2-Isolation valve Switches:
They control the flow of bleed air between the left, right and center systems via the crossover manifold.
Switch IN -The isolation valve is open.
Switch OUT -The isolation valve is closed.
VALVE -The isolation valve is not in the commanded position or is in transit.
Note: The center isolation valve is left OPEN for all normal operations.Left and Right isolation valves are only opened on the ground for engine starts and to provide APU bleed air for pack operation. They are normally closed after both engines are started.