As you can see from the photo, the Capitol Building is the center of the bull's eye from the air. In fact the street system radiates outward from that point. A cross hair if you will. On a clear day, such as. Sept 11, there is no way that even a goat herd could miss it.
Just to the left of the Washington Monument you can make out one of the bridges on the Potomac River which is the flight path ( rt to left in the photo) for National Airport (not far beyond the left) on the river. By the time planes reach the Monument they are already on the landing approach and very low. As you can guess one could make a quick banking left turn and there's no chance of stopping impact.
