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Avionics master needed?

snoop9erdog

Well Known Member
Hi All. I'm finalizing and wiring up all my Garmin boxes, plus I have IBBS with PFD,GTN625, ARINC, ADAHRS, EMS, and MAGnetometer on the IBBS, most which take minimal amps. Items in an emerg to get down. MFD is on the Garmin Gad 27 Keep Alive. G5 (another contingency) is on separate backup battery.

Startup procedure is to switch on the IBBS backup battery first to check that those items come up and check charge, then Master/Batt/Alt on, so with at least 14.4 V or more the systems runs off main buss. IBBS switch stays on and only kicks in backup once the main batt is rundown/depleted to a min voltage.

So the question is with that startup process is an avionics master even needed? My avionics master is essentially my backup battery switch that powers up most everything, minus my Comm1 because it has no secondary power feed.

Convince me that in 2023 an avionics master is needed. Id appreciate your thoughts
 
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I think the main purpose of an avionics master switch is to protect avionics from big spikes in voltage that can occur as the engine is starting or stopping. I don't claim to understand what in the system can cause those, but....

An older airplane I had for awhile had a Terra transponder with an internal fuse that would routinely blow if you shut the engine down with the avionics master on. I'm sure it's the case that the Terra transponder was an unusually poor device (a POS really) that was overly sensitive, and very likely that most modern avionics boxes have much better power supplies that are less likely to be damaged, but I still think it is good practice to be able to isolate those systems.

No doubt some EE guys will enlighten us as to why, or why not.
 
I like to have a avionics master to allow quickly shutting down a large segment of powered items and wiring that could be causing a smoke or fire event inflight.
 
If I turn the avionics master on after I start the engine it will be up to 30 seconds during EFIS boot-up before I can see the oil pressure after the engine has started, so I turn the avionics master on after the Master Switch before startup. I turn it off before shutting the Master Switch off after shut down. I don't know if that's necessary anymore these days in a diode-protected circuit with modern avionics, but I have been told that the big voltage spikes that might harm avionics are more likely to occur with the collapsing circuit as the Master Switch is turned off. Not that they will, only that they could. Used to be mantra, now I'm certainly prepared to believe that it's just legend.

I'm old school, but I guess I'd prefer a separate avionics master switch. It is on my list of things to ask Stein at the next Fairbault fly-in, however.
 
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My avionics shop is owned by a former Garmin engineer. I had this discussion with him when I was in the design phase. His opinion was that we install avionics masters just because we've always done it that way, not because they are necessary with todays avionics. He was adamant that there is 0% chance of modern Garmin avionics being damaged by being on during startup/shutdown.

He did recommend the ability to reboot anything with a processor in case it locks up, and with Garmin that's pretty much everything.

Based on those criteria, I elected to not have an avionics master. Just an IBBS for AHRS and engine sensors, then breakers for stuff I want to be able to reboot or load shed individually.

I will admit that I was initially resistant to the idea, but after thinking about it for a while I don't really see a downside and it simplifies the system design quite a bit.
 
No avionics master

I dont think a avionics master is needed for modern Glass. I have a Garmin panel with the GAD27 box. The box has a power circuit that prevents the critical boxes from browning out during engine start. But I have circuit breakers for the major boxes; they have been useful for hangar power ups when I dont need everything on.
 
Yes, it seems that the EMS (or EFIS with engine info displayed) is the one box that you want to power up with the main master, not on an avionics master. That way it is up and running for engine start.

If I turn the avionics master on after I start the engine it will be up to 30 seconds during EFIS boot-up before I can see the oil pressure after the engine has started, so I turn the avionics master on after the Master Switch before startup. I turn it off before shutting the Master Switch off after shut down. I don't know if that's necessary anymore these days in a diode-protected circuit with modern avionics, but I have been told that the big voltage spikes that might harm avionics are more likely to occur with the collapsing circuit as the Master Switch is turned off. Not that they will, only that they could. Used to be mantra, now I'm certainly prepared to believe that it's just legend.

I'm old school, but I guess I'd prefer a separate avionics master switch. It is on my list of things to ask Stein at the next Fairbault fly-in, however.
 
