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Thread: Transponder altitude question on garmin 327

  1. #1
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    Default Transponder altitude question on garmin 327

    Sorry if these are dumb questions.....Not very educated on transponders.....looking for some clarification/education.

    I hooked up my used garmin 327 to my used MGL oddessey. Quadruple checked the wiring and all is good......A question I have is, the pressure altitude on the Garmin 327 is based on 29.92, so unless the pressure in my area is 29.92, it's not going to display my actual elevation of 250' on the pressure alt display?

    What the EFIS has displayed when I adjust for field elevation or local pressure is not going to match the pressure alt display on the garmin? Unless everything is 29.92?

    So an example is today, I set the EFIS to 29.92 and it shows zero as my elevation. (actual pressure was 30.19) My garmin displayed 0 ft pressure alt. So this seems like it matches. As I increase the Baro setting on the EFIS, it changes, but the Garmin pressure alt does not. Makes sense that this stays constant. But.... how sensitive is the pressure altitude supposed to be on the Garmin? Yesterday, pressure was 30.39 and my Garmin 327 was still displaying 0 ft. Today it is 30.19 and still showing 0 ft. Seems like the pressure alt should be changing with that much of of a difference? Or am I way off base here?

  2. #2
    Senior Member efwd's Avatar
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    Default Re: Transponder altitude question on garmin 327

    I could be mistaken but I believe that is by design. i.e. Just as when you reach 18000ft you put 29.92 in. Now all aircraft at 18k plus are using the same setting for good reason. I believe ADSB utilizes this same method for all of us on the system. Imagine if ATC was looking at their screen (info being fed to them from aircraft transponders not radar) and what they see is crappy reliable info that was incorrectly dialed in or not by the pilots.
    Eddie Forward
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    SS7, 912iS, Garmin G3X

  3. #3
    Senior Member Delta Whisky's Avatar
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    Default Re: Transponder altitude question on garmin 327

    From Flying Magazine: Mode C altitude transmissions are independent of the barometric altimeter. The transponder can get its information from one of two sources: an encoding altimeter, which transmits a pressure altitude reading to the transponder, or — more commonly — a blind encoder, an altimeter without needles or adjustment knob permanently set to 29.92 (pressure altitude). In either case, the altimeter setting does not affect the altitude the transponder sends. ATC’s computers apply the current altimeter setting to the pressure altitude received, converting it to msl (which should match your indicated altitude). Therefore, you cannot fool ATC by resetting your altimeter (of course, you could always fly the airplane a little higher or lower — that’ll fool ’em!). ATC will most likely ask you to “stop altitude squawk” if your indicated altitude and the altitude received by ATC differ by 300 feet or more. The transponder transmits pressure altitude to ATC in 100-foot increments.

  4. #4
    Senior Member 109JB's Avatar
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    Default Re: Transponder altitude question on garmin 327

    The GTX 327 has to be set up for the altitude input you are using. For example, I hooked mine up to my Garmin G5X using the serial output on the G5X to the serial input on the GTX 327. For this combination I had to change the altitude source. I can't remember exactly, but I had to set it to either "Shadin" or "Icarus" input. I believe it was Icarus. This is detailed in section 5.2.5 of the installation manual that you can find at this link,

    https://chapters.eaa.org/eaa245/-/me...90A075FC0.ashx

  5. #5
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    Default Re: Transponder altitude question on garmin 327

    Thanks all.

    I set up the Ikarus output on the MGL and the selected Ikarus input on the 327. I am using pin 19, the RS 232 in and it's turned on in the 327.

    What's kind of throwing me off is I can go into settings on the EFIS and adjust the altimeter calibration, and it will effect the 327 pressure ALT. As I go up, so does the pressure ALT.

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