Seems like one of those things that needs a test world airline with god mode switched on, so you can simply add & subtract planes from an airline and see exactly what commonality does. Or get the formula out of the underlying code.

Well, this is not a black box, that we can't look inside. Sami has the code, and it is not that difficult to look at the formula directly. The data we already have:

MT5, Jumboshrimp: 376 planes. 4 fleets = 355% of 3 fleets.

MT5, Jumboshrimp: 680 planes. 4 fleets = 676% of 3 fleets.

clearly shows that the formula does a complete opposite of what it is supposed to. There is a lot more commonality with 680 aircraft in 4 fleets (simple average of 170 aircraft per fleet) than in fleet of 375 aircraft in 4 fleets (~94 aircraft per fleet).

And the fact that number of fleets is almost completely ignored up to ~100 aircraft:

JA5, Sanabas, 115 planes, 4 fleets = 129% of 3 fleets.

JA5, Sanabas, 108 planes, 4 fleets = 121% of 3 fleets.

JA5, Sanabas, 121 planes, 4 fleets = 101% of 3 fleets

indicates that this is an area that can be exploited. After all the discussion about ways to limit growth of airlines, the most reasonable one of all is to make it expensive to get 5 or even 10 fleets initially, the system makes it possible (or even encourages) players to get a lot of fleets, expand quickly, and start to consolidate number of fleets when the number of aircraft gets to 200-300 range.

I think this really needs to fixed. In my opinion, this is the #1 priority (of all the items I see in Feature Requests and in Bug forum). The current formula is not only contrary to the real world, it is contrary to game playability, and one of the objectives AWS has had, which is to slow down the initial rush, prolong the build up phase, and to encourage careful fleet planning.