737NG Air cart/normal vs Crossbleed
Hello all,
Reviewing FCOM 1 I found:
Start no.1 first(Obvious reason that ground equipment will obstruct no.2)
Use normal start procedure... which means Isolation valve switch OPEN.
Then; for a crossbleed start it says the isolation valve must be positioned AUTO. Since the pack switches will be OFF, the actual valve should still be open.
Why is the isolation valve to be set to open during a normal and ground start and to AUTO for a crossbleed start?
Just wondering what would make the big difference, it seems to be pointless to move the switch at this stage.
Thanks in advance!!