I understand the
the shuttle valves. These valves are designed such that the priority system ( in this case B) has a larger surface area on its pressure face than the alternate system
functioning, and that's a step forward, but forgive me, I'll insist as this matter is not 100% clear to me.
Correct me if I'm wrong:
When B pressure is available, the ALTERNATE SOURCE SELECTOR VALVE (not the
normal/alternate shuttle valves ) is closed and the alternate brakes system is
not pressurized.
If B pressure falls below 1500psi, the ALTERNATE SOURCE SELECTOR VALVE opens and allows A pressure to the alternate brakes.
Is that valve electronic, as per avioniker's post, or is it working on the larger surface on one side principle as per mono's post?
If both normal brakes and alternate brakes were always pressurized, the
priority function could be accomplished by the NORMAL/ALTERNATE SHUTTLE VALVES (the ones close to the wheels on the diagram), but I think that's not the case, because this priority function is accomplished by the ALTERNATE SOURCE SELECTOR VALVE.
I still have a problem with the parking brake!
The parking brake is set by blocking the return of fluid in the return line (and yes, by blocking the pedals with a pin).
When we use system A pressure only (ALTERNATE BRAKES), we must trap oil pressure in the A return line, right?
But I don't see the blocking valve on the ALTERNATE BRAKE SYSTEM SCHEMATIC.
I seem to understand you are telling me there's only
one PARKING BRAKE VALVE (a blocking valve), and it's located on the B return.
So my question is: how can we block the A return?
There
must be a blocking valve on the A return as well!!