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I'm a CFII and for training GPS approaches with advanced students, I like to occasionally turn off the WAAS receiver on the 430/750/G1000/etc. when they are not looking, to simulate a WAAS failure and make them notice they need to change their minimums.

Now, ADS-B out depends on a WAAS GPS-receiver. When I intentionally turn off WAAS, does that turn off ADS-B out as well? Would I be technically non-ADS-B-compliant in 2020, when flying the approach without WAAS?

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    $\begingroup$ I think ADS-B usually uses a separate GPS receiver, in which case there would be no effect. That may be installation dependent. $\endgroup$
    – GdD
    Commented Dec 18, 2019 at 20:33
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    $\begingroup$ @GdD Yes, it is, but on systems with a Garmin GNS430W, the 430W can feed the WAAS signal to a Garmin ADS-B like the GTX345, this is how it is in my aircraft. It also feeds GPS signal to our G5. $\endgroup$
    – Ron Beyer
    Commented Dec 18, 2019 at 21:36
  • $\begingroup$ How exactly do you disable WAAS on a 430? It’s not something I’ve come across in the manuals or heard about before. $\endgroup$
    – JScarry
    Commented Dec 18, 2019 at 22:04
  • $\begingroup$ Your ADSB Out device may or may not have an independent position source; if you don't know, find out before doing this again in rule airspace. $\endgroup$
    – StephenS
    Commented Dec 18, 2019 at 22:14

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This depends on the ADS-B receiver that you have, but if it is still receiving position information, but the WAAS portion is missing, you will get a lower NAC/NIC/SIL in the broadcast message. If you disable the GPS completely, the ADS-B will continue to function however the SIL for position will be the lowest level (likewise for NAC/NIC). The ADS-B will continue to function (as in send messages) but the position reports will be unavailable.

It's important to note that your transponder is still a Mode-C transponder, even if the ADS-B fails (completely or partially). You should inform ATC that you have an ADS-B or GPS failure and they will inform you if you are cleared to continue or not.

Really though as far as this is concerned, creating an actual failure is very much not recommended. If I were you, I'd just say "let's pretend that you have a RAIM message about your WAAS, what are your new minimums?" Creating an actual failure is something like pulling the bulb out of one of the gear indicators. It might be a "teachable" moment at first, but really just creates a distraction that can be done in other ways. What happens if the circuit breaker fails and the weather actually drops? Now you have a real problem...

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  • $\begingroup$ In fact the transponder will not only be still a Mode C transponder but even a Mode S transponder ‘cause neither of those is depended of ADS-B or ADS-C. $\endgroup$
    – pcfreakxx
    Commented Dec 20, 2019 at 8:15
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Interpreting your question more broadly along the lines of "would I be non-compliant with the ADS-B Out Rule":

RTCA's DO-260, the standard that defines requirements for 1090ES ADS-B Out and In systems, states the following in section 2.2.3.3.2.12:

The ADS-B Transmitting Subsystem shall terminate broadcast transmissions of the Airborne Position Message when position (latitude/longitude) and altitude data are not available for a period of 60 seconds.

Note: For the Airborne Position Message, altitude data alone is sufficient to maintain broadcast of the Message once the Message has been initiated. When only altitude data is available, the Airborne Position Message continues to be transmitted even after 60 seconds. However, if the altitude data is not available for 60 seconds, then the Airborne Position Message transmission is terminated and the conditions for start-up defined in §2.2.3.3.2.1.2 are necessary to resume the transmission of Airborne Position Message.

Similarly, for the velocity messages:

The ADS-B Transmitting Subsystem shall terminate broadcast transmissions of the Airborne Velocity Message if input data necessary to update the subfields of the Airborne Velocity Message [...] is not available for a period of 2.6 seconds.

Interestingly, however, there are additional messages (e.g. status messages, identification messages, etc.) that will continue to be broadcast even in the absence of usable position information:

The ADS-B Transmitting Subsystem shall not terminate broadcast transmissions of Aircraft Identification and Category Message even if input data necessary to update the Message is not available.

Comparing this against the text of the ADS-B Out rule, §91.225:

§ 91.225 [...]

(b) After January 1, 2020, and unless otherwise authorized by ATC, no person may operate an aircraft below 18,000 feet MSL and in airspace described in paragraph (d) of this section unless the aircraft has equipment installed that

(1) Meets the performance requirements in [...] TSO-C166b [...]

(2) Meets the requirements of § 91.227. [...]

(f) Each person operating an aircraft equipped with ADS-B Out must operate this equipment in the transmit mode at all times [...]

So, one could say that you have a properly certified ADS-B transmitter installed, you are operating it in transmit mode, and it is actually broadcasting messages; therefore you are in compliance with the rule.

That said, I am not sure I would risk my CFI ticket on this argument...

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Two points - 1. compliance with 91.225 requires performance that meets TSO-C166b - that TSO invokes DO-260B. Earlier versions of ADS-B Out do not comply. 2. Compliance with 91.227 requires broadcasting (approx.) 18 message elements, including position and velocity.

So, while the OP may be compliant with the equipment requirement of 91.225, he/she would not be compliant with 91.227.

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