Two Avionics Masters

[petrolero]

I've never understood how this is possible...

My airplane has two avionics master switches. Each one seems to work completely independently from the other. That is, flipping one on while the other is off still turns everything on. And vice versa... same thing.

One switch is on the side breaker panel and the other was obviously installed at a later date.

I asked my avionics shop about this when they were installing my GTN-750 and I don't think they ever quite answered it to my understanding.

What's going on there? I haven't spent a lot of time researching this because it ain't broke, but I've always wondered.

 

[Greg Bockelman]

They are wired in parallel. If one switch fails, you still have the ability to power the avionics buss through the other switch. Not common, but I have seen it before.

 

[petrolero]

They are wired in parallel. If one switch fails, you still have the ability to power the avionics buss through the other switch. Not common, but I have seen it before.

That makes sense now that I think about it. thanks

 

[James331]

Two different avionics busses?

 

[dell30rb]

They are wired in parallel. If one switch fails, you still have the ability to power the avionics buss through the other switch. Not common, but I have seen it before.

I have seen these before as well. One is "Master" the other "Emergency Master"

You could switch the labels and never know, they both do the same thing.

 

[Non Compos Mentis]

The airplane I fly has this.

I'd hate to loose all avionics at once because a silly switch failed. If so, flip the other, and all is right with the world.

Just simple redundancy. Two nav/coms, HSI plus back-up VOR, standby vacuum pump, not to mention hand-held nav/com & GPS.

 

[Stewartb]

Backup? How many times have you had a switch change state while not changed by you? If the switch fails won't it happen on the ground when you try to change its state? Redundant aviation masters is a little nuts. I know some of you guys have them. If you were designing your dream panel would you do it again?

 

[petrolero]

Backup? How many times have you had a switch change state while not changed by you? If the switch fails won't it happen on the ground when you try to change its state? Redundant aviation masters is a little nuts.

Switches can wear out.

In that case, rather than change state uncommanded, I would imagine that it would fail to change state when commanded. Likely on the ground.

The switch on the breaker panel is a rocker made of plastic but doesn't seem fragile at all. The other switch under the center stack is a beefy metal toggle - that's the one that the previous owner said he used 100% of the time. The previous owner was not the one who had it installed, however.

Hard to imagine either one failing, but someone felt the need to install that extra switch.

 

[Non Compos Mentis]

How many times have you had a switch change state while not changed by you?

Never- Yet.

Redundant aviation masters is a little nuts.

If I'm in the clouds, hard IFR, do I want to take a chance on loosing all communications, GPS, VOR, DME, and my wonderful moving map all at once because one little cheap switch overheated?

If you were designing your dream panel would you do it again?

Why not? The "main" switch is right next to the avionics stack, the second is hardly noticeable on the far right lower side.

I've never had a vacuum failure- Yet. Should I toss out the standby vacuum pump?

For a VFR-only airplane, maybe no big deal. In the clouds, I want all the help/back-ups/redundancy possible.

 

[Jim Rosenow]

Our '86 R182 has an avionics master on the side panel with the breakers, and a standard split battery/master switch lower left on the main panel. The avionics master on the side panel does nothing without the split master on first. Based on a half dozen R182's I've seen, I think that's standard for the breed.

We power up the 'main' master, start up, and then when everything's settled down current-wise, fire up the avionics master. Eliminates spikes to the good stuff.

Jim

PS- Just looked it up in the Information Manual. The schematic says the main master powers the main bus, and the avionics master the...wait for it....avionics bus, including the a/p, and HSI. The avionics master is actually a jumper between the main bus and the avionics bus. To the OP...I'd be looking for a 337 that explains the variance from the TC.

I've never understood how this is possible...

My airplane has two avionics master switches. Each one seems to work completely independently from the other. That is, flipping one on while the other is off still turns everything on. And vice versa... same thing.

One switch is on the side breaker panel and the other was obviously installed at a later date.

I asked my avionics shop about this when they were installing my GTN-750 and I don't think they ever quite answered it to my understanding.

What's going on there? I haven't spent a lot of time researching this because it ain't broke, but I've always wondered.

 

[JimNtexas]

have seen these before as well. One is "Master" the other "Emergency Master"

You could switch the labels and never know, they both do the same thing.

Our club's Archer is wired with two avionic master switches. I think somewhere along the line a pilot had the avionics master fail, which turned off all blinky lights.

 

[petrolero]

Our '86 R182 has an avionics master on the side panel with the breakers, and a standard split battery/master switch lower left on the main panel. The avionics master on the side panel does nothing without the split master on first. Based on a half dozen R182's I've seen, I think that's standard for the breed.

We power up the 'main' master, start up, and then when everything's settled down current-wise, fire up the avionics master. Eliminates spikes to the good stuff.

