The black box (flight data recorder) is not the same thing as the radar ID transponder. The transponder sends the identifying data (I believe the military version is called Friend-or-Foe) that associates the plane ID with the radar blip---without it, all the radar sees is a reflection that could be anything: a balloon, a flock of birds, etc.
It is possible to turn off the transponder---it happened both on flight 370 and during the 9/11 event. I don't think it's possible to turn off the FDR, but the data is not available until they find the actual box, which of course has not happened yet in this case.
I believe the speculation around the unlucky flight MH370 and its Human Beings load, has overpassed any tolerable assumption of what is credible and what is just pure fantasy, but the question is more technical and refers to the technology inside an aircraft, so we know superficially that computers have the main role, and said computers because there are few redundants or in other words, if one fails there's its twin taking over the reins of governing the air giant, there are black boxes, OK, but WHY a giant of the air gets lost ????? Is it too far from aviation engineering to provide an electronic system maybe based on GPS system, to transmit position data that cannot be deactivated by anybody either pilots or cabin crew.... ?????
@Janine: The recording of the CVR & FDR can not be disabled. If you have meant the transponder, I completely agree with you. I don't find any reason why there is a provision kept for someone in the cockpit to be able to turn the transponder off for the comercial flights. I was under the impression that the transponder cannot be turned off the same way as the recording by CVR & FDR but I was surprised to learn that it could be turned off by the pilot or anybody in the cockpit who is educated about it.
The military designation of the transponder is IFF "Identification Friend or Foe". Depending on the level of sophistication, a transponder replies to a pulse from ATC's Secondary Surveillance Radar with a 4-digit code, (Mode A, the basic level), possibly altitude, (Mode C), and additional information, (Mode S). On request from ATC ("Squawk Ident"), the pilot can make the response pulse distinctive, for identification. Special codes are available to identify hijacking, communications failure, and Mayday situations, in case radio communication is impossible or difficult.
It is necessary for the crew to be able to switch transponders to a non-responsive mode, because otherwise the signals can overwhelm the system, (e.g. around airports).
The FDR (Flight Data Recorder) records the data from the aircraft's systems and controls, while the CVR (Cockpit Voice Recorder) records the sounds in the cockpit for the last hour (I think). This provides a record of the crew interactions, (and sometimes vital clues as to explosions, decompressions, engine failures, and other events). The crew have no control over these records.
@perl_geek: "It is necessary for the crew to be able to switch transponders to a non-responsive mode, because otherwise the signals can overwhelm the system, (e.g. around airports)."
I was searching for an answer to the question why there is a provision kept for the crew to switch the transponder off. Thanks for the answer to that. A continuing question that occurs to my mind now: is the intention not to overcrowd controller's radar screen or not to "overload" the system with too many signals from all different flights?
Its possible that the tower can request to turn off the Transp. and nobody else, if too many signals coming in. Both the pilot and tower can turn it on only. Also about the recoding box should go off and wake up on demand when request comes, so battery life is preserved.
The transponder signal takes up space on the ATC radar display. (A controller's screen has to show many miles in less that a metre.) If too many signals come from the same point (as far as the screen resolution is concerned), they overlay and blur each other.
The transponder is switched from "standby" (warming up) at the runway threshold, just before takeoff, and back to standby, or off, after landing.