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As the SOP says in Preflight Procedure – First Officer:

"ISOLATION VALVE switch – OPEN"

In this way, when engine starts, the ISOLATION VALVE is OPEN not AUTO.

The PACK valve is switched to OFF, under AUTO can also make the ISOLATION VALVE open.

SO why is the ISOLATION VAVLE switched to OPEN instead of AUTO?

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  • $\begingroup$ What SOP are you looking at? If we're talking PREFLIGHT rather than Before Start, it could be so the APU bleed can provide air to both packs between the time the APU is running & when it's time to start engines. $\endgroup$
    – Ralph J ♦
    Jul 26, 2020 at 12:21

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Position OPEN is guarantee correct work supply air bleed, AUTO is not. For safety.

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  • $\begingroup$ That isn't really correct. Open is open, Closed is closed, assuming that the valves are working; Auto drives the valves according to logic applied to the other switch positions, but (again, assuming that the valves work), it's fully as reliable as Open or Closed. Normally, you want that logic working for you; if you don't then use a different positionto accomplish what's desired. $\endgroup$
    – Ralph J ♦
    Aug 1, 2020 at 1:38

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