I think there are a lot of questions out there because this type of event is extremely rare so the actual outcome is just as rare. Let me put it this way, we know what happens to buildings when they are hit with an earth quake because of engineering but also because we have seen the out come many of times and learn from those experiences. A 767, loaded with fuel, hitting a building is extremely rare and the out come has only been seen twice (the twin towers). With that, while engineering can tell us a lot, the real after math also speaks volumes. Unless we hit similar building with similar planes a few more times we may never know what really does happen. A 767 can weight up to almost 400,000 lbs and carry up to 24,000 gallons worth of fuel. Throw it at a building doing a few hundred mph and who knows, maybe the building will fall.....