In JA, I am slowly phasing out my old Constellations. I just terminated the leases on 4 of them, and took before and after shots of commonality costs. Two pics atttached, first is planes, 2nd is engines. Before on the left, after on the right.
Plane part is completely as expected. 3 groups save ~3.3% each, because of the difference in the size of 4th fleet penalty between 116 & 120 planes. Connies save more, both the 3.3% saving on 4th fleet and a saving from having less planes in the group. So no worries there.
Engine part is strange. Wright engines save money, as expected, because I've just dropped 4 planes & 16 engines. Ivchenko & Kuznetsov stay identical, the steadily growing % size of 4th fleet penalty doesn't appear to apply to engines, so that is also as expected.
What's weird is that the P&W engines have got cheaper, by 6.5%. The number of engines hasn't changed, so they should stay identical, same as the other two groups. I don't think it's the 4th fleet penalty being applied, as I'd expect a 3.3% drop then.
So for some reason, having 4 extra constellations with wright engines was causing the P&W engines in my Martins to cost more. I believe that is a bug.