I can't seem to nail this down. Not considering any wiring or anything else-let's look at the basic theory of a electrical system with the UL Power engine.

One issue is that the engine needs the ECU and pumps to run. I may even throw in there the EFIS since it has all the engine instruments.

But I'm thinking of flying along and I smell electrical smoke. I'd want to turn off the avionics master right? Some might even say turn off the master switch.

I'm trying to figure out the very basic architecture of the electrical system where I can turn things off in a hurry, but keep power to the essential stuff to keep the engine running.

So in the diagram below, I have the ECU, pumps, