Boeing BOP
 

Got a question about the isolation valve.

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. 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. For the live of me I can't figure out why? Anybody got a good answer on that one?

Thanks in advance