Jim

PS- Just looked it up in the Information Manual. The schematic says the main master powers the main bus, and the avionics master the...wait for it....avionics bus, including the a/p, and HSI. The avionics master is actually a jumper between the main bus and the avionics bus. To the OP...I'd be looking for a 337 that explains the variance from the TC.

I have the split (red) main aircraft master switch in addition to the two avionics masters. Neither avionics master will energize the avionics bus without at least the battery half of the main (red) aircraft master switch ON.

The 337 explains and authorizes what was done, but doesn't explain why.

 

[Jim Rosenow]

I have the split (red) main aircraft master switch in addition to the two avionics masters. Neither avionics master will energize the avionics bus without at least the battery half of the main (red) aircraft master switch ON.

The 337 explains and authorizes what was done, but doesn't explain why.

At the risk of boring everyone else...what's it say?

Jim

 

[kontiki]

I'm kind of curious what it shows in your POH and maybe even the applicable AMM.

When folks go jungle and rewire things they may not think it through. My airplane didn't originally have an avionics master from what I see in the POH or the AMM diagrams. I can't see how it would affect any of the published emergency procedures and from a failure mode analysis standpoint, I don't see any new critical failure modes introduced, so I'm OK with it.

From the history I'm guessing it was done when the GPS system was installed, however those installation records are thin. The 337 was written to conform a previous installation, and I don't have a copy of the Garmin installation instructions they conformed it to. I suspect it would not show where the system draws it's power, beyond some reference to a breaker or wire size in any case.

I've run across other wiring changes that I put back the way it's shown in the AMM because I do see potential for failure modes that could be a real safety problem.

 

[ahkahn]

I've never understood how this is possible...

My airplane has two avionics master switches. Each one seems to work completely independently from the other. That is, flipping one on while the other is off still turns everything on. And vice versa... same thing.

One switch is on the side breaker panel and the other was obviously installed at a later date.

I asked my avionics shop about this when they were installing my GTN-750 and I don't think they ever quite answered it to my understanding.

What's going on there? I haven't spent a lot of time researching this because it ain't broke, but I've always wondered.

Yep, my plane has this too.

Interestingly enough, right after I got my PPL about 7 years ago, I flew a G1000 plane. Right before I left to come back home, fired up the plane, then fired up the G1000 and the darn thing was dead as a doornail. Screwed around with it for a few minutes, but could not get those darn screens to come on. Had to fly the entire plane on the backup gauges... was ****ed. Turns out it was the avionics master switch that had gone bad....

-Andrew

 

[flyingron]

Mine has a normal used relay to turn on the avionics bus, but there's also a "pulled" circuit breaker that you can press in to turn on the avionics if the relay won't close for whatever reason.

 

[Pegasus]

I have two switches, it eliminates a single point of failure. Cheap redundancy.

 

[kontiki]

Another thing that might be valuable to know is if the switch(es) are current protected or not. The one in my airplane has the number 35 stamped in the end of the toggle lever. I looked up the specs on the switch and found out out it's got internal current protection (a circuit breaker). I would have preferred a god mil spec switch and a conventional breaker, but I haven't found a spec for any simple toggle switches larger than 20A.

The problem with commercial parts is that the manufacturer can change the design at any time and the P/N will stay the same. The other problem is the theory that consumers regulate the quality of commercial parts. That is if it turns out to be unreliable, word gets around and folks stop buying it. In this application, I want a part that qualifies to a spec for so many cycles by design.

The other issue I had with mine is that if it trips, there is no popout indicator like most airplane breakers have and that 35 A is too big to protect the 12 Ga. Wire (per AC 43-13) that supplies power to my avionics bus. I also didn't think much of the way someone snaked it up between the instruments either, it should have been secured with other wires in a bundle.

I'm in the process of reviewing my installation, and working up a load analysis to see just how much the load is on that bus. I'm guessing I'm right around 20 A. They didn't put the CDI and autopilot on it either. Both radios, the GPS and the Aspen are on it though.


Sent from my iPad using Tapatalk

 

[Jim Rosenow]

On the R182, the avionics master switch is also the circuit breaker for the entire avionics bus. The information manual specifically notes that if it opens, try re-setting it once (only) and then "do not reset it". Shunting around that protection without knowing the cause is.....discuss.

2nd (back-up) switches...redundancy if the switch breaks?...I suppose. Any serious redundancy would take a lot of switches. IMHO...YMMV

What might be an interesting scenario to consider, based on the schematic, would be to wire a 2nd 'avionics master' directly from the battery to the avionics bus (suitably protected). It looks to my untrained eye that then, by shutting off the Gen side of the double rocker aircraft master switch, and shutting off the factory avionics master (which is just a pass-thru from the main bus) one could eliminate battery drain from anything on the master bus while powering the minimum avionics to complete a flight. This in case of an electrical issue during flight, of course. Seems awfully complicated...would be an interesting placard, tho!

