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The flight school I usually rent from is under the shelf of class C airspace. If you're going out to a practice area, the standard procedure is to contact approach control and get a squawk code, so they can better coordinate traffic. However, it's not required, since the practice areas are outside the class C.

At the moment, none of the school's planes have transponders*. One of the instructors said not to contact approach for now, because without a transponder, they wouldn't be able to do anything anyway and it'd just annoy them. On the other hand, during my last practice flight, I heard** them vector an aircraft around some unknown other aircraft. I suspect the unknown aircraft was me, which made me think that it would be better to talk to them after all. That way, I could (for instance) tell them my altitude, and they wouldn't have to vector an aircraft that was (say) 5,000 feet above me. But, back on the first hand, if I'm just a primary contact, they have no way of knowing which blip is talking to them, so maybe they'd have to vector the traffic anyway.

So, in that situation, is it better to contact ATC or not?

* Since this came so soon after the switch to ADS-B, and since it affects all aircraft at the flight school, I rather strongly suspect that they just got a bad batch of ADS-B transponders and had to send them back. That's just my theory, though, I don't know for sure one way or the other.

** I tuned in the approach frequency, even though I didn't contact them.

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    $\begingroup$ 'they have no way of knowing which blip is talking to them'. There are ways to correlate blips to aircraft on the frequency. For example by giving an unambiguous position report, or by making identifying turns. The blip can then be tagged with the proper identification. $\endgroup$ – DeltaLima Jun 15 at 10:46
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    $\begingroup$ I believe that unless the IFR traffic reported you in sight that they would still be required to vector the other aircraft to maintain lateral separation. Your verbal reporting of altitude is helpful to enhance their situational awareness, but unless they can verify on their scope the information is “for reference only”. And even if your mode C is working, if you aren’t talking and squawking you are unverified. This is the difference between “traffic 12:00, 2500 feet” and “traffic 12:00, indicating 2500 feet.” If you had no transponder the call would be “reporting 2500...” $\endgroup$ – Michael Hall Jun 15 at 15:51
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Every US controller I've ever heard from says they'd prefer to be talking to every aircraft in their airspace, period. This lets them know your intentions and allows them to move you around if needed, which saves them far more time than it costs giving you traffic advisories.

If you don't have a transponder, they can still tag your primary target with your tail number. Since you can't squawk or ident, they'll need a turn and/or an exact position for positive identification before they're allowed to do that. Once you're tagged, though, you look the same to them as someone with mode A only.

If you're practicing maneuvers, I suggest letting them know what to expect with something like "maneuvering at or below 3500 ft." That's a good idea even if you have mode C, but it's critical without it.

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They'd rather talk to you, even if they can't do much for you. If you report over XYZ at 4500 heading east, they can correlate that to a primary target & note that it's N1234S "over there". They may have to call you again if traffic is heading your way at 7,500' to confirm your altitude, but unless they're busy, that's no big deal. And if the targets are pointed at each other, it's a HUGE help for the controller to know he can call YOU, by callsign, and have contact.

During a TRACON tour a couple years ago, they played a tape of scope video + audio of a situation that got really tense because the controller couldn't reach to a VFR target & find out his intentions. Their plea was, call us, let us know you're there.

If the frequency & airspace are really busy with IFR traffic that's well away from you, then that's probably a different case. But that doesn't sound like what you're describing.

Easy enough to call them up, and when things are quiet, ask if they would prefer hearing from you each flight until the transponders get fixed. If they say no, go with that. But I suspect they'll be glad to have you listening on frequency.

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