already confounding mystery. why didn't any emergency beacons send a distress signals? they're called elts and officials say there were four of them on the plane. >> the emergency low indicator transmitter is a small radio transmitter that sends a signal detected by satellites in the event of an airplane crash. >> reporter: signals sent by elt on to satellites are relaid by monitoring stations on ground. it's not related to the pinger from the black boyes boxes containing the voice recorders. they are to ping, circumstances which could have played out with flight 370. experts say there are other possible scenarios with the missing plane where the elts would not have worked. >> if the plane crashes with too much velocity into the surface so that the entire plane is disintegrated. also, if the plane lands gently enough, for instance in the case