Unrealistic elements (SPOILERS)
Post 9-11, it cannot be said "that's not possible" or "that would never happen".
However....
1. Post 9-11, hijacking a plane only to crash it into a city is lame
2. Once it was apparent the plane was going to crash wouldn't the tower have radioed the plane would be destroyed in air?
3. The three terrorists did not seem a realistic trio. What was Daniel doing with them? Anyone spending time with Vedat would have known he wasn't a killer and would not be of real use.
4. On a plane full of Westerners and Christians, the second victim they chose would be a Turkish Muslim? Particularly since Vedat was a Turkish Muslim, Daniel should have realized the consequences of selecting someone so close to home with Vedat.