More Transponders

[Serco Stuffer]

Pilots (sorry guys - real ones) can your big nasty plane thing squawk two codes at the same time? Two boxes transmitting? Especially interested in Boeing but anything big and noisy.

[HugMonster]

Nope.

Usual installation is two TXPDR boxes under the bonnet, controlled by one panel on the dashboard, so only one squawk code can be selected, but sent by one of two boxes, selected by a switch on the panel.

[HEATHROW DIRECTOR]

Hug's answer puts the mockers on my own idea as to why our "TCAS" sometimes indicates that an a/c is conflicting with itself. I've experienced it a significant number of times but nobody was able to give me a reason why. A label goes red - heart rate quadruples - and the little box which shows the callsigns of the a/c about to hit has two identical callsigns and only one a/c is involved!

[HugMonster]

Perhaps I should elaborate on that answer a little.

I have in the past flown smaller aircraft that actually had two entirely separate TXPDR installations. (Dunno if they used the same aerial or not).

This is, I have to stress, a very rare situation. I've seen it on one or two aircraft, and the only one I've actually flown with such a set-up gave its last squawk moments before ploughing into the Cumbrian countryside about 10 years ago.

[Serco Stuffer]

Heathrow Director.

Thats very interesting as conflict alert at LATCC can do the self same thing. I will contact someone concerning TCAS having the same problems - when I find out who!

[2Donkeys]

GA dual-transponder installations typically consist of two physical transponders that share the same aerial. They do this by being connected to the aerial coax, via a box which detects "collisions", and effectively masks the second box if the first one is selected to either ON or ALT.

Lots of room for confusion here, in that there are still two sets of 4 digits staring at the pilot. Its just that in the case when both boxes are selected to ON or ALT, the first will be the only one that shows.

A similar coax masking arrangement is used for TCAS installations, preventing an aircraft's TCAS antenna(s) from detecting its own transponder return.

-2Donkeys