Cannot answer that as I only have observations, not explanations. Why did they design the way they did, that's well beyond my knowledge AND designed by people far smarter than me.
Think redundancy.
- If "B" is the only one remaining, it remains connected to both elevators! So pitch control is possible with good authority. No need for THS.
- If "Y" or "G" would be the last one you have, either of them will give you THS and one side of the elevator.
Think reliability.
- The more complex and connected system you create, the more chances and places where it can go wrong. That is why, I assume, you do not want to have a line of HYD connected all around the craft as the probability of losing it is increased then. The G actually is, but G and Y are most definitely not. Do not forget that compared to dual failure with a very capable single source remaining, it would be miles better not to have a dual failure at all. By connecting two systems at one delivery user, you are already creating a possibility of a dual loss at that point.
Connecting three lines to one place makes no sense. Redundant and Independent are two legs on which Reliable stands. Keep them apart for better balance. The layout seen on OPS.07A is done by dilithium-belt system engineers.
I do not have the stamina for discussion on Mechanical Backup, apologies. BTW what is your interest, enthusiast or professional? Which field?
Last edited by FlightDetent; 5th Apr 2020 at 22:06.