I couldn't catalog every gripe I had with "Flightplan" in the review without giving away the twists, but here are a few of them. Some won't mean much until after you see the movie. And BEWARE! They're full of spoilers. You've been warned!
Once the whole plot is revealed, it becomes obvious that it was dependent on the incredible unlikelihood that not one single person on the flight saw the missing girl at the gate or on the plane. The whole thing would fall apart if even one passenger said, "Yeah, I saw her." What kind of idiot villain comes up with a plan like that?
What's the villain's motive? Why such a complex ransom plan for so much money? Why not commit a simple kidnapping for a couple million dollars instead of this ridiculously complicated scheme involving staged suicides, staged disappearances, falsified passenger manifests, predicting the behavior of scape goats -- and doing it all with 400 captive-audience witnesses?
If the baddie is planning to kill the girl anyway, what's the point of keeping her alive, then leaving her unsecured (what about turbulence?) in the nose of the plane (which in the real world would have no heating)?
OK, I admit this one is completely extraneous and apropos to nothing, but what airline would paint their planes an ominous, uninviting dark gray?