Avionics Bus.What to put on it?

I have the following:
G5
GDU460
GDU470
GSU25
GAD29
GEA24
GMU11
GNX375
GTR200B

Should all go on the Avionics bus?
I have a TCW IBBS as well. The wiring diagram supplied by Stein has the GDU460, GDU470, GEA24, GSU25, and GMU11 connected to it. I was thinking of leaving the G5 on the main bus. It has its own backup battery and thus would be independent of both aircraft power and the IBBS should anything bad happen to either.

I don't read Garmin, what are all those things?

The question is, do you really need that avionics bus, or are you just adding a failure mode. I received the following from Garmin when I was designing my system, which was similar to yours, apart from the fact that I didn't use the separate battery (I had backups in the G5 and Aera 660) and a GAD27 for light/trim/flap control and low voltage ridethrough. Then again, looking at your list you don't have a GAD27, so you may need a separate but to feed from your IBBS to the avionics to keep your instruments alive on start. I do admit the GAD27 is great the way it takes care of this.

Tom.
RV-7, with a lot of Garmin fruit.

>>> From: G3Xpert <[email protected]>
>>> Sent: Monday, 17 October 2016 1:18 AM
>>> To: Thomas Mills; 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,
>>>
>>> Steve
>>>

Yes, I understand and agree the Avionics master switch is not required these days. But it is a nice convenient method of turning everything on and off. Yes, I’ve read Bob’s book, understand the single point failure modes, and have what I believe are good enough backups (IBBS, G5 battery, always take my hand held radio flying) to mitigate the possibility I lose everything if the Avionics master switch/bus stops working. And, I have 25 years of flying in many types of small aircraft of various ages. All of which had Avionics masters and none of which failed on me or my flying buddy’s that owed some of them. So yes, I agree it can be a single point failure. But it’s in my airplane and staying that way

Any comments on what to have wired to it? My Kitfox was all steam gauges so only the radio, transponder, and encoder we’re wired to the Avionics Bus. The original panel in the Rocket had first gen Dynon boxes that were all self contained. The Garmin system has many more boxes that need to be powered and this is why I ask the original question.

Have you considered something similar to what Cessna puts in the G1000 piston singles? It's a spilt switch like the alt/batt rocker most of us are familiar with so that 90% of the time you hit both halves and don't even think about it. I can't remember exactly what's on which side, but basically it divides stuff up so that failure of either 1/2 will leave you with a working nav/com and enough other stuff to get by.

I can't speak to the specific Garmin boxes you mentioned, but in general, if I wanted an avionics master and was confident that I had my failure modes covered with either a split switch, battery backup, or both, the only thing I wouldn't have on it would be the circuits required for engine monitoring so I could see oil pressure and system voltage/amps on startup.

F1 Rocket said:

I have the following:
G5
GDU460
GDU470
GSU25
GAD29
GEA24
GMU11
GNX375
GTR200B

Should all go on the Avionics bus?
I have a TCW IBBS as well. The wiring diagram supplied by Stein has the GDU460, GDU470, GEA24, GSU25, and GMU11 connected to it. I was thinking of leaving the G5 on the main bus. It has its own backup battery and thus would be independent of both aircraft power and the IBBS should anything bad happen to either.

Click to expand...

I generally leave the G5 on the main bus, everything else on the Av bus.
You don't need both screens or the GMU on the BU bat, if you actually go on the BU bat in an emerg you need to save energy for PFD only.
I also don't use the feedthrough on the IBBS as this prevents powering off those items unless you turn off the master.

I also like to split the AV bus into "avionics" and "radio", this way you can control power to the radios independently (the GTN's have no power sw) and it also splits the load between two switches (no relays for me).

To do an ?Avionics Master? switch, I strongly recommend doing two. Run the left EFIS, Comm #1, XPDR, Audio Panel, trim and such off one, the right EFIS, Comm #2, autopilot servos, flaps and such off the second one.

Run non-panel stuff off your main buss and single master, such as starter, pitot heat, alternator, landing lights, NAV/Strobe, fuel boost pump, the USB charger points and whatever else is drawing power not panel related.

You can take this a step further and use ?Avionics Master Relays? that connect to the battery(s) instead of the main buss. This will remove any fault on the main buss from taking down your panel. Such a setup enables POH emergency action steps such as ?on any electrical fault, open the Master relay(s). This single action will isolate the majority of likely electrical problems but not take power from the panel.

As a data point, over the last 20 years I?ve had switches fail. I?ve never had one of these simple 30 amp cube relays fail (use real relays not auto store stuff - for $10 you get very nice silver contact relays). The switches on panel that control these relays have ~100ma current load - they will fail mechanically before contacts become pitted.

Carl