Mode C question

Discussion in 'Pilot Training' started by Adamoh, Nov 18, 2015.

  1. Adamoh

    Adamoh Pre-Flight

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    What happens if you fly with your Mode C off or in standby in a Mode C area?
     
  2. BillTIZ

    BillTIZ Final Approach

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    MMM... Off, standby, on, altitude, test, is the normal switch positions for old ModeC transponders.

    So what are you really asking.

    if you leave it off, or standby, you are not transmitting, it's as if you have no transponder. Skin paint, primary radar return only.

    Turn it to "on" and they ATC, see your code, but not your altitude.
    "alt" gets code and altitude.
     
  3. BillTIZ

    BillTIZ Final Approach

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    If you departed from a Class D or higher airport, you may be asked to check transponder, before they let you change frequency.
     
  4. docmirror

    docmirror Touchdown! Greaser!

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    The radar service for the mode C area doesn't like it(typically approach/departure). Your lat and long location are determined by your mode A from the TXP but the altitude is determined from the azimuth reflection of the radar(s) in use. Some are better than others. You will be reported as 'traffic xxxx, altitude 2600 feet unverified', meaning they are not getting an altitude output from your TXP. If they know who you are, you may get asked to cycle the mode C, you may be asked to verify altitude, you may be asked to leave the mode C area, or you may be told to call this number.
     
  5. Adamoh

    Adamoh Pre-Flight

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    As I was departing class D they didn't let me know. They usually do when one of the rental airplane's transponder is faulty but not today. They told me when i called them up
     
  6. mkosmo

    mkosmo Pattern Altitude

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    Interesting. I thought unverified was a mode c return they hadn't been able to verify since nobody is talking with the target rather than an azimuth estimation.
     
  7. docmirror

    docmirror Touchdown! Greaser!

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    Just an FYI, not all class D airports are equipped with mode C interrogating equipment, and mode C is not actually required in class D airspace, providing the class D airport does not underlie a mode C area. An example would be Chino. Class D airspace there, however mode C is not required to operate from Chino. However, Chino may have altitude interrogating equipment, I don't actually know.
     
  8. docmirror

    docmirror Touchdown! Greaser!

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    I believe that is another valid example. If they are getting a mode C return, but are not talking to that traffic, it could be reported as unverified. the azimuth primary return is even less verified. :D
     
  9. Ravioli

    Ravioli Final Approach PoA Supporter

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    I'm curious about this.

    If I were to ever take off with the txp not set to ALT, it would probably be in STBY (purely hypothetical of course :))

    Tower politely reminds me to check it. So, in STBY they saw nothing, and in ALT they may see only mode A? That could mean that if it was set ON they would see the A but no C and neither of would know.

    Is this all part of the reasoning for the recent guidance to always leave the transponder set to ALT?
     
  10. roncachamp

    roncachamp Final Approach

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    In air defense radar systems but not in ATC.
     
  11. roncachamp

    roncachamp Final Approach

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    If your transponder selector is not in the ALT position the encoder will not send altitude information. If you're talking to ATC the controller will tell you he's not receiving altitude. You will then move your selector to ALT, and all will be right with the world.
     
  12. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    I've several times operated in the class B with no transponder at all. What was amusing was one time the primary radar was also out at IAD at the time, so essentially I was invisible to them.
     
  13. jordane93

    jordane93 Final Approach

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    Pretty much.
     
  14. docmirror

    docmirror Touchdown! Greaser!

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    Thank you, I stand corrected. Do you get primary return with any altitude info at all?
     
  15. roncachamp

    roncachamp Final Approach

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    Nope.
     
  16. Witmo

    Witmo Pattern Altitude

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    You're a bit confused (or I am). If your transponder has an "Alt" setting, by its selection, you are squawking both Modes A and C. There's no way you can only squawk altitude without also squawking mode A (the four digit code). You can squawk just mode A, ususally the "On" setting for the transponder. In Stby, you just power the unit but don't respond or transmit. The reason ATC always wants you squawking both Modes A and C is because they can fix your position in three dimensions vs. two. If they determine that your altitude readout is incorrect, they'll ask you to turn off mode C so that it doesn't confuse them or their computer.
     
  17. BillTIZ

    BillTIZ Final Approach

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    Where is the BS flag when you need it?
     
  18. Ravioli

    Ravioli Final Approach PoA Supporter

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    I don't think either of us are confused. Part of the thread was about equipment differences at the tower vs departure which I didn't know about.

    So in theory you could take off set to "On" and tower wouldn't notice since they see the mode A and Departure wouldn't see your altitude. That was the interesting part for me.
     
  19. roncachamp

    roncachamp Final Approach

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    If the tower has radar the typical difference in the equipment is the scope in the tower is usable in daylight while the scope in the radar room needs to be in the dark.
     
  20. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    And even the dark radar room is pretty much a historical thing. Many of the displays used these days will work just fine in normal room illumination (a room full of windows like the tower is a different story). But, controllers have this history of sitting in dark rooms. What I could never figure out is the guys in the FSS sitting in windowless dark rooms looking at their computer monitors. Crikes, it's not ATC and a window might help these guys ability to relay weather.
     
