Ouch...my head hurts!!!
Hmmm,
First of all, why do you have to ask such difficult questions? I'm going to have to take some Tylenol to get rid of the headache brought on by racking my inept brain for an answer to your question
Our study guide says the following...maybe it will help you visualize what happens, maybe it won't:
1. All valves associated with the 10th stage bleeds are spring-loaded CLOSED and require air pressure for them to move open.
2. On the entire Bleed Air panel, the L & R 14th stage valves are the only ones that are spring loaded OPEN, and they too need air pressure to move closed.
3. Because all valves need air pressure to move them, they do not always move when the appropriate switch is pressed. It is easiest to remember that when the switches are selected, IN is OPEN and OUT is closed. The vavlves will move when the air pressure reaches them.
The book also states that for a crossbleed start the operating engine's RPM needs to be at least 85% N2 or higher, so that would be another way to indirectly check for appropriate pressure without actually checking the ECS synoptic page.
So in other words, the 10th stage ISOL valve is normally closed to separate the left and right side distribution ducting. It opens automatically during engine start to ensure a supply to both engines, regardless of pneumatic source.
Hope all this helps...think I'll go apply a cold washclothe to my head now.
