I seem to constantly run out of time when it comes to getting my transport to the terror site, even when I leave right away. No terror site as yet appeared within my radar range (bad luck I guess). So I end up having to fly halfway around the world to the place, only for it to be nuked an hour before my troops arrive. I'd suggest either extending the time limit for getting there, or do what they did in the original XCOM games and keep the crash/terror site location while a transport is actively flying to it.