say pingers, these are in the flight data records. this is what alerts people to the flight data recorders. >> it's the sound. >> are they test before being put into the data recorders and boarded on the aircraft? >> they're tested in that they have a machine that will test them to see if they ping. there's no way to check the load of the battery. >> really? david gallo, if the pingers have failed, or obviously limit a life span, what's then the best way to go about finding the black boxes? >> good old systematic mapping of the sea floor. line by line, step-by-step. it takes a bit longer but that's what happened in air france 447. >> now in that case, the pingers were no longer working, not because of any default that we know about but simply because it took so long to actually -- it took some two years to actually find the location underwater of where the plane was and where the boxes were, correct? >> yes. the pingers were long past gone. but i went back and looked at