B737 Isolation valve AUTO position

[Cakov]

I'm having trouble understanding the reasoning behind the AUTO position of the isolation valve in the B737NG pneumatic system.

I understand what pack and bleed switch positions make the valve open or close, but can't find an explanation of why.

If you were to turn off one PACK switch with the ISOLATION VALVE switch in AUTO, the isolation valve would open and let the two engines bleeds share the load of the remaining pack. Makes sense to me.

However, why does the isolation valve open automatically if you were to turn off one of the ENGINE BLEED switches? This would leave one engine bleed trying to supply air to both packs. The operating manual states that one engine should at no times be used to run both packs.

Hope this makes sense to someone. Thanks.

[LEM]

Hi Cakov, to me also that's a mistery!
As you say, there is no reason for that, and it could even be annoying to have one bleed trying to supply both packs.
We also have to bother turning it to close for a no engine bleed take off, instead of just turning both bleeds off.

We've probably missed something, but what?
:confused:

[Alaskan Timber]

This is because of wing anti-ice.

If you get a problem with one bleed due to bleed trip off or eng. failure, you have to switch off the pack on the affected side.

The wing anti-ice situation though is a bit contradicting. In the bleed trip off c.l. is written 'avoid icing conditions', in the eng. fail./shutdown c.l. not. May be because N-1 you are altitude limited (weather).

[LEM]

Hi Alaskan Timber, we understand the logic with the pack switches.

What we're having trouble with is the logic with the bleed switches, in relation to the isolation valve automatically opening.

Nothing to do with antiice or the cklists you mention :confused:

[Alaskan Timber]

Hey LEM,

This is a precaution. If a bleed trips off while in icing conditions, your wings will still equally be anti(de)-iced. Apparently it is not a big thing to fly with 2 packs on 1 bleed for a couple of minutes.

This is all my own theory. So if I am wrong, I am bracing for impact.

[alexban]

LEM ,as AT said in case of bleed trip off light you'll have to select also affected side pack off- so no 2 packs on only one engine bleed. That's qrh.
The valve will open automatically when you select bleed off-when you select manually the switch to off-it won't trip by itself- so you'll have ram air for systems that require it i.e. hydraulics pressurization,water,etc.
Due to high bleed load it is not recommended to use wing antiice,it can result in the second bleed to trip off --loss of presurization.
It is stated in the qrh that you should avoid iceing conditions for this reason.
Brgds Alex

[Ka8 Flyer]

Not really an expert on the 737, but have you considered the idea that the hydraulic reservoirs need to be pressurized?
On the 767, each pneumatic duct will pressurize its respective hyd reservor - maybe this applies for the 737 aswell? (2 ducts - 2 hyd systems)
Should a hyd reservoir not be pressurized for a longer time, cavitation may occur (especially at high altitudes) which may cause damage to the hyd pumps.

Just a theory...

Regards,

Mark

[Funnel Cloud]

Alaskan Timber,

You mentioned the situation of a bleed trip off.
However, with the ISOLATION VALVE in AUTO position, the valve will only open when one of the BLEED AIR switches is selected off, the manual doesn't mention anything about a trip off.

Just like Cakov, I've been struggling with this question for a long time. I've asked several people on the line, but nobody has given me a reasonable answer so far.

Regards,
FC

[LEM]

Exactly, gentlemen. Some of you are missing the point.

Once again, we are asking about the positioning of the bleed switch to off.

For what reason this should automatically open the isolation valve?
As stated, it could even be annoying!

AT & Alex If a bleed trips off while in icing conditions, your wings will still equally be anti(de)-iced.
Initially, if you don't touch anything, NO!
The isolation valve will remain closed.
It won't automatically open.
How do you force it to open? By turning the pack switch OFF, not the bleed switch.

Again, in the nonnormal situations we are talking of, we have to move the PACK SWITCH, NOT THE BLEED SWITCH, to off!

[Cakov]

Agreed - if you turn off the pack switch, the isolation valve opens. (And this is what the QRH tells us to do for a bleed trip off.)

So we are still no further forward in resolving why Boeing included the automatic opening of the isolation valve if you turn off a bleed switch.

Or am I missing something? (Thoroughly possible, I hasten to add!)

[Alaskan Timber]

Hey LEM, Funnel Cloud and Cakov,

Your remarks regarding the eng. bleed or pack 'switch' postion with isolation valve switch in 'auto' are correct. My memory is back on track.

[alexban]

Ka8 is of course right..
There are systems on a plane,other then air conditioning or anti-ice that require air from the bleeds to properly work i.e. hydraulics or water system should be pressurized to properly function.
If you have a bleed trip off-the valve will remain closed,if in AUTO.
Then you select the pack switch to off-thus causing the valve to open (if in AUTO)
The reason for this is, not to be able to use the wing antiice-this is said to be avoided by the QRH (might cause the other bleed to fail)
The reason is to have pressure in the ram on both 'sides' ,thus being able to pressurize both hyd sys,water sys..
And you won't have at any time two pack's on one source,due to the qrh instructions to close affected pack.
FC,LEM I hope this helps..
Brgds Alex

[Alaskan Timber]

Hey Alexban,

Don't think this is the main reason. In case of a bleed trip off you leave the eng. bleed 'switch' in the on position. Only the affected pack is switched off. This opens the isolation valve aswell (in auto).

Actually there is not one situation (in the QRH) were an eng. bleed 'switch' is placed to off, with the isolation valve switch left in auto.

