I would have to side with the idea that Osama, and others who planned this had no idea that the buildings would come down. I think they would have wanted the burned out hulks to remain in the great satans skyline.
Our countries reation and call for war are a much bigger blowback to them than they had in mind. They wanted us defeated and frightened. Not empowered. We are so much more dangerous now than ever.
Jet engines do not burn avgas, which is similar to the gasoline we use in our cars. The fuel is more like kerosine, which does not evaporate easily (without heat). High octane avgas was often "used" in hotrods int he 50's and 60's.