Avionics Emerg Switch

[TheTurtle]

Anybody know what this does?  I'm in the process of purchasing and wondered what it is.  I searched around but dont see exactly that switch.

Its in the bottom right in the attached picture.

[Hank]

That would be an unusual place for an ELT switch . . . Mine is near the right edge but up high in plain sight, not hidden at the edge of the panel behind the yoke.

[TheTurtle]

i asked the owner if it was elt and he said it was some kind of backup avionics bus or something, he wasnt sure.

[Vance Harral]

That looks like a backup avionics master switch, given that it's immediately adjacent to the regular avionics master.  The idea is that two switches are wired in parallel, and either one can enable the avionics bus.  If one switch fails, the other one may be used to keep the power on.  Typically used in an installation where a high-current switch directly connects power to the avionics bus (as opposed to systems where the avionics master is a low-current device that controls a relay).

Edited July 24, 2016 by Vance Harral

[TheTurtle]

great info thanks. 

[211º]

If you purchase that aircraft, I'm curious to hear about your use/integration of the Brittain AP. Mine appears identical - especially if there is a "Pitch pull-on" button somewhere else.

Sent from my iPad using Tapatalk

[TheTurtle]

its in escrow now so I hope everything works out so I can let you know

[carusoam]

Great questions for the PPI to review.  The electrical drawings should be updated to include the various switches. A separate switch to turn the radios on is greatly appreciated. A back-up switch to power the bus is interesting, but not usual.

Vance explained the built in safety using the relays pretty well.

Best regards,

-a-

[jetdriven]

The radio master switch applies 12v to the avionics master relay.  The normal, spring loaded position of this relay is closed.  So if the switch fails the relay depowers and turns on the avionics.  

[flyboy0681]

19 minutes ago, jetdriven said:

The radio master switch applies 12v to the avionics master relay.  The normal, spring loaded position of this relay is closed.  So if the switch fails the relay depowers and turns on the avionics.  

True statement as we are experiencing this right now, and have in the past. Regardless of the radio master position, the circuit stays open. We had the relay cleaned about two years ago and the problem started up again last month. Oddly, the first indication of trouble is when the main master is turned on, the KAP 150 autopilot breaker pops.

[Vance Harral]

 

On 7/25/2016 at 7:55 AM, jetdriven said:

The radio master switch applies 12v to the avionics master relay.  The normal, spring loaded position of this relay is closed.  So if the switch fails the relay depowers and turns on the avionics.  

Careful, not all avionics master switch installations use relays.  In older airplanes, sometimes an avionics shop will simply wire a large, high-current switch (or two) directly between the main bus and the avionics bus.  There are pros and cons to this approach vs. using a relay.  I won't get into that debate right now, but if you're debugging an avionics master issue, you really need to get behind the panel and see what the switch is connected to.  There may or may not be a relay.

[tony]

When I was designing my panel update,  incorporated two avionics master switches in parallel as well.  I saw the switch as a single point failure that was easily mitigated.  

[carusoam]

The technical advantage of the relay is... it can quickly close the switch to avoid sparking and melting of the hand thrown switch.  

The risk of sparks and melting gets greater with age (of the switch) (not the user )

Include this detail when balancing the single point of failure argument.

PP ideas only, not an electrician...

Best regards,

-a-

[Vance Harral]

13 minutes ago, carusoam said:

The technical advantage of the relay is... it can quickly close the switch to avoid sparking and melting of the hand thrown switch. 

All switches designed for high current and/or inductance are spring-loaded, whether they're relays or hand-operated switches.  The typical non-relay avionics master switch has the same "quick" open/close behavior as an avionics master relay.

[Vance Harral]

9 hours ago, tony said:

When I was designing my panel update,  incorporated two avionics master switches in parallel as well.  I saw the switch as a single point failure that was easily mitigated.  

Just make sure you actually use the "backup" switch from time to time.  If you don't, the contacts tend to oxidize, such that the backup will fail you when you really need it.

Some folks argue a single DPST switch is a better solution than two SPST switches for exactly this reason: both sets of contacts are exercised every time the switch lever is operated.  It's a good theory, but you're unlikely to notice when/if one set of contacts goes.  That effectively puts you back to having a single SPST switch.