At what point does it go from being assur to being mutter? That's the real question.
In the airport? If it's YT in the city, you can't do melacha. Which in this case would include entering/leaving the tchum and a bunch of other stuff... So this scenario would be asur.
On the plane I would assume it goes based on where you left from.
This kinda question usually comes up with fasting where there's a change happening in both the place you are coming from and the place you are going to, and the Question becomes which location to go by. Here there's no change on the flight itself, it's not YT for you when you take off...
But AYLOR.