I agree with John. Modern avionics should be well protected against over-voltage, mostly because it’s so easy and cheap these days. I’d be more worried about low voltages - switching power supplies that stop switching, or fluorescent backlighter tubes that keep trying to strike back on - which a GAD 27 type device hopefully prevents.
I have a GRT EIS (engine instruments) box on the main buss, separate from the avionics buss. It does re-boot on engine starts but is so simple electronically that it re-boots in 2 seconds - and shows a big red light if there’s no oil pressure.
 
Here we go again, a quick search will give you some different opinions, personally I put avionics masters in all panels I build.
 
Why would anyone be one broken switch away from a dark panel in IFR?

What I do:
- Half the panel (one EFIS, one Comm, etc.) on one battery. The other half on the other.
- Either side can be powered from either battery - multiple backup modes.
- Engine startup is the left EFIS on (to get EMS data), the right EFIS is off, as well as the touchy GTN-650.
- Engine start is via a master solenoid on each battery feeding the starter solenoid. Easy.

Carl
 
An avionics switch is not needed. It is a matter of personal preference.
If you want one, install it. If you don't install it, then it can not fail.
 
KISS is good.
My version is 1 fuse panel (Bussman 15401) for everything (fuses & relays). No avionics switch. IBBS functions as the Essential Bus using the second power input on the Garmin LRU's.

Hopefully you will never need the IBBS as the sole providor of power in an emergency but one function that you will use every time you start is that it will power units during start if bus voltage drops below 11v. The GAD27 also has a "keep alive" circuit for the same purpose. This will keep units from rebooting due to low voltage during start.

I had two LRU's with no second power input I wanted on the IBBS. Easily accomplished by splicing both power wires together to form one input wire and a diode/fuse to keep from backpowering the bus from the IBBS. (see positions F39 & 28). The dual power LRU's perform this same function internally.
 

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Adding more complexity increases risk IMO, to each his own though, no need to rehash this again.

Walt, I'm confused. In post #9 you said you do put avionics master switches in the panels you build. Then in posts #11 and #13 you seem to imply ( KISS) that you don't? I must be misunderstanding when I think that 'simple' implies no un-needed switch?
 
Here’s Garmin’s position on the issue after I asked the question with respect to my G3X/GAD27/GTN build. I do have a 3 position master switch however (off/batt/batt+alt) so I can run the avionics on the ground with the engine off without causing alternator draw.

From: G3Xpert <[email protected]>
Sent: Monday, 17 October 2016 1:18 AM
To: Thomas ; G3Xpert
Subject: RE: Avionics master/bus

Hello Thomas,

It is not necessary to have these avionics devices powered off during engine start. As you know, the keep alive feed from the GAD 27 keeps the PFD/GEA24/GSU25 from resetting during engine cranking.

The attached page from Bob Nuckolls book explains that an avionics master is not required.
http://www.aeroelectric.com/Book/AEC_R12A.pdf

Thanks,
 
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My avionics shop is owned by a former Garmin engineer. I had this discussion with him when I was in the design phase. His opinion was that we install avionics masters just because we've always done it that way, not because they are necessary with todays avionics. He was adamant that there is 0% chance of modern Garmin avionics being damaged by being on during startup/shutdown.
Well we just had a Garmin 240 audio panel go TU when the pilot tried to start the engine (IO390) with a very weak battery. The voltage sag was low and long enough to kill the G240 and a few of the EMS sensor input locations. Just another situation where reality supersedes theory.

Here we go again, a quick search will give you some different opinions, personally I put avionics masters in all panels I build.
Agree 100%. IMHO You don't "need" something, until you need it. :cool:
 
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This!

And next, define “avionics”…… :)

Truly the first question to be answered. Definitely a fuzzy line these days.

I define as anything that is not flight critical therefore my “avionics master” is one switch that I can flick to get down to min power draw. Definitely not typical definition of avionics master but I have steam gage backups for everything flight critical so my avionics master is pretty much my glass stuff. This still brings lots of questions. Is a radio flight critical especially if you have some sort of malfunction happening? What if having engine issues does that make your engine analyzer flight critical so you can best monitor progression of issues with more than just oil pressure and temp gages? And many other questions
 
Walt, I'm confused. In post #9 you said you do put avionics master switches in the panels you build. Then in posts #11 and #13 you seem to imply ( KISS) that you don't? I must be misunderstanding when I think that 'simple' implies no un-needed switch?

