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FAA Order 7110.66E says

Code "0000" should never be assigned or used.

and

Code 0000 must never be assigned

However, NASA recommends that crews set 0000 after landing to help detect when a clearance has not been received.

Resetting the transponder to 0000 (four zeros) after landing can help you, or the next crew, detect lack of a PDC. Additionally, should a flight depart without setting an appropriate IFR code on the transponder, ATC will be more likely to quickly detect the problem. (Setting 1200 on the transponder may lead a controller to believe the target is normal VFR traffic.)

The idea is to set a code that could never be correct. If you set 0000, a casual glance at the transponder will indicate "Some part of our preflight did not get accomplished" rather than "Ah the other guy must have set our code already".

I set my transponder to 1200 after IFR flights, and am considering changing to follow NASA's recommendation of 0000 instead, but I don't want to create headaches for ATC if they see it one day on their scope.

Why should you never squawk 0000 in the USA?

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    $\begingroup$ “Ah the other guy must have set our code already". I cannot fathom anyone actually making such a reckless assumption. Because I would automatically think “ah the other guy must have forgotten to squawk 1200 after landing...” Sure, 0000 would add another layer of cheese to Reason’s model, but is IFR traffic taking off with a 1200 squawk (inadvertent VFR?!) really a problem deserving of such (forbidden) mitigation? If you fly for a carrier that swaps crews you will likely have it in your SOP to always file IFR, in which case resetting 1200 works fine. $\endgroup$ Feb 14 at 16:27
  • $\begingroup$ @MichaelHall In this case, "the other guy" is the other pilot on the same crew, not the previous pilot who has now left the aircraft. Setting 1200 could work the same as setting all 0's, either conveys "you haven't set your assigned code here yet," but turning all 4 dials counterclockwise to the stops is more certain than dialing in 1200. $\endgroup$
    – Ralph J
    Feb 15 at 4:22
  • $\begingroup$ @RalphJ, did you clarify with Steve on the side that that’s what he meant? Because if so, with a two person crew it ought to be even more easy to avoid taking off with the wrong squawk set, right? Anyway, most newer digital ADSB compliant transponders I have seen use a single “VFR” button to roll in 1200 with a single push, making it very easy to be certain you aren’t IFR any more. $\endgroup$ Feb 15 at 4:44
  • $\begingroup$ @MichaelHall - Although Ralph did not clarify with me, that interpretation is correct. "The other guy" is intended to mean the other pilot on the same crew. $\endgroup$
    – Steve V.
    Feb 15 at 4:47
  • $\begingroup$ @MichaelHall Transponders in airliners have no such VFR button (we never fly VFR, and if we did we'd still get flight following), and the control heads in them aren't all that new. $\endgroup$
    – Ralph J
    Feb 15 at 4:49

2 Answers 2

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You've quoted the E revision; the latest (as of writing this) G revision (issued September 30, 2021) may have the answer:

This revision reserves Code 0000 for certain Automatic Dependent Surveillance-Broadcast (ADS-B) units that do not detect and set the ATC-assigned beacon code unless the aircraft is in secondary surveillance radar (SSR) coverage. Broadcasting “0000” on power-up will allow ATC automation systems to process the ADS-B data for presentation to air traffic controllers. Similar to a change made in 2015 for beacon code 1200 for VFR aircraft, this revision allows VFR gliders that may be in contact with ATC to remain on beacon code 1202.

It's possible this reservation for those certain ADS-B units was planned for all along and was settled on last September. How those ADS-B units differ, and how other countries handle the same issue (if such units are used outside USA), I don't know, but it's a step in the right direction (anyone feel free to expand on the above in your answer).

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  • $\begingroup$ It’s possible they decided to take advantage of ATC never assigning those codes for a new purpose, but I remember reading 0000 and 7777 were reserved (for the military?) back in the 1990s, before ADSB was invented. $\endgroup$
    – StephenS
    Feb 15 at 16:11
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The National Beacon Code Allocation Plan (FAA Oder 7110.66), states that Code 0000 should never be assigned or used, as you point out in your question. That's why you should never squawk 0000 in the USA. Apparently code 0000 has an application beyond normal ATC purposes.

The article you link to in your question does not appear to be a "NASA" recommendation. Rather it appears to be a feedback recommendation from the author based on PDC issues identified thru ASRS (from reports on the ASRS system) published and reported by the NASA ASRS system.

Unlikely that this is an endorsement by NASA. Plus the article is almost 30 years old.

Better pilot IFR Predeparture clearance verification procedures (following SOP's) would seem to be the best course of action.

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    $\begingroup$ This does not answer the main question: why should code 0000 never be assigned or entered into a transponder? $\endgroup$
    – randomhead
    Feb 14 at 1:32
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    $\begingroup$ But you can read between the lines and determine the reason for the altitude assignment rule: to ensure safety by preventing aircraft from colliding. What is the reason that JO 7110.66 forbids the use of 0000? That is the question. $\endgroup$
    – randomhead
    Feb 14 at 2:24
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    $\begingroup$ @randomhead Because any system of codes needs at least one special code to denote "this isn't a valid code" and zero works nicely because it's trivially simple and obvious. All of the software you use relies on the fact that zero is a special number. $\endgroup$ Feb 14 at 5:52
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    $\begingroup$ @CareyGregory Since one can squawk Standby, why does one of the 4096 available codes "need" to be a "this isn't a valid code"? Sure, it's handy to be able to "zero out the transponder" and know that you still need to enter your assigned code into it (since we won't be assigned 0000), but what is behind the stern warnings to everyone never to do this? If VFR were 0000 instead of 1200, you could use the same "don't have an assigned squawk set" reminder, and ATC could assign all 4096 codes (minus the specials like 7500, 7600, 7700, 2000, etc) instead of just 4095 that they actually have. $\endgroup$
    – Ralph J
    Feb 15 at 4:16
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    $\begingroup$ @CareyGregory that 0000 is reserved for testing seems perfectly reasonable. Do you have some information to back up that assertion? $\endgroup$
    – FreeMan
    Feb 15 at 12:15

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