B737 Isolation valve AUTO position
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.