I tested the aircraft carriers and I was surprised to see that, while they have the same stats as the brigantines, they really do work as carriers: they can't move infantry units but really can move planes. Why don't you release them properly ? Their initial purpose of "allowing planes to attack further" sure is in fact useless, but they do something great instead: reinforce a town with no/not enough runway, with basically any number of planes. Remember that Tuey asked so often years ago about when this unit would be released, maybe finally releasing it would bring her back (and her donations) ? I can message her on facebook about it.
Really from what I tested, they work just fine, they just need to have real stats and cost. Thus, air transport (those that one achievement talk about) should work just fine too, you don't even have to make new sprites for them, you can use those of the empty air traders.