View Full Version : Switchin' it off!

12th Apr 2002, 13:36
I have heard there are plans to create transponders that cannot be turned off. I also read that this was what happened on Sep 11 - the hijackers simply switched the transponders off. I imagine the primary radar would have known that the aircraft were there, although it wouldn't have been able to identify them. Is this true, or just another myth? :confused:


12th Apr 2002, 20:26
You would think the ATC systems could mark a non-transponding return for various reasons - transponder failure, airspace transgressor etc. They must have noticed the loss of transponding.

I'd be interested on opinions why the hijackers drew attention to themselves by doing this. Maybe they wanted to give the impression of major electrical failure so they wouldn't have to respond to ATC calls with a suddenly different accent.

12th Apr 2002, 20:50
In many parts of the world, some of them very heavy usage areas, there is no primar radar cover, so if the transponder goes off, no one in the control centre can see the aircraft!

It may be visible to the military, and sometimes, if the centre concerned has primary radar, they will still get a return, but it's far less common that most people realise, and I'm not being alarmist here.

In many cases, that's why the MEL requires 2 transponders on the aircraft, so that if one fails, the flight can continue without problem.

So, transponder that can't be turned off may be a good idea, but then again, if there's a serious electrical problem with said transponder, how do we then disable it? Pull the entire flight deck power? I don't think so.

Also, the Txponder needs to be off when on the ground to avoid horrendous problems for ATC with conflict alerts due to separation problems. Thare are other problems, but these 2 spring to mind most rapidly.

As always, quick fixes may not necessarily be the answer.

13th Apr 2002, 11:47
No point having a transponder that you can't turn off, because it will still have a circuit breaker which can be pulled and you can turn it off that way.

13th Apr 2002, 16:48
What about GPS linked transponders, i.e. satelites tracking aircraft. It would give greater coverage and of course you could make them so you couldn't turn them of :) .