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In the US, would FAR 91.227, or any other regulation or law, forbid use of ADS-B-out in an unregistered ultralight? Could the N-number field simply be left blank, without violating any regulation or law?

What about a transponder with no ADS-B-out?

One possibly relevant detail: FAR 91.1(e) exempts ultralight vehicles from all of Part 91.

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It looks like using ADS-B without any identifier would be contrary to 47 CFR 87.107(a), which requires all aircraft transmitters (with some exceptions, which don't include ADS-B) to include an identification, usually the registration.

§87.107 Station identification.

(a) Aircraft station. Identify by one of the following means:

(1) Aircraft radio station call sign.

(2) The type of aircraft followed by the characters of the registration marking (“N” number) of the aircraft, omitting the prefix letter “N.”

[...]

(3) The FAA assigned radiotelephony designator of the aircraft operating organization followed by the flight identification number.

(4) An aircraft identification approved by the FAA for use by aircraft stations participating in an organized flying activity of short duration.

14 CFR 91.227 does allow for a "TSO-C154c self-assigned temporary 24-bit address" rather than the actual N-number but that part doesn't apply to ultralights anyway. It appears to be a reference to 87.107(a)(4) above.

Transponders are exempt from the requirement to broadcast identification, per 47 CFR 87.107(d).

Based on all that, and unless there's an FAA regulation about it (I couldn't find one), I'd say that a transponder is allowed on an ultralight (if you can operate it within the weight limit) but ADS-B is not.

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One of the required datasets of an ADS-b transmission is it has to have either the aircraft callsign listed on the flight plan, or the aircraft registration number. An ultralight by definition would not have this since all Part 103 ultralights are unregistered. Transponders do not have the same requirements nor dataset transmissions. Although both ADS-b and transponders are even used for ground equipment at airports.

Per 14 CFR Part 91.227

(d) Minimum Broadcast Message Element Set for ADS-B Out. Each aircraft must broadcast the following information, as defined in TSO-C166b or TSO-C154c. The pilot must enter information for message elements listed in paragraphs (d)(7) through (d)(10) of this section during the appropriate phase of flight.

(1) The length and width of the aircraft;

(2) An indication of the aircraft’s latitude and longitude;

(3) An indication of the aircraft’s barometric pressure altitude;

(4) An indication of the aircraft’s velocity;

(5) An indication if TCAS II or ACAS is installed and operating in a mode that can generate resolution advisory alerts;

(6) If an operable TCAS II or ACAS is installed, an indication if a resolution advisory is in effect;

(7) An indication of the Mode 3/A transponder code specified by ATC;

(8) An indication of the aircraft’s call sign that is submitted on the flight plan, or the aircraft’s registration number, except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address;

(9) An indication if the flightcrew has identified an emergency, radio communication failure, or unlawful interference;

(10) An indication of the aircraft’s “IDENT” to ATC;

(11) An indication of the aircraft assigned ICAO 24-bit address, except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address;

(12) An indication of the aircraft’s emitter category;

(13) An indication of whether an ADS-B In capability is installed;

(14) An indication of the aircraft’s geometric altitude;

(15) An indication of the Navigation Accuracy Category for Position (NACP);

(16) An indication of the Navigation Accuracy Category for Velocity (NACV);

(17) An indication of the Navigation Integrity Category (NIC);

(18) An indication of the System Design Assurance (SDA); and

(19) An indication of the Source Integrity Level (SIL).

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    $\begingroup$ But aren't you quoting from something in FAR Part 91? $\endgroup$ – quiet flyer Jan 31 at 18:26
  • $\begingroup$ The bolded paragraph 8 suggests it may have ADS-B and use the temporary self-assigned address, no? $\endgroup$ – Jan Hudec Feb 1 at 13:58
  • $\begingroup$ I think this answer would be improved by providing a source for the quoted content-- $\endgroup$ – quiet flyer Feb 1 at 20:06
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    $\begingroup$ @Warner Conrad There does not appear to be a rule or regulation prohibiting it. There just does not seem to be a need for it. You can get authorization to fly into A, B, C, D, and E to the surface for an airport without the transponder or ADS-b. Whether this makes it easier to get authorization is a better question. But, it does not replace the need to get authorization since the aircraft would still be flying under Part 103. $\endgroup$ – Dean F. Feb 1 at 20:16
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One other thing to consider - many here are stating that ultralights "aren't" registered. This as I read from the EAA is incorrect.

It is true that ultralights don't register with the FAA, but ultralights can be registered through the voluntary registration system provided by the EAA. More than just issuing a registration number the EAA states the number should be on the ultralight's tail just like an "N" number. The EAA also states the registration as well should be used when making radio calls. The number however is not an "N" number but instead is an "E" number very similar to the "N" number format. Instead of keying up at a non-towered airfield with "Field name, Ultralight entering down wind for runway xyz, Field name" the pilot would state the ultralights manufacturer such as "Field name, Badland Echo 8743 Alpha Romeo, entering downwind for runway xyz, Field name".

Since there are transponders and ADS-b that are extremely small and even a few that use Bluetooth to connect to a phone or tablet, the question is would this be allowed and can the "E" number be used instead of the "N" number?

Add on to this, if the answer is that this would be allowed or isn't prohibited would an ultralight with such equipment be allowed to file flight plans and request flight following?

With the technology jump over the past 10 years, safety equipment like this has become smaller and cheaper. The same can be said for ultralights which are once again becoming popular because of their low cost. I need to add the the EAA will not just issue an "E" number on request. There are pilot training requirements with a CFI before the EAA will issue the registration. This to me is ideal and similar to how amateur radio operators have been self regulating for years. It's the right direction to go for ultralights since the FAA changed the rules for SLA aircraft and almost killed ultralights.

I can see this voluntary system of registration come into play along with the use of transponders, ADS-b, other safety equipment, allowing flight plan submission, and flight following a major benefit to ultralight pilots looking at upgrading to SLA or PPL. I'm certainly not advocating to allow ultralights to have the current flight restrictions changed. Those limitations should remain, but allowing any type of flying machine to use identification equipment helps everyone in the air as well as those that control airspace.

If you don't think so then I ask the question, why do all with the exception of the very smallest drones (UAV's) need ID systems now, but not ultralights?

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    $\begingroup$ "More than just issuing a registration number the EAA states the number should be on the ultralight's tail just like an "N" number."-- I have never heard of anything like this. Will have to look into it and educate myself. Thanks for contributing. $\endgroup$ – quiet flyer Feb 21 at 2:04

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