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737OPR
13th Sep 2007, 20:34
Got a question about the isolation valve in the B737. I asked the question earlier but no response, still curious though.

Recently we switched to BOP. In the old procedure we always put the isolation valve in auto during engine start. According to boeing procedures we now put the isolation valve in open during engine start.

Okay, got no problems with that, in both cases the isolation valve is open anyways because the packs are off so it doesn't matter. But during a cross bleed start the FCOM now says to put the isolation valve in auto after #1 engine has been started(on an external source). and before you start engine #2. I can't figure out why? Anybody knows?

Thanks in advance

Mr.Brown
16th Sep 2007, 06:31
If I remember correctly the isolation valve(on the 737CL) is electrically controlled and if in "AUTO" with ANY of the following switches in the off position the valve will open: L/H Bleed, R/H Bleed, L/H Pack, R/H Pack.
So all I can think of is that it makes sure your packs are off prior to attempting a cross bleed start, however it does only take one pack to be off to open the isolation valve so I can't really see it as the reason. Sorry
:confused:

InSoMnIaC
17th Sep 2007, 16:25
after a ground air assisted start of Eng 1 you must disconnect the ground air source and use bleed air from Eng 1 to start Eng 2 (ie crossbleed start)

if the isolation valve is closed then the Bleed from No 1 eng is isolated from the No 2 eng (ie unable to crossbleed start)

so thats the reason why the isolation valve switch much be placed in Auto to enable bleed from eng 1 to start eng 2.

According to boeing procedures we now put the isolation valve in open during engine start. - that doesn't sound right. It should be switched to AUTO

737OPR
17th Sep 2007, 16:31
Nope, sorry, it does say to put in in open in the NP's, not auto!! That's so strange, switch in open or auto, it doesn't matter, because the valve is open anyways due to the fact that the packs are off for engine start.

Hence my question

repulo
17th Sep 2007, 19:32
Maybe this will help.
After all the problems with the air conditioning contributing to some recent accidents, Boeing changed a lot of procedures (Checking the press mode selector at the preflight...) This is one of the last aircraft where you donīt have an eicas that will warn you if something had been forgotten.
With switching the isolation valve to the open position you will end up in the same config in which you would have been with a normal engine start. (Since the NG can supply both packs on the ground, hence you need the valve in open) So if some start problems happen, the QRH can be applied right away. QRH is always assuming switches to be in the usual position. Technically it does not matter at all.
Hope that helps
repulo