There seems to be an option in the GTX 330 settings that keeps to unit from powering on when receiveing power from the power system.

I flew in two different airplanes and one immediately powered on the transponder, the other one keeps the transponder turned off, until I press ON.

Looking thru the Pilot's Guide and Installation Manual didn't give me any clue.

Can someone help? Seems like an undocumented feature.

  • $\begingroup$ In the manual I see the device does have the OFF button on the panel. Have you tried to switch it off with that button before removing the external power? After powering up, it would be logical for the device to return into the state where it was when the power was removed. $\endgroup$
    – h22
    Jul 21, 2018 at 13:34
  • $\begingroup$ It turns on automatically when a certain altitude has been reached. I try to keep it also from that behaviour. $\endgroup$
    – David
    Jul 21, 2018 at 13:36
  • $\begingroup$ @David It doesn’t turn on automatically when takeoff is sensed, it changes from GND to ALT. The FAA recommends that you always be in ALT mode—even when on the ground—since many airports can now track you on the ground. There are settings that let you configure it to always be in ALT mode. You are required to be transmitting whenever you are in the air, so why would you want to turn it off? $\endgroup$
    – JScarry
    Jul 21, 2018 at 14:29
  • 1
    $\begingroup$ I don't know, but I suspect it's a question of how the unit was installed. The manual says (emphasis mine) "The GTX 330 transponder is powered on by pressing the STBY, ALT or ON keys, or by a remote avionics master switch (if applicable)." It seems possible that one aircraft has it connected to the avionics master and one doesn't. $\endgroup$
    – Pondlife
    Jul 21, 2018 at 15:51
  • $\begingroup$ You might get a more useful answer if you can explain why you want to do this. And if the unit is on its own circuit breaker, you can always just pull that out. $\endgroup$
    – Pondlife
    Jan 27, 2021 at 17:24

1 Answer 1


If pin number 1 on the back of the GTX330 unit is connected to power (+), the unit will turn on. If it is not, you will have to manually press the on button. If you remove power from pin 1, it will not turn it off.

See the installation manual, which is findable by a simple Google/Bing search, page D-1, figure D-1, Note 3.


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