  21. Velocity173

    Velocity173 Touchdown! Greaser!

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    If tower gets a radar feed, then yes, they would notice. If you're on a discrete, you'll still tag up but with no altitude. Whether or not they care or have time to tell you to squawk altitude is a different story.

    Departure cares because they need to validate your altitude in order to provide vertical separation in airspace that requires vertical separation. In order to do that, they either need to see your altitude on the ground be less than 300 ft of the elevation or you report to them in flight and it's less than 300 ft from what you report. They can still separate you vertically without mode C but it makes it a pain on their end.
     
  22. roncachamp

    roncachamp Final Approach

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    I think there are still plenty of radar rooms where turning the lights on turns the radar display into a big yellow-orange disk.
     
  23. Velocity173

    Velocity173 Touchdown! Greaser!

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    Chattanooga for one. Still use old Amber displays. ASR-8 too! Every PAR I've seen doesn't do well in light either.
     
  24. Jaybird180

    Jaybird180 Final Approach

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  25. James331

    James331 Ejection Handle Pulled

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    You're not saying they'll try to do the "possible pilot deviation..." because of a transponder readout, which for all they know could be a unknown issue for the PIC.
     
  26. AKBill

    AKBill En-Route

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    So I live in Alaska and have not had to use my mode C for years. I think Anchorage is the only airport in Alaska that requires mode C.

    Juneau is my home base class D. Every so often I hear tower tell some one traffic 3 miles to the South, or something like that. Is the tower picking up mode C to report traffic? I think you need to be above 10,000 feet to be picked up by the nearest radar.

    Second question is when operating at a Mode C airport do you turn your transponder to standby on the ground, taxi for departure, then turn to alt after departure?
     
  27. Jaybird180

    Jaybird180 Final Approach

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    See my link above. I almost got murdered over it.
     
  28. bobmrg

    bobmrg En-Route PoA Supporter

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    You leave it ON all the time unless told by ATC to switch to STBY. AIM 4-1-20(a)(3).

    I learned to fly out of the small boat harbor at JNU.

    Bob Gardner
     
  29. James331

    James331 Ejection Handle Pulled

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    Yikes!

    That was in the SFRA though, I wouldn't think you'd have received the same treatment had it been in airspace outside of the SFRA


    That's just the AIM though, not a regulation.

    Mine goes to stand by automatically for ground ops and ALT after I takeoff, not worth reprogramming it over a AIM recommendation IMHO.
     
  30. Jaybird180

    Jaybird180 Final Approach

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    Then don't bring it into the SFRA. You're required to keep it reporting Mode-C from landing until you reach parking.
     
  31. James331

    James331 Ejection Handle Pulled

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    I avoid airspace like that like the pleauge, same deal with LAX, NYC, etc.

    Landed in KLAS once, taxiing out with a plane that had the same type of transponder, ground asked me to cycle the transponder, oops, hit the button, that was that.
     
  32. PPC1052

    PPC1052 En-Route

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    "Cessna 1234, check your transponder, I am not getting a signal."
     
  33. PPC1052

    PPC1052 En-Route

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    Yeah, but wasn't that in the DC SFRA and/or FRZ? That's a little different.
     
    Last edited: Nov 20, 2015
  34. Jaybird180

    Jaybird180 Final Approach

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    Really? I almost got blasted to bits when I was TALKING to the controller, he knew who I was AND I was DEPARTING the SFRA!!!!

    I followed the procedure to the letter (and to the best of common sense).

    and keep in mind, I'm also a FRZ vetted pilot and they knew this. When I call to file (or get squawk), my number pops up on Caller ID and they already have it associated with my aircraft ID and Pilot profile. They KNEW with 99.9999999% certainty that I was no threat.
     
  35. petrolero

    petrolero Pattern Altitude

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    Mine goes to "ground" mode based on speed and squat switch which means it will respond if interrogated I believe.
     
  36. dukeblue219

    dukeblue219 Line Up and Wait

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    Can you reference that? I don't recall it from my SFRA course nor could I find anything online. Obviously you keep the discrete code until shut down and never use 1200, but I'm 95% sure our GTX-330 goes to "ground" automatically on landing. Ground is different than standby in that it still responds with some mode s data, but I believe it does not send out the mode c altitude number.

    Given the enormous number of transponders that automatically switch between ground and altitude modes, I'd think this rule would have to be more prominently emphasized for SFRA operations or we'd hear about busts far more often.
     
  37. Jaybird180

    Jaybird180 Final Approach

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    Only citation I have to offer is the requests from Potomac to "keep the code on the ground."
     
  38. BillTIZ

    BillTIZ Final Approach

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    Airport now uses radar to track aircraft while taxiing. Keep transponder on at all times.
     
  39. James331

    James331 Ejection Handle Pulled

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    Right now I don't fly to any airports which require it
    If I go into one I'll hit the button after I touch down to keep it on. Come to think about it, I'll have to double check if mine goes to standby or ground automatically.