The Chances of this happening are slim, but say it is July 5'th. A Singapore A380 carrying several hundred 4'th of July tourist from Singapore, loaded with jetful (enough for the long trip half way around the world) took off from JFK International Airport in New York. The controller clears it for takeoff. If You look at the headings of the runways at JFK and LGA and them being extended another several dozen thousand feet or so, You may notice how they cross each others paths. Lets say A Us Airways A321, Packed with tourist from New York (who spent there 4'th of July in Washington DC), was on the baseleg part of it's approach to LGA. Say these planes were bound to cross each others paths at the same time. The TCAS system on tthe A380 says to keep climbing. On the other plane it tells the pilots to desend quicker. But at its already low altitude they don't want to desend quicker. So The pilots of the A321 just decide to let the Singapore plane Climb out of its way. But the A380 is to big to move out of the way in time. Both planes collide at about 2,600 feet.Over such a densley populated area this sort of aircrash could potentialy kill up to one thousand. This sort of thing probably won't happen, and if it did it doesn't mean it would be with these two planes, airlines, etc. But It is possible.