Sorry for the confusion, it was in ref to Carls's method of "risk mitigation" by having dual busses/2 batteries with half a dozen contactors and associated switches to control it all, in which case you maybe could forgo the "avionics buss" because you added "buss 2" instead.

A lot of folks here IMO basically say it looks good on paper and in theory it should be fine, my experience, about 40+yrs worth, is from working on this shi*, take it or leave it, don't really care.

And just another thought which I've posted before, if aircraft manufacturers agreed that we didn't need all this avionics buss stuff and they could save a bunch of money by forgoing it, why does every certified aircraft still have them?

From a strictly personal perspective, if you think I'm going to take the risk of an unstable voltage damaging my 40K worth of equipment in my panel, you're mistaking. The Honeywell switches I use to power the avionics buss (no relays) are not cheap (the same switches we used in the B727 and most other airliners), but in all my years at the airlines I cannot remember seeing one fail, and most of them get operated multiple times every day/360, more use in a year then we will see in the lifespan of our simple little aircraft.

I also primed the inside of my entire aircraft with Akzo.
 
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Even Dynon mentions NO PROBLEM starting with it powered up at start, please
remember a lot of things are done same way as always , WITHOUT ANY VALID REASON !
 
If I turn the avionics master on after I start the engine it will be up to 30 seconds during EFIS boot-up before I can see the oil pressure after the engine has started,.

I would argue that as long as the engine was making good oil pressure at shut down, there is about a 0% chance it won't make pressure at the next start up. Nothing in that system can really fail while the engine is not running, with the exception of somehow draining the oil without your knowledge.

I turn the EFIS on after startup along with the avionics via an avionics master, as I have seen brown outs blow electronics in the past, though suspect my modern stuff can handle it more gracefully, but I am not willing to take the chance to save the .5 seconds it takes to flip that switch or the hour it took to add it during the build. Brown outs are nasty and create all sorts of bad things in electronic systems.

The EFIS is far more modern than the 430/530 is. I don't see how anyone would turn on a $10,000 device before an engine start. Just doesn't make sense to take that risk, no matter how small it is.

Larry
 
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And just another thought which I've posted before, if aircraft manufacturers agreed that we didn't need all this avionics buss stuff and they could save a bunch of money by forgoing it, why does every certified aircraft still have them?

Hey Walt, you know the answer to any aviation question about why things are always done a certain way is "because we've always done it that way!" ;)
 
Well we just had a Garmin 240 audio panel go TU when the pilot tried to start the engine (IO390) with a very weak battery. The voltage sag was low and long enough to kill the G240 and a few of the EMS sensor input locations. Just another situation where reality supersedes theory.

Agree 100%. IMHO You don't need something, until you need it. :cool:

And there you go, point made.
 
Hey Walt, you know the answer to any aviation question about why things are always done a certain way is "because we've always done it that way!" ;)

While there is a lot of that in aviation, forcing us to hang on to stuff we shouldn't, but a lot of it is acquired knowledge that shouldn't be forgotten. I forget who said it: "If you forgot history, you are destined to repeat it." The challenge is picking through it all to see what is obsolete due to new tech or methods and which are not.

Larry
 
Sorry for the confusion, it was in ref to Carls's method of "risk mitigation" by having dual busses/2 batteries with half a dozen contactors and associated switches to control it all, in which case you maybe could forgo the "avionics buss" because you added "buss 2" instead.

A lot of folks here IMO basically say it looks good on paper and in theory it should be fine, my experience, about 40+yrs worth, is from working on this shi*, take it or leave it, don't really care.

And just another thought which I've posted before, if aircraft manufacturers agreed that we didn't need all this avionics buss stuff and they could save a bunch of money by forgoing it, why does every certified aircraft still have them?

From a strictly personal perspective, if you think I'm going to take the risk of an unstable voltage damaging my 40K worth of equipment in my panel, you're mistaking. The Honeywell switches I use to power the avionics buss (no relays) are not cheap (the same switches we used in the B727 and most other airliners), but in all my years at the airlines I cannot remember seeing one fail, and most of them get operated multiple times every day/360, more use in a year then we will see in the lifespan of our simple little aircraft.