If the OP's switch should happen be wired this way, none of the lights should work if the switches are placed in the configuration above.

I'm not an A&P or EE, nor did I stay at a motel of any sort last night.

Jim

 

[eetrojan]

Does the second redundant switch ground a single avionics bus relay, or does it ground a second redundant relay?

 

[petrolero]

Does the second redundant switch ground a single avionics bus relay, or does it ground a second redundant relay?

I don't know for sure.

As it happens now, if one or both switches is "on" then the bus is energized.

That seems consistent with the switches' being wired in parallel but it may also be consistent with what you suggest too - not sure. Would it behave the same way if they both operated one relay?

 

[eetrojan]

I don't know for sure.

As it happens now, if one or both switches is "on" then the bus is energized.

That seems consistent with the switches' being wired in parallel but it may also be consistent with what you suggest too - not sure. Would it behave the same way if they both operated one relay?

I think it would behave exactly the same way if everything was working, but in my mind's eye, having the extra switch wouldn't restore the avionics power if there is only one avionics relay and that's what failed.

 

[Greg Bockelman]

I think it would behave exactly the same way if everything was working, but in my mind's eye, having the extra switch wouldn't restore the avionics power if there is only one avionics relay and that's what failed.

Which would sort of defeat the purpose of a redundant switch, wouldn't it?

 

[eetrojan]

Which would sort of defeat the purpose of a redundant switch, wouldn't it?

Not if you experience a bad switch rather than a bad relay.

However, assuming that a Cessna 172P and 172S are representative of the OP's 1979 Cessna TR182, the avionics switch doesn't appear to drive a relay.

In both the P and S-model, the "avionics switch" directly connects the avionics bus to the primary bus.

Never mind...

 

[kontiki]

Not if you experience a bad switch rather than a bad relay.

However, assuming that a Cessna 172P and 172S are representative of the OP's 1979 Cessna TR182, the avionics switch doesn't appear to drive a relay.

In both the P and S-model, the "avionics switch" directly connects the avionics bus to the primary bus.

Never mind...

If it's a current protected switch (that is contains a built in trip free circuit breaker), no relay, and there is a short circuit, you have no indication.

A built in device may have opened to protect you from a potential fire and all you may know is that the switch position is now irrelevant, if you think to experiment in the confusion. Hopefully you are not flying at night or IFR.

So, if you now close a second switch, (that may or may not be current protected), you may have set yourself up for a more complicated emergency involving smoke or fire.

How your power circuits are connected and what devices protect you from over current conditions and which ones indicate a tripped condition and which ones do not could affect how you apply emergency procedures. Particularly if it was someones bright idea to make it better than original. Most hackers do not perform a good failure effects mode analysis. They get something to work and feel like a genius.

 

[flyingcheesehead]

Interestingly enough, right after I got my PPL about 7 years ago, I flew a G1000 plane. Right before I left to come back home, fired up the plane, then fired up the G1000 and the darn thing was dead as a doornail. Screwed around with it for a few minutes, but could not get those darn screens to come on. Had to fly the entire plane on the backup gauges... was ****ed. Turns out it was the avionics master switch that had gone bad....

What kind of plane? The G1000 isn't supposed to be completely wired through the avionics master. Normally, both displays should come on when the master is turned on, and only a few extra things like the #2 Nav/Com/GPS and the XM datalink come on with the avionics master.

What engine indications did you have without the G1000? Was your flight even legal?

 

[kgruber]

On most modern avionics installations, pulling the avionics circuit breaker puts power to the avionics bus bar. No need to power up with the avionics power switch, as it gets completely bypassed. Beech....Mooney


Great backup!

 

[Greg Bockelman]

On most modern avionics installations, pulling the avionics circuit breaker puts power to the avionics bus bar. No need to power up with the avionics power switch, as it gets completely bypassed. Beech....Mooney


Great backup!

Hmm. Having a hard time wrapping my brain around this. Seems like pulling a breaker is supposed to REMOVE power from the bus, not connect it. What am I missing? What is the purpose?

 

[kgruber]

Hmm. Having a hard time wrapping my brain around this. Seems like pulling a breaker is supposed to REMOVE power from the bus, not connect it. What am I missing? What is the purpose?

On most newer aircraft.....even old, but good installations.....pulling the avionics CB removes power from a relay that closes and powers up the avionics bus. Purpose.....emergency DC power to the avionics bus.

Tom Clements explains it MUCH better(as usual!)

http://www.beechtalk.com/forums/viewtopic.php?f=21&t=51252