Isolation valve, B737-800

[KristianNorway]

Hello brainiacs

Can anyone here tell me why the isolation valve opens with AUTO selected and an engine bleed air switch set OFF?

Often, in the QRH, you offset this function by selecting isolation valve switch CLOSE before selecting a bleed air switch OFF. It would also connect one bleed air source to two packs in flight, something which is not desired.

[M.82]

Hi,

To prevent a possible aircraft depressurization is my answer.

[aviatorhi]

AUTO does not mean it does what is best, it just means the logic is automated to the following:

ISOLATION VALVE - AUTO ... Closes the isolation valve if all Engine Bleed Air and Air Conditioning Pack Switches are ON. Opens the Isolation Valve automatically if either Engine Bleed Air or Air Conditioning Pack Switch is positioned OFF.

I believe the purpose is to insure that both sides of the manifold are pressurized for startup (and then automatically separated as packs are brought online), that the right side is pressurized for a bleeds off takeoff, etc. etc. Abnormal procedures are... well abnormal. Personally, I much prefer the good old fashions "on/off" switches on the 707/727, nothing convoluted about those.

[framer]

To prevent a possible aircraft depressurization is my answer.

Is there a particular scenario where the logic will do this? ie crew forget to turn one bleed on or similar? I'm struggling to think of when it would prevent a depressurization because the logic works on the switch, not the pack or the bleed itself.( keep in mind though that I've always been a bit slow.)
Cheers, Framer

[KristianNorway]

I can't see that that would be correct either. Do you care to elaborate?

[Schiller]

If, as I assume, the 738 has the same system in this regard as the 732, the reason for the IV opening on selecting either engine bleed air switch OFF is to avoid assymetric wing de-icing. It's true that this will give a single source to both packs, which is undesirable because now neither pack will work efficiently on the reduced flow, but the QRH will correct this. However, you'll still have the sufficient air from the one engine that's now supplying it to maintain pressurisation; it doesn't matter whether that air has passed through two packs or one.

[bungeeng]

Well, I'm not rated yet but I'd say:

Left engine bleed on, left pack off, right pack on, right engine bleed off.
With the Isolation Valve closed, no packs are running.
In auto, the right pack will be supplied by the left engine.

But this seems to be a very artificial case, I agree...