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If you look at shows like Air Crash Investigation, which surely take some creative license in how events are portrayed, they often portray the disappearance of an aircraft, or specifically loss of its transponder return, as the radar return simply disappearing from ATC's radar scopes.

My question is: is that actually the only thing that happens when ATC secondary radar loses an aircraft's transponder return, when the aircraft is not actually on approach to land (and thus losing line of sight to the radar would be normal and expected, and/or that radar return is no longer relevant for air traffic control purposes)?

In general, humans are better at picking out things that are there than they are at picking out things that aren't there, even if they were there previously. It's also very rare for an aircraft to lose its transponder, let alone crash. Therefore, some kind of positive notification that a transponder return was lost when it shouldn't have been, possibly after a short delay (to account for those aircraft where the transponder is set to standby while a new code is being selected) would seem helpful.

Yes, it's the controller's job to keep track of the aircraft they are in charge of, but especially with modern, digital radar systems, it seems like something like flashing a distinct symbol where the aircraft was last seen, or sounding an alert chime, or just about anything, could make their job a lot easier the very few times that an aircraft transponder return disappears when it shouldn't have.

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    $\begingroup$ In the US, when on flight following while practicing approaches, I had the transponder go out because of water in the antenna. ATC lost contact with us and had us do a couple of turns to headings so that they could identify us on primary radar. They were able to vector us for approaches, so their system had some way of keeping track of us even though they couldn’t pick up our transponder. $\endgroup$ – JScarry Jul 21 at 15:56
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    $\begingroup$ At least some ATC radars are perfectly capable of picking up objects that don't have transponders. I've had them notify me about targets that turned out to be hang gliders, or even flocks of birds. $\endgroup$ – jamesqf Jul 21 at 19:01
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It depends on the system. Assuming only secondary radar is available, in some systems, the track will indeed just disappear when radar contact is lost. Other systems will put the track in "coasted" mode, continuing to show the track (but with another track symbol), waiting to pick up the transponder again. The track will then be terminated after a specified time limit.

Of course, if primary radar coverage is available - which works independently of aircraft equipment, such as transponders - the system would still be able to track the aircraft, but automatic flightplan correlation would most probably be lost, along with mode C data (altitude). So all flight information would vanish from the screen and the controller would have to re-identify the flight using proper procedures.

Producing an alert when a track is lost could be useful, but it could also potentially result in a lot of nuisance alerts, particularly in areas with spotty radar coverage. So alerts should only be shown for areas and levels where radar coverage is expected to be perfect, otherwise the controllers would likely develop a habit of ignoring such alerts. Also remember that radars lose contact with flights all the time - as they move out of the range of the radar. No ATC station has global radar coverage. Showing an alert every time an aircraft leaves the area of radar coverage would also be pointless, and the range of a radar can vary from day to day depending on atmospheric conditions, so defining a precise area in which alerts should be suppressed might not be a trivial task.

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    $\begingroup$ Nice answer! Only thing to add is, that most of the airspace is covered by primary radar as well. In that case the radar label world disappear and so does all the Information (squawk, level, callsign, mach number, heading,...) but the target with Speed and speedvector would Be tracked normally $\endgroup$ – pcfreakxx Jul 21 at 15:41
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    $\begingroup$ "otherwise the controllers would likely develop a habit of ignoring such alerts" I'm sure that, if the desire to do so is there, there would be a way to unobtrusively indicate to the controller that the transponder return has been lost. Even just changing the symbol to add a question mark (or whatever else is unused and clear) could unobtrusively draw attention to a specific radar return. As for when aircraft leave the radar coverage area, that sounds like a place where adaptive thresholds could be used to good effect. Neither seems an insurmountable problem in the least. $\endgroup$ – a CVn Jul 21 at 18:07
  • $\begingroup$ @aCVn what happens in a primary capable facilitiy is that the symbol will change to denote the loss of the secondary radar return, AIUI $\endgroup$ – UnrecognizedFallingObject Jul 22 at 0:20
  • $\begingroup$ @aCVn Not saying the problems can't be solved. Just explaining how it works. $\endgroup$ – J. Hougaard Jul 22 at 5:49
  • $\begingroup$ @pcfreakxx, if the track had a flight plan correlated with it, some information (callsign, cleared level, etc.) won't be lost. And besides, most of the airspace is certainly not primary radar-covered. All oceans, for example; most of Siberia and Australia... $\endgroup$ – Zeus Jul 23 at 1:47

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