The isolation valve separates the left, from the right side of the bleed manifold. It is powered from AC Transfer Bus 1 but can also be manually opened/closed by a control lever, accessible in the left air condition bay. Because it’s AC power* it will fail in the selected position when power is removed. When the Isolation switch is in AUTO, valve opening relies on the so-called “corner switch” positions. They are the Pack and Bleed switches, when neither switch is in the OFF position, the isolation valve is closed with its switch on AUTO.
On
the other hand if any corner switch is selected to OFF the isolation valve
opens in the AUTO selection.
When a Pack switch is OFF, the Isolation valve opens to create equal performance of the engines.
When
a Bleed is selected OFF the Isolation valve opens to allow air from either side
of the manifold to be used for the Bleed off side WTAI.
Note the isolation valve logic is related to switch position, so a tripped pack or bleed will not open the isolation valve when in AUTO. After flight the Isolation valve should be selected OPEN just in case you need to battery start engines when there is no APU or external electrical power available. The ground air connection is located on the right side of the manifold, close to engine #2. When N2 >20% there is no personnel allowed in the vicinity of the engine inlet so you have to start engine #1 first. When this would be a battery start you’ll need the isolation valve to be open, so when you removed AC power with the isolation valve switch OPEN, the valve is still in the open position.
* A general rule for
electrical power is; “AC lies, DC dies”.
This is also nice to know for analog instruments, an AC powered instrument stays where it lost power and a DC powered instrument will drop of.
This is also nice to know for analog instruments, an AC powered instrument stays where it lost power and a DC powered instrument will drop of.
