Flight Simulator is not a game, but an actual learning tool aimed at teaching you the fundamentals of flight. As someone who has run through most of the lessons on FS, I have to say I have a far better idea of what keeps the thing in the air than I ever did just understanding the physics of it. It is no replacement for the real thing though is it? I can therefore find it perfectly plausible that terrorists might have learnt the fundamentals using FS in their own home country, safe from potential capture, prior to entering the US and learning to fly for real. It no doubt would have accelerated the learning curve. As for the WTC bit... that's rubbish. Fly FS for two seconds and you know perfectly well that it cannot in any way prepare you to be able to fly a 200 tonne aircraft at 600mph into a building. You have no repsonsive feedback and no inertial effect on your body, making it impossible to fly extreme scenarios like that.