I can't answer your specific question about how the camera decides when to adjust ISO versus aperture in Tv mode, but I do shoot a couple of air shows each year (three so far this year, with two more coming at the end of September), so I can offer some advice there. I usually find it best to set exposure compensation to overexpose by about 1 stop, maybe more if there are clouds in the sky. Even with a deep blue sky, you're typically shooting up into the sky so some or all of the plane is in shadow. When you add clouds or haze, you're shooting against a fairly bright white background so you want the plane to be properly exposed.
Complicating matters further is that the planes are moving across the sky, so one second the lighting might be perfect, the next second you're shooting toward the sun.
I'll also second what PhotosGuy said about prop aircraft. For prop aircraft, I'll set for Tv with a shutter speed of no faster than 1/200 or 1/250. I find that's typically fast enough to *almost* freeze the propeller so you can see that it's there, but it leaves enough blur to look natural.