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On the PA-28-181 that I fly, there's a circuit breaker called Audio/Mkr. What is that? In the aircraft, there's also a dual GNS430 if that makes it any more specific. I was thinking it could be controlling the speaker on the aircraft?

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  • $\begingroup$ The electrical diagram in your POH should identify exactly the equipment plugged into that breaker if it was from the factory. What year PA28 do you have? They weren't great with manuals in the early years... $\endgroup$ – Ron Beyer Mar 30 '17 at 12:01
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This circuit breaker is responsible for your Audio Control Panel. It's called Audio/Mkr. because the Audio Control Panel in most aircraft includes the Marker receiver and displays (maybe you noticed the three colored lamps on the left side reading O, I and M, colored white, blue and orange).

If the circuit breaker is out you loose your audio on the one hand side, on the other side you also loose your ability to identify Marker Beacons (functionality explained by Carlo).

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It's the circuit breaker, most likely, for power from the avionics master bus to the audio panel as well as the marker beacon receiver.

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  • $\begingroup$ What's a marker beacon receiver? $\endgroup$ – nyorkr23 Mar 30 '17 at 5:14
  • $\begingroup$ The marker beacon receiver used to be used for ILS approaches as a position cross-check, now it's essentially obsolete. Often built into the audio panel. $\endgroup$ – pericynthion Mar 30 '17 at 5:26
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    $\begingroup$ @nyorkr23 Marker beacons were (are) low power transmitters that transmitted a signal that causes a marker beacon receiver to beep as you pass over it. There were outer markers, middle markers, and inner markers in the order of their positioning along the ILS approach path, each marker transmitting beeping of a different sound and rapidity. There were a lot more outer marker installations than middle, and a lot more middle than inner. ILS installations that had all 3 were relatively rare. I encourage you to post your comment question as a regular question for a more detailed answer. $\endgroup$ – Terry Mar 30 '17 at 6:37
  • $\begingroup$ @nyorkr23 Here's a link to the Garmin GMA 340 Audio panel/marker beacon receiver. You should have something similar at the top of your radio stack. $\endgroup$ – Gerry Jan 15 '18 at 19:22

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