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There are three standard transponder codes for exceptional situations: 7500 (hijacking), 7600 (radio failure), and 7700 (general emergency). Which code do you use if multiple situations apply? For example, if a hijacker smashes your co-pilot's head into the radio, do you squawk 7500 for the hijacker, 7600 for the smashed radio, or 7700 for your co-pilot's head injury?

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    $\begingroup$ Trying to decide if this is a serious question or a troll.... $\endgroup$ – John K Feb 8 at 23:42
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    $\begingroup$ @JohnK, the example may be less than serious, but the question itself isn't. For example, you could reasonably find yourself in a situation of "radio failure + minor emergency". $\endgroup$ – Mark Feb 8 at 23:52
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    $\begingroup$ If I ever find myself in that situation, setting the appropriate squawk code will probably be fairly low on my list of priorities 😀 I'd say 7500 > 7700 > 7600 but ultimately only the pilot (or one of the pilots) on the spot can decide and any of those will get ATC's attention. FWIW, none of the 9/11 flights squawked 7500, as far as I remember. $\endgroup$ – Pondlife Feb 9 at 2:34
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    $\begingroup$ I'd use 7500 when hijacked, no matter what else happens, 7600 if only my radios are broken but I am continuing the flight normally, 7700 if I have a radio failure and am in an emergency. You could also consider squawking 7600 first until you are in proximity to where you want to land, then switch to 7700 to show ATC: please also roll the trucks for me. $\endgroup$ – Jan Feb 9 at 8:27
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There is a trail of canceled AC's here that all say "only available within the FAA network" that might cover the topic if you can get ahold of them.

I found a brief mentions of it in this AC relating to ADS-B and its integration with transponders.

Operational procedures regarding the transmission of the generic emergency code (i.e., 7700) in cases when the flightcrew actually selected a discrete emergency code (e.g., 7500, 7600);

According to that language you basically use 7700 unless its explicitly one of the other two cases. In your example I would say a hijacking supersedes all but as with many things its a case by case kind of things.

Ultimately the codes are there as somewhat of a backup to actual radio telephony and cover 3 important cases the way I see it. In an actual scenario you use the code that is most applicable to the situation as the pilot in command sees it.

7500: The aircraft has been hijacked, they may not know what I (the pilot) is fiddling with so Im sending you this code since I cant say anything on the radio

7600: My radios are down, I may not be able to send or receive transmissions. There may be nothing else wrong with the aircraft in which case radio out procedures prevail. The tower may need to get a light gun out. In IMC see this question.

7700: Somethings going on this is how Im telling ATC for what ever reason chatting on the radio is insufficient. As to what the advantages are see this question.

This ICAO document on reply codes states that

ATS authorities should establish the procedures for the allotment of SSR codes in conformity with Regional Air Navigation agreements, taking into account other users of the system.

2.1.4.2 The following Mode A codes shall be reserved for special purposes

2.1.4.2.1 Code 7700 ... {2}7600 ... {3}7500 ...

So what will happen when you use the code is somewhat up to local jurisdiction and you should know what will happen when you enter the code to help you (as the pilot in command) decide which one to use.

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  • $\begingroup$ If in doubt, squawk 7700. Even in a lost comms situation, you might choose to squawk 7700, even though 7600 is more precise, if you, the PIC, believe that an emergency situation exists simply because of the lost comms themselves, or if you're unsure of the local jurisdictional conventions. $\endgroup$ – ammPilot Feb 9 at 6:53
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Regarding the USA:

First, remember that those codes are intended for situations normally considered emergencies.

If any apply, you likely have an emergency, and then you also have the decision-making freedom of the pilot in command during an emergency per CFR 14 part 91.3 meaning you can choose the one you consider to be the most pertinent.

If i were being hijacked and had a gun to my head and then my engine failed, I would squawk 7500 because I would want search and rescue to know that wherever my craft went down, there's likely someone holding us hostage.

If my radio is out in IFR and my engine fails, I would squawk 7700 so that they don't assume I just landed in a grass field intentionally in response to a failed radio. I want them to understand that generally, there is a big problem with my flight before going off-radar.

If I'm hijacked and my radio goes out, I would choose to squawk 7500 because my bigger concern is the hjack, and most likely, authorities would assume that my lack of radio contact was due to the hijacking anyway.

So, the short version:

If you have an emergency you should squawk what you think is best to keep yourself and your passengers alive.

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