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In almost every light aircraft I've ever flown, I recall their being a rocker switch marked "Emergency Avionics". Here's an example:

enter image description here

It's never really occurred to me to ask an instructor/owner what it does nor when to use it. Can somebody here explain what it does exactly?

Also, it appears to always be an aftermarket addition (the switch never quite matches the rest of the panel), could there perhaps be a reason why this second avionics master was added?

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  • $\begingroup$ My light aircraft doesn't have a switch like that... $\endgroup$ – Ron Beyer Feb 25 at 13:21
  • $\begingroup$ I have no such switch in my plane. I do however have 2 Avionics Master switches wired in parallel, either will power up the Avionics power buss. They were installed in place of an Avionics Master relay (which was removed) that could potentially fail closed and let high, spikey voltage from the alternator onto the buss during startup (or engine shutdown) and damage the avionics. I turn one of them on after the engine is started, and back off before the engine is shut down. $\endgroup$ – CrossRoads Feb 25 at 13:25
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    $\begingroup$ In what type of aircraft was this picture taken? Between aircraft with avionics from steam to Garmin, Avidyne, and Dynon, this is the first time I’ve seen an “Emergency Avionics” switch. $\endgroup$ – Dean F. Feb 25 at 13:41
  • $\begingroup$ @DeanF. This is a PA28, and I've certainly seen it in other PA28's. Don't recall seeing one in C150152/172's but haven't flown one in a number of years. $\endgroup$ – Jamiec Feb 25 at 13:59
  • $\begingroup$ I Have flown various Archers, Cherokees, and Warriors. I wonder if this was specific to a certain run of model years. Any clue? $\endgroup$ – Dean F. Feb 25 at 14:06
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A quick google will turn up some answers. The consensus seems to be that it is just another switch in parallel with the Avionics Master Switch, such that, if the master switch fails, the "emergency" switch can be used to connect the juice instead! The point being that the master switch is a single point of failure for the entire avionics suite (radios, transponder and nav) - if that switch fails in flight, you lose everything!

EDIT:

From the Piper Dakota (PA-28-236) Maintenance Manual:

Electrical power for various avionics components is controlled by an avionics master switch located near the top of the instrument panel between radio stacks. It controls power to all radios through the aircraft master switch.

An emergency bus switch is provided to supply auxiliary power to avionics bus in event of a radio master switch circuit failure. The emergency bus switch is located behind lower right shin guard, to left of the circuit breaker panel.

enter image description here

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    $\begingroup$ This is largely unfounded and likely incorrect (insofar as this serves as the backup for the master avionics switch as a component). $\endgroup$ – verandaguy Feb 25 at 15:47
  • $\begingroup$ The Dakota POH you referenced says that the Master and Alternator switches are mechanically interlocked. Could the Emergency Avionics switch be in case you have to disengage the alternator due to excessive amps or voltage? In which case, you would still be able to manually select battery power to the avionics when the Alternator/Master combo switch were turned off. $\endgroup$ – Dean F. Feb 26 at 0:50
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I have not encountered this personally myself, but a Google search indicates that the emergency avionics switch is usually used in setup where a relay is used to control the power to the avionics bus. If the relay were to fail the emergency avionics switch would bypass it. Relays allow the reduction of high current wiring having to run to the panel with a high current switch. Instead the switch is a low power switch which drives a relay which would be located closer to the actual avionics bus so not as much wiring would be needed. More high current wiring increases the risk of hazardous electrical shorts and electrical fires, however mechanical relays do fail eventually. So the emergency avionics power switch will bypass the mechanical relay in case it fails.

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  • $\begingroup$ That seems to be self-defeating. You have to run the high-current wire to the emergency switch anyway, so all that does is add one low-current wire. $\endgroup$ – HiddenWindshield Feb 26 at 0:34
  • $\begingroup$ @HiddenWindshield: I agree which I think is why most planes don't have this configuration. There are places where a relay would make sense, like the starter circuit, but I don't think a relay on the avionics bus makes much sense. $\endgroup$ – DLH Feb 26 at 16:51

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