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I know that in Mode $3$ (which includes Mode A and Mode C) the aircraft transmits $12$ information pulses. That corresponds to $2^{12}=4096$ squawk combinations in Mode A which are sent to ATC to identify the aircraft. In Mode C, all $4096$ combinations are used to send the pressure altitude of an aircraft to the ATC.

Since there are $12$ usable information pulses, when operating in Mode C, there is no way to transmit the squawk code, which in my mind raises an obvious question: What if I suddenly squawk $7700$ due to an in-flight emergency? Or $7600$, radio failure? If I continue to operate in Mode C, it seems that there is no way that ATC would know that I am in any kind of trouble. How is this issue resolved and is my understanding correct?

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  • $\begingroup$ Mode 3/A and Mode C are not the same thing. $\endgroup$ – StephenS Sep 20 '20 at 15:26
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Squawk codes (Mode A replies) are transmitted in response to Mode 3/A interrogations. Altitude codes (Mode C replies) are transmitted in response to Mode C interrogations.

Both Mode A and C replies use the same 12 pulse encoding, but the radar that asked the question knows how to interpret the answer (assuming there is no FRUIT or garbling, but that is another topic).

If the transponder is in altitude reporting mode (i.e. it replies to Mode C interrogations), it will also reply to Mode 3/A interrogations.

Thus, if you squawk 7700 and you transponder is switched on (either Mode A only or Mode A/C), then code 7700 will be the response to the next Mode A interrogation from a radar. If your transponder responds to Mode C interrogations as well (i.e. altitude reporting is on), the radar will receive altitude information in response to Mode C interrogations.

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