldn't be able to do an APU bleed takeoff, which leaves one engine with no service bleed.

The VBV is like a surge bleed valve but it can modulate to provide better control.

 

Good to know. Thanks! Sadly, our books are preeeety thin on the topic.

[jns278 0]

Any pack failures available should be visible in the CDU menu. Pack doesn't need to be capitalised btw, it isn't an acronym.

I did not find any pack failures options there...

[kevinh 409]

I did not find any pack failures options there...

...which means there aren't any programmed in. If there's a failure there has to be a means to clear it.

[Copper. 78]

The vague terminology in the FCOM could be masking an underlying reason (known only unto Boeing), however, it is commonly accepted as being an advisory note, highlighting an unintended and possibly an uncertified consequence (at time of design) that could result from that configuration.

 

It is also considered advisory on the basis that, normally:

- the hsv is ordinarily closed under similar circumstances I.e. A higher pressure from 9th stage downstream of the hsv will cause the hsv to close

- there are sufficient protections engineered into the bleed system to detect and prevent damage resulting from overpressure, back pressure, over temperature (to a reasonable extent)

- the air pressure fed to the hsr downstream sense line from the apu bleed wouldn't be dramatically different to the air pressure that would ordinarily be in the sense line as if the pressure were coming from the 9th stage

 

In summary, its likely that the note exists because the unintended consequence of that configuration was not part of the systems original certification or design, and it is largely inconsequential, but noteworthy.

---------

To some of your other questions - I don't see what value you'd get by simulating the above symptoms of that configuration, and I guess PMDG thought the same, which is why you probably can't replicate it, because, even if they did, there'd be negligible cockpit affects assuming that configuration. If you went outside of that configuration, well, that would just be too stupid to consider. The procedures are written not to protect the health of either the engine, or, the apu (which is still an engine), but the procedures are written to protect both.

To your question about how valves are simulated on the ngx - There's a limit to the extent in which you can expect a $60 product to perform... It isn't a real valve... It's isn't a real airplane. So obviously valves aren't going to modulate as they open and close, because they physically aren't, because it's a simulator... To emulate it would, again, be stupid for entertainment purposes.

 

Lastly, the engine performance will not suffer because of the apu bleed closing the hsv.

[kevinh 409]

Okay, I should not have opened mouth before engaging brain.  The FCOM although only a schematic representation does indicate the 9th stage valve is a pressure valve, or a modulating valve, which adds bleed air to the system as required. It is DC operated and the controller also cuts back high temperatures. Believe me, a high temperature high pressure DC operated pressure control valve is a very expensive item. Of course, not much compared to engine costs but considerable engineering goes into an item like this.  I was wrong, not solenoids.

The 9th stage bleed modulating valve is purely pneumatic, no electrical control. The engine bleed valve (which also modulates to limit duct pressure) is DC controlled though. Perhaps you were thinking of that?