I also primed the inside of my entire aircraft with Akzo.

I'm gonna add to the bit of drift, Walt is Walt. We've been in both disagreement and violent agreement in the past. I respect it and will gladly buy him an adult beverage or two if I ever get to meet him. I like and appreciate his position of high quality (with relatively high current capacity) mechanical switch preference versus relays. I'll utilize it whenever I can.

KISS is an awesome approach. Sometimes it is beautifully disguised. What I believe is being missed from the wider discussion is the "E" portion of FMEA here. The two aforementioned positions are much closer than they would first appear. The Froehlich dual bus (specifically NOT main and Avionics buses) may appear to increase complexity but the extra cross-feed componentry does not increase any related failure effects. Parallel back-ups versus serial primary components for off-nominal conditions -> restoring redundant functionality, convenience, etc. They could be safely eliminated.

The dual architecture diagram could be redrawn without said cross-feeds. Replace the relays with Walt approved switches. Now its basically two independent, non-connected, operator action-free sources that power separate primary/back-up MMIs.

Perversely (key word with emphasis), it's a vacuum system with an electrical back-up arrangement from the steam 6-pack days. An elegant KISS, my brothers.

Certainly not expecting a lot of agreement on this one but at least consider the position. As previously stated, the positions might be closer than they would first seem.

Edit = "Walt is Walt" could come off wrong. Should have said he has a passion for his positions. No problem here.
 
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I would argue that as long as the engine was making good oil pressure at shut down, there is about a 0% chance it won't make pressure at the next start up. Nothing in that system can really fail while the engine is not running.
The EFIS is far more modern than the 430/530 is. I don't see how anyone would turn on a $10,000 device before an engine start. Just doesn't make sense to take that risk, no matter how small it is.
Larry

Not necessarily. Example; I had an old Continental A75 in my T-Craft. If that engine sat for an excessive period of time, the oil pump would drain back and loose it's prime creating 0 oil pressure at start-up.

Not saying that this is common or that is likely to happen. Just an example of; Things can happen.
 
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Primer War

I’m fairly new to the EAB community but have a lot of experience with flight electronics. I am amazed/horrified at the complexity of electrical systems in RV’s. Lots of circuit breakers and switches. In some cases the complexity and the limited space end up with electrical systems that are more likely damaged during maintenance and repair than they are in flight.

Modern electronics are pretty fault tolerant. AC 43.13 and even the AeroElectric Connection are pretty dated. The energy required with the new stuff is so small. The biggest power draw on my VFR airplane is the master relay.

When you are thinking about adding a control or redundancy you owe it to yourself to do some type of risk analysis where you assign a score to how likely the failure is and the consequence of that failure. Multiplying those two numbers gives you a relative idea of if it’s a problem or not. I think some of these controls could increase the pilots workload during an emergency.

I don’t think a significant voltage spike at startup or shutdown can happen. I couldn’t find a avionics manufacturer that warned about it.
Mike
 
Not necessarily. Example; I had an old Continental A75 in my T-Craft. If that engine sat for an excessive period of time, the oil pump would drain back and loose it's prime creating 0 oil pressure at start-up.

Not saying that this is common or that is likely to happen. Just an example of; Things can happen.

Ok, so what would be done differently if you had an OP gauge indicating 0 just after start up in this case? I am guessing you just sit there and wait for the pressure to come back. Struggling to see what else could be done about it. In that case, being advised of the low OP a few seconds earlier didn't change anything beyond letting you know that it happened. I am not suggesting the OP indication is not necessary, just saying that not having it for 15 seconds rarely creates issues.

Larry
 
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I don’t think a significant voltage spike at startup or shutdown can happen. I couldn’t find a avionics manufacturer that warned about it.
Mike

Please see Galin's post on the previous page and please re-assess your opinion. That was from a brown out, which is far more likely than a voltage spike during start up. Not sure why an avionics manufacturer would provide a warning when most every certified plane has an avionic master switch and a POH that says to have it off during start up.

I believe that most well designed modern electronics (modern designs not necessary recently manufactured) deals with brown outs pretty well. That is not the case for 20 or 30 year old architectures.