It is not always possible to avoid icing conditions, in this case wing anti-ice could still be required. Above fl 350 wing anti-ice could also cause a bleed trip off with both bleeds working.

Coming back to the question of Cakov. IMHO the function of the eng. bleed 'switch' position in relation to the isol. valve (in auto) is just a back up function. In case an eng. bleed switch is for what ever reason in the wrong postion during moderate icing, then at least both wings are de-iced. Further you could use this function, on the ground, as a handy tool (several possibilities to control the isol. valve).

Once again this is all theory. Nothing found in the books about this.

[alexban]

AT -my friend,the fact that the bleed switch is left in the ON position after 'bleed trip off'light doesnt's mean that you'll have bleed pressure from that engine.
So bleed trip off ---no pressure on that side of ram. (no matter what you do with the bleed switch)
Next step from QRH ---select pack affected side OFF--valve in auto will open--pressure again for the systems I've told you before.
About the bleed switch relation to the isol.valve ,hmm.
In flight ,as you say ,if by whatever reason you select bleed off-then you'll have two packs on the same source.If you add also anti-ice I bet you'll fail the other bleed also.
So,as LEM said ,annoying.:confused:
On ground,you'll have one pack operating (classic)--valve is already open.(in auto)
Selecting bleeds to off ,indeed will command a 'special'high flow on the operating pack,but has nothing to do with the valve (it's already open)

[Alaskan Timber]

Hey Alexban,

You edited your post after my latest reply. I am 100% sure you wrote initially that after a bleed trip off you select the 'bleed' switch to off. You changed that to 'pack' switch. My reply was partly referring to that. Makes me look a bit like a fool don't you think so ? Editing is ment for grammar corrections, not to change the contents of your post.

Were did I write that after a eng. bleed trip off light with the bleed switch on, bleed pressure is still available from that engine?

Referring to my theory about the bleed switch position as a back up (isol. valve sw. in auto). Indeed it is just a theory. For the rest
I am fully aware again (as I already wrote) about the isol. valve position with the switch in auto in relation to eng. bleed switch or pack switch postion.

[alexban]

AT,indeed you are right.After reading your answer I've realised,not only that I've written 'bleed' instead of 'pack' ,but also that I missread the question initially asked by cavok.So,i've edited my previous answer, although I was tempted to delete it,but you've already answered it.And I usually try to write correct stuff,even if I'm tired,so I had to correct my answer.
Regarding the original question I'll try to dig further maybe I'll find some answer.

[Alaskan Timber]

Hey Alexban,

No problem and thanks for explanation. I am digging aswell, no solid answer found yet. Maybe we better call mr. Boeing.

Good luck.

AT

[Terraplaneblues]

Assume that airplane was configured with wing TAI "on" at the time that another problem caused the crew to switch "off" one of the engine bleeds.

At no time during the QRH actions to resolve the bleed fault would the airplane be asymmetrically wing anti-iced, i.e. one problem doesn't give you a much bigger problem.

I think this may be the reason for the auto function of the isolation valve being associated with engine bleed switch position.

[Zenj]

Alaskan Timber

You wrote this
==
Above fl 350 wing anti-ice could also cause a bleed trip off with both bleeds working.
==

I have seen this info elsewhere twice and it was also stated that you can even lose pressurization of the aircraft , when this happens, now where is this source stated officially by Boeing ?

Zenj

[Alaskan Timber]

Hey Zenj,

FCOM (AOM) volume 1 suppl. procedures page 16.9.

rgds,

AT

[Zenj]

AT

Thanks

Zenj

[junior_man]

I think the "why" you are looking for is this:
The 737 was designed a long time ago. To get rid of the FE some things had to be "automated". When you get a bleed or pack trip, you do not have to change any switch positions. If you go up and start moving switches, then they are assuming you are intending to do something, like wing anti ice.
Next Gen is designed to be flown as a common fleet with the old ones. That is why some of the things that could have been improved were not.

BTW there is a 727 hiding inside most Boeings. Just automated in some.

[Zenj]

Alaskan Timber

I do not see this info at all in FCOM Vol 1 SP16

Zenj

======

Hey Zenj,

FCOM (AOM) volume 1 suppl. procedures page 16.9.

rgds,

AT

[Popolama]

==
Above fl 350 wing anti-ice could also cause a bleed trip off with both bleeds working.
==

I have seen this info elsewhere twice and it was also stated that you can even lose pressurization of the aircraft , when this happens, now where is this source stated officially by Boeing ?

Zenj

QRH BLEED TRIP OFF June 6,2003
"CAUTION : Use of wing anti-ice above approx FL350 may cause bleed trip off and possible loss of cabin pressure.

hope that helps...

[Alaskan Timber]

Hej Zenj,

Both my FCOM (AOM) vol. 1 for the 700 and 500 mention this on page SP 16.9. Also in the QRH as mentioned by Popolama.

A.T.

[Zenj]

Popolama and AT

It's not that I disagree with you about this but I need it mentioned in the Boeing documents so that I can show to my other fellow crewman.

I have the QRH for B737-200 from old Air Tanzania on NON-NORMAL CHECKLIST section page NNC2.3 BLEED TRIP OFF dated 5th October 2001

and I have

QRH from B737-200 from new Air Tanzania ( same with SAA) same checklist but with date of April 4th 2003

Both they do not mention the FL350 and icing with bleed tripping

Same with FCOM page 16.9

If possible please scan it and send it to my e-mail

Thanks

Zenj