[RogerBW]

I think one big question at this point is:

"Should a single aircraft only ever require a single GURPS skill to fly it in all its modes?"

And I think the answer must be "no", because we have things like Ospreys and Harriers and stoppable-rotor helicopters that have multiple distinct – let's call them "flight modes", one helicopter- or vertol-style for hovering and low-speed flight and one airplane-style for faster flight.

Mind you, I'm working right now on an adventure set during the Falklands War, and one of the stock PCs is a Harrier pilot who's been shot down. I'm not going to insist that he have Vertol as well as High-Performance Airplane even if that's what he should have, because his piloting skill in this adventure is only going to be useful for knowing things about aviation, not for flying.