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737: 1 Pack + 2 Bleeds. Why Not?  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (2 years 1 month 4 weeks 21 hours ago) and read 3616 times:

What is the reason not being able to run one pack from 2 x engine bleeds. Is it that one pack cannot simply cope with the amount of bleed air coming from 2 sources of bleed?

Also, why is it that there is a lot more backup for the air conditioning of the CONT CAB/cockpit ie bigger source of air and temp control. What was Boeings thinking behind this, pilots up the front are flying the plane so we will make sure they always have suitable hair.

Of course no problem with it but just curios, why such emphasis on air going to the cockpit.

Is it the same for newer Boeings?

9 replies: All unread, jump to last
 
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 1, posted (2 years 1 month 4 weeks 20 hours ago) and read 3599 times:

Quoting smartt1982 (Thread starter):
why such emphasis on air going to the cockpit

I suspect you nailed it with this:

Quoting smartt1982 (Thread starter):
pilots up the front are flying the plane

On the other hand...

Quoting smartt1982 (Thread starter):
so we will make sure they always have suitable hair

In my experience they haven't been entirely successful on that front.


User currently offlinetb727 From United States of America, joined Jun 2005, 1586 posts, RR: 9
Reply 2, posted (2 years 1 month 4 weeks 20 hours ago) and read 3593 times:

Quoting smartt1982 (Thread starter):
What is the reason not being able to run one pack from 2 x engine bleeds. Is it that one pack cannot simply cope with the amount of bleed air coming from 2 sources of bleed?

You'll probably fry the pack, it's just too much air. You also can't run 2 packs off 1 engine bleed, at least on the 727, not enough air in that case. You can run both packs off the APU bleed though.



Too lazy to work, too scared to steal!
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 3, posted (2 years 1 month 4 weeks 19 hours ago) and read 3557 times:

Quoting smartt1982 (Thread starter):
What is the reason not being able to run one pack from 2 x engine bleeds. Is it that one pack cannot simply cope with the amount of bleed air coming from 2 sources of bleed?

In addition to the capacity problem, the bleeds will never be at exactly the same pressure so one will always overpower the other if they're ganged together.

Quoting smartt1982 (Thread starter):
Also, why is it that there is a lot more backup for the air conditioning of the CONT CAB/cockpit ie bigger source of air and temp control.

You can't meet the smoke containment or smoke clearing regulations if you feed the flight deck with the same supply as the cabin. You need the flight deck to have cleaner (generally no recirculation), high volume (lots of flow), and higher pressure (make sure flows go out of the flight deck, not in) air.

Also, the cooling for the flight electronics is supplied by flight deck air drawn through the panels by cooling fans. You need to send enough extra air to the flight deck to meet the cooling demand plus all the other needs.

Quoting smartt1982 (Thread starter):
Is it the same for newer Boeings?

Basically, yes, although the 787 doesn't have a bleed system so it doesn't have the same pack limitations.

Tom.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
Reply 4, posted (2 years 1 month 4 weeks 15 hours ago) and read 3465 times:

Quoting David L (Reply 1):
Quoting smartt1982 (Thread starter):
so we will make sure they always have suitable hair

In my experience they haven't been entirely successful on that front.

It is a problem if you have to wear a hat.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineTristarsteve From Sweden, joined Nov 2005, 3998 posts, RR: 34
Reply 5, posted (2 years 1 month 4 weeks 6 hours ago) and read 3327 times:

Quoting tdscanuck (Reply 3):
In addition to the capacity problem, the bleeds will never be at exactly the same pressure so one will always overpower the other if they're ganged together.

The reason for not mixing bleed sources is that Boeing are considering the failure of the bleed NRV in the engine. If the other bleed source is higher pressure then this bleed air can be fed into the engine.
If all the NRVs are working normally, there is no problem mixing bleeds. (but you have no way of knowing)


User currently onlineyeelep From United States of America, joined Apr 2011, 653 posts, RR: 0
Reply 6, posted (2 years 1 month 3 weeks 6 days 21 hours ago) and read 3158 times:

Steve, I agree with your reasoning. However I am not familiar with the acronym NRV. Are you referring to the 5th stage check valves? I'm guessing NRV stands for something like non reversion valve.

Quoting tb727 (Reply 2):
You'll probably fry the pack, it's just too much air.

You won't destroy the pack using dual engine bleed source. Each engines bleed pressure is controlled through the pressure regulator and shutoff valve (PRSOV) to limit output pressure to around 42 PSI. The volume of air is limited by the pressure and the diameter of duct leading to the pack.


User currently offline737tdi From United States of America, joined Sep 2007, 810 posts, RR: 1
Reply 7, posted (2 years 1 month 3 weeks 6 days 8 hours ago) and read 3013 times:
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Something y'all may not realize???? There have been some very catastrophic engine failures due to dual bleed malfunctions (737NG). Our engine run checklists were changed and all of us run/taxi designees were told to stress the importance of the dual bleed light on engine run/taxi operations.

I.E.: While taxing at higher then idle throttle, or engine high power runs a failure of the bleed air system on one engine when the bleed air switch/engine bleed switches are improperly selected will cause a melt down (my words) of the opposite engine. I have pics. but I can't show them. Turbines completely fried, melted, fubar.

By the way, this only happens in ground ops.. It won't happen at all if you follow the checklist!!!!!!!!!!. Just a little nugget of info..

737tdi


User currently offlinetb727 From United States of America, joined Jun 2005, 1586 posts, RR: 9
Reply 8, posted (2 years 1 month 3 weeks 5 days 18 hours ago) and read 2890 times:

Quoting yeelep (Reply 6):
You won't destroy the pack using dual engine bleed source. Each engines bleed pressure is controlled through the pressure regulator and shutoff valve (PRSOV) to limit output pressure to around 42 PSI. The volume of air is limited by the pressure and the diameter of duct leading to the pack.

Ahh yes, that's true, I was thinking of a different problem I had with a 13th stage bleed valve.



Too lazy to work, too scared to steal!
User currently offlineHAWK21M From India, joined Jan 2001, 31679 posts, RR: 56
Reply 9, posted (2 years 1 month 3 weeks 5 days 7 hours ago) and read 2797 times:

Quoting yeelep (Reply 6):
I am not familiar with the acronym NRV

Non return valve or Check valve.....allows flow in one direction only.



Think of the brighter side!
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