Why is the ISOLATION VAVLE switched to OPEN during ENGINE START on Boeing 737? As the SOP says in Preflight Procedure – First Officer:
&quot;ISOLATION VALVE switch – OPEN&quot;
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?
Position OPEN is guarantee correct work supply air bleed, AUTO is not. For safety.