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I understand that blocks of transponder codes are assigned to different area controls. I am curious if the block 7701 to 7777 is in use anywhere? Maybe the emergency code 7700 needs some extra protection?

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    $\begingroup$ According to AOPA, 7777 is used by military for interception. $\endgroup$ – mins Sep 19 '15 at 19:31
  • $\begingroup$ I do not understand that. Should I set 7777 if I realise I am being intercepted? $\endgroup$ – Wirewrap Sep 19 '15 at 19:31
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    $\begingroup$ Maybe it's used by the interceptor. Otherwise, I read different things for code 7776 and 7777, about being used to monitor SSR. Also it seems many pilots receive 77xx (except 7700) for regular use, from time to time. On the other hand 7700 is not used for each emergency, ATC is able to associate an emergency sticker to a regular code, however 7700 (and other special codes) initiated by the pilot trigger an alarm at ATC. $\endgroup$ – mins Sep 19 '15 at 19:55
  • $\begingroup$ @Wirewrap I was trained to handle being intercepted as follows: If talking to ATC, do not change your transponder code. Notify ATC that you've been intercepted. If not talking to ATC, squalk 7700. Being intercepted meets the definition of an in-flight emergency. $\endgroup$ – David Schwartz Apr 27 '17 at 21:24
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Yes these codes are used. Only 7700 is used to denote emergencies, and usually only if you aren't in contact with ATC, or can't get their attention(frequency is packed). Codes in this range are allocated according to the National Beacon Assignment Plan in the US. There are other codes that are assigned for various issues, 7777 as mentioned in the comments is assigned, and I've assigned others in that range before, same with the 7600 and 7500 ranges.

If you're already in contact with ATC, and they have you tagged, it is very easy to mark you as an emergency and have it light up on everyone in the facility's scope.

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    $\begingroup$ Just one thing to be aware of when you are assigned a code in the 7500, 7600 or 7700 blocks; always make sure that as you're punching up the transponder code, you never have the display set to the reserved code even for a second. For instance, if you're squawking VFR (1200) and are told by the tower to squawk 7732, after you've set the first digit to seven, when setting the second digit you'll transition through 7500 (hijack), 7600 (no comms) and 7700 (emergency), lighting up the tower's systems. $\endgroup$ – KeithS Sep 21 '15 at 15:32
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    $\begingroup$ To avoid this, get in the habit of setting the digits in reverse order when moving from VFR to a tower-assigned code, and (if you don't have a one-button VFR function on your transponder) setting the first digit first when moving to VFR. There can still be some gotchas, but overall you're much less likely to create a false alarm. $\endgroup$ – KeithS Sep 21 '15 at 15:34
  • $\begingroup$ You should still use the emergency code even if you are in contact with ATC, because you will also light up on other facilities' screens then. Unless you are already in an airport control zone, there will be at least one handover before landing. $\endgroup$ – Simon Richter Sep 29 '17 at 9:45
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    $\begingroup$ Newer units have pushbuttons for code entry, they don't seem to change what is being squawked until the 4th digit is pressed. See GTX345 at Garmin.com for an example. See at 1:25 into this video where 1200 goes away and the code starts going in. youtu.be/-BHp47RJbYk?t=75 See that? The nice picture went away when I changed from Answer to Comment. $\endgroup$ – CrossRoads May 4 '18 at 18:31
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Airman's Information Manual AIM 4-1-20 e.2 says, "Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations." (AIM 2016 edition, seems to be the latest edition as of 2017 Sep)

AOPA flight training magazine Nov 2017 advises temporarily putting your transponder to standby while changing numbers so that you don't accidentally trigger alerts in towers by passing through the special 7xxx numbers.

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