I've experienced significant dread before international trips. Usually it only happens a night or two before the trip, but it can be enough to keep me awake. It's usually not related to anything in particular, and there's very little specifically related to fear of a crash or something like that. Usually it has more to do with things falling through with the itinerary. So I think it's kind of like the common nightmare of walking into a classroom for an exam where you haven't attended class for the previous 6 weeks... i.e., for me it seems to be related to a fear of being under-prepared. And since I put a good deal of planning into most trips like that, when the time comes that I can't plan anymore, it does seem natural that my subconscious would start to panic a bit.

The morning of departure, all fears vanish.

More recently, as I now have two very young children, I've been on a few international trips on which my wife has not joined me for at least part of the trip. I have another one starting in mid-May. And for those trips (as for the one pending), I notice a smaller sense of impending doom that crops up farther in advance. I think that has more to do with control issues, related to the fact that I'll be so far away from my family for an extended period.