Larry
 
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Ok, so what would be done differently if you had an OP gauge indicating 0 just after start up in this case? I am guessing you just sit there and wait for the pressure to come back. Struggling to see what else could be done about it. In that case, being advised of the low OP a few seconds earlier didn't change anything beyond letting you know that it happened. I am not suggesting the OP indication is not necessary, just saying that not having it for 15 seconds rarely creates issues.
Larry

When oil pressure didn't come up within a few seconds, I shut down. I asked the mechanic on the field. He immediately knew the problem. I primed the oil pump, propped the engine again, had immediate oil pressure, and no damage.
 
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When oil pressure didn't come up within a few seconds, I shut down. I asked the mechanic on the field. He immediately knew the problem. I primed the oil pump, propped the engine again, had immediate oil pressure, and no damage.

Ok, fair point. I get it. Is this an issue with Lycomings? I have never heard about it before. On two overhauls, they seemed to self prime and produce decent pressure in about 10 seconds at cranking RPM.
 
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Not necessarily. Example; I had an old Continental A75 in my T-Craft. If that engine sat for an excessive period of time, the oil pump would drain back and loose it's prime creating 0 oil pressure at start-up.

Not saying that this is common or that is likely to happen. Just an example of; Things can happen.

This is why I have the "Low Oil Pressure" annunciator light powered off the main bus and driven by the oil pressure switch.

Also reminds me I left the master on....
 
Please see Galin's post on the previous page and please re-assess your opinion. That was from a brown out, which is far more likely than a voltage spike during start up.

I’m not convinced. The device mentioned has a voltage regulator like all the other modern devices that goes to 0V output when the voltage goes below the dropout voltage.
Mike
 
This is why I have the "Low Oil Pressure" annunciator light powered off the main bus and driven by the oil pressure switch.

Also reminds me I left the master on....

I hadn't thought of that. I have an AG6 annunciator on the main buss and Low Oil Pressure, as well as "OK" and "High Oil Pressure" are available inputs. I'll have to track the wiring down and see about wiring it in.
 
Ok, fair point. I get it. Is this an issue with Lycomings? I have never heard about it before. On two overhauls, they seemed to self prime and produce decent pressure in about 10 seconds at cranking RPM.

If you will re-read my post, you will see that I only used this as an example that "Something CAN happen", not that this particular thing may happen. But that unexpected things can happen.
 
Bob Nuckolls advises that if you have an avionics master, it's a good idea to exercise the power switches on devices that have them from time to time. It seems that if the contacts are not used, they can corrode.
 
I don’t think a significant voltage spike at startup or shutdown can happen. I couldn’t find a avionics manufacturer that warned about it.
Mike

Uhm, It can and does - that's why we *should* install snubber diodes across all inductive loads.

For grins/giggles someone should measure the size (Volts) and duration of the spike caused when the Starter Motor stops, the Starter Solenoid Opens, the Starter Relay Opens, the Master Relay Opens --- oh wait, someone did https://www.youtube.com/watch?v=43HFng0CVKg&ab_channel=electronicsNmore

Galin's issue in all likelihood was caused by these spikes (Galin - are the diodes intact?), Voltage sag/low voltage issues cause problems for motors, not linear/logic circuits.
 
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G3X and VP

I have the G3X system powered by my Vertical Power box. My panel was built by Stein. To start I switch my master on, wait for G3X to boot up, turn to engine page and turn the key. I figured Stein and his guys knew what they were doing and it has worked great for 125 hours of a rookie pilot! I have had to reboot with the engine running with no issues.
 
Chime tone

I preferred to have a tone to indicate low or no pressure. My chime becomes active the moment my master is on and oil pressure is low. If for some reason oil pressure does not come alive within the first few seconds of engine start the chime will give me an audible alert. If I forget to turn off the master the chime will let me know I’ve forgotten to turn the master off. Works well.
 
Galin's issue in all likelihood was caused by these spikes (Galin - are the diodes intact?), Voltage sag/low voltage issues cause problems for motors, not linear/logic circuits.
There are no protection diodes anywhere in this airplane. FWIW the EMS module was an MGL unit. Two different manufacturers had the same result at the same time.
 
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