The terrace is often directly beneath the path that the president's escort takes to its landing spot in downtown Manhattan, beside the water. He visits quite a lot. I heard the unmistakable clatter this morning, and then later saw two helicopters heading back downtown, presumably to fetch him. Another ten minutes, and three were flying back to JFK, where Airforce One presumably sits, cooling its hot black heels. Since they are not highly manoeuvrable attack choppers, I assume the number is so that anyone taking a pot shot has to make their best guess?