Smaller non-commercial aircraft loaded with a couple hundred pounds of explosives and taking off from municipal airports and attacking sporting events in nearby open stadiums. Of course these flying bombs would be a lot smaller and less powerful than Sep. 11, but still one such suicide attack could easily kill hundreds at a professional baseball or college football game. The scariest part is that it could only be stopped by on-the-ground security because with thousands of airports and hundreds of stadiums it's physically impossible for the US Air Force to be everywhere at once and be able to shoot down the airplane.