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Why mode A not C?

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  • Why mode A not C?

    Hello, why do we, for example in unlawful interference ,need to set mode Alpha rather than Mode C, the transponder to 7500. ?

  • #2
    have a read at the following buddy, it should explain your question:

    http://code7700.com/transponder.htm

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    • #3
      Well, I'm interested in the answer as well.
      r10bbr, I don't find much information in the link you indicated

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      • #4
        Anyone else?

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        • #5
          Mode 'A' or more correctly Mode 3A is the 4 digit code you squawk and Mode 'C' is your flt level read out.

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          • #6
            Sorry, it is still a blurry situation for me.

            Let's suppose you are in cruise in a 737 with the Transponder in mode C (XPDR I believe)
            Suddenly a terrorist of some kind start taking hostages in the cabin.
            What should be understood by "set the transponder to mode A code 7500" :

            - Leave the transponder to the actual mode C (XPDR) and squawk 7500 (an A mode code), or
            - Turn the transponder knob to the actual mode A (ALT RTPG OFF) and then squawk 7500 ?
            Last edited by ArnaudN; 08-08-2018, 16:51.

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            • #7
              John CTKI Thanks John - but what I would like to know is, why do we not keep the transponder on Mode C when squawking 7500 (or 7700 for that matter) but turn it to Mode A?

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              • #8
                You would normally be squawking both Modes A and Mode C. So when changing to squawk 7500 on Mode A you would normally still squawk Mode C unless instructed otherwise.

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                • #9
                  Oh OK, it makes much more sense in this way.

                  Myself, and I think Transalp too undestood this on first read to turn the transponder to mode A and then squawk 7500, effectively disabling the altitude info transmission. Which doens't makes much sense since it deprives ATC from valuable infos, and probably the aircraft from its ACAS system.

                  Reading through the law text again, it is a very subtle difference between "mode A code 7500" and "mode A , code 7500" It could be worth adding a little highlight about this in the courses.

                  Thanks for the Clarification John !

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