A minor SPOILER here.
A very good premise that gradually - and inevitably - drifts into teen horror film territory. I say inevitably because there is a group of teens involved so obviously there can be no long-term seriousness. Still, I was a little surprised just how unseriously the movie ends. When one compares the excellent beginning with the ridiculous horror/comedy ending, it is like looking at two different movies. It's a pity that the premise wasn't taken and "assigned" to a cast of adults, i.e. that a "serious" horror film wasn't made. Nevertheless, FD is never dull; even the obviously weaker second half. The flight disaster scenes are terrific, the murder scenes are very good, and there is real interest as to what is going to happen next, in spite of a premonition that the conclusion can't and won't be satisfactory. The conclusion is almost like a mockery of all the previous happenings (the movie turns from horror into the horror comedy genre). Easily the dumbest scene, however, isn't the ending but the way the aggressive teen turns around his car to beat up the movie's main hero, and nearly kills the bicycle-driving teen - while completely ignoring the fact that he nearly killed him and caused an accident! That was really, really dumb.