pinger, the signal coming from the black boxes contain pg the flight recorders. these beacons are designed to be activated if they send extreme deceleration or impact. experts say there are other possible scenarios with the missing plane where the elts would not have worked. >> if the plane crashed with too much velocity into the surface so that the entire plane is disintegrated also if the plane lands gently enough for instance if the plane is set down gent ri enough that there wasn't sufficient g force to activate the elts. >> other reasons why the beacons wouldn't have worked. a crash could have broken the elt's antennas. also there's a time lag after impact. it takes 50 seconds for the first signal to be transmitted. >> even after it hits the water. it has to stay afloat for more than 50 seconds or they won't sound either. >> so either two elts placed by