Hello housecarl
This was a lot of discussion when it seems people don't really read your posting. Don't get bothered, it's just part of the pprune package.
I'm not sure whether your captain is rationalizing or prioritizing correctly, but technically what he is doing is correct. Here is how I think:
If the differential pressure between LDG ALT and FLT ALT is more than 7.8 psid it will maintain this pressure on the inner window when flying above 28.000'. If you then select a FLT ALT below 28.000', it will regulate to 7.45 psid. In other words you get a lower differential pressure, and thus a lower total pressure on the inner window.
On the 738 we see this all the time during cruise climb. Up to and including 37.000' it regulates to 7.8 psid, and when setting FLT ALT to more than 37.000', say 38.000', you see the cabin descending to create the computed pressure of 8.35 psid. Looks somewhat weird the first times when you are expecting a climb.
The problem with this is that a lower differential pressure than the 733 was designed for at those altitudes gives you a higher cabin altitude. Maybe not significant, but still.
What I do get is that he wants to better his chances and reduce the load on the inner, load bearing window, but this is not supposed to be necessary, and it lowers the pressure in the cabin.
This is as you see a tradeoff. Whether one thing is better to do than the other I don't know, I haven't been in this trade for a very long time myself.
Personally, because of my lack of experience, I would not regulate the pressure because it is not mentioned in the QRH.
I hope that helped. Cheers
