As a French nuclear submarine arrived off the coast of Brazil to join the effort to locate the black box from Air France Flight 447 on Thursday, aviation experts stressed the necessity of recovering those cockpit recorders in order to learn what exactly brought down the Airbus A330 and the 228 people on board. In past inquiries into airline disasters, investigators have been able to figure out the cause by piecing together clues from the wreckage itself, sometimes without information from the black box. But after 10 days of searching, the authorities combing what’s believed to be Flight 447’s crash site, some 700 miles out to sea, have come up with only 41 bodies and relatively little of the plane’s wreckage.