Aircraft have floating, jettisonable radio beacons (Emergency Locator Transmitters) that transmit on the international guard frequency 121.5MHz (with intentiona

Aircraft have floating, jettisonable radio beacons (Emergency Locator Transmitters) that transmit on the international guard frequency 121.5MHz (with intentional harmonic distortion that allow reception on the military guard frequency of 243MHz). That frequency is monitored by SARSAT (Search and Rescue Satellite) in North America -- the middle of the South Atlantic may not be monitored, since the automated search notifications would not necessarily reach anyone able to do much about it. Local air control stations will attempt to determine the position, but ultimately it's usually someone with a portable directional receiver that pinpoints the actual source of the signal. (Occasionally a unit will begin transmission due to mishandling or malfunction, and I spent far too much of my time in the air force trying to sort through ejection seat packs and spare ELTs to find an overambitious backup gone wrong.) ELTs are also normally located on any floatation rafts the aircraft may be carrying. The aircraft itself will also carry an underwater acoustic beacon (pinging at about 40KHz) or two so that underwater wreckage can be located once the general area has been established by the ELT that should have been jettisoned on impact. Both items (the ELT and the UAB) have limited battery life -- they pump out huge signals relative to their size. By convention, the UAB is located in the same general area as the FDR and CVR longitudinally. In the case of the Air France flight incident, it would seem that the ELT did not work or was at a significant distance from the crash site by the time the search reached the position. That the black boxes (the FDR and CVR) are expected to be recoverable at this point seems to indicate that the UAB did work (it would have been detected by a sonobuoy dropped by the search airplane, which looked to me like it was equipped with anti-submarine warfare equipment, judging by the MAD boom on the tail).