I was playing as Trevor and saw that he has a helicopter and plane at the Sandy Shores?? Airfield. I think that's awesome. I can get in an air vehicle whenever I want and they'll respawn. Then I got the S&F mission as Franklin to get to the top of the MazeBank building to skydive off of it. I immediately remembered Trevor's airfield, and went there as Franklin, only to find nothing there but the Stunt Plane. I switch to Trevor, go there, the vehicles are there. Switch back to Franklin and they disappear. If a vehicle is there for one character it should be there for all of them.
At first I was very pissed, I had no way to get on top of MazeBank, I have no idea where to steal a helicopter in the game. But there's a happy ending. I remembered I bought the Collector's Edition, and I called for the Blimp and used that. Much slower, and a long ass drive to get to it in the first place. Trickier to land without it falling off, but it worked.
Still though, if Trevor has airplanes parked at his airfield... (Not actually HIS airfield for me, I haven't bought it yet) those airplanes and helicopters should still be there even if you come back as Michael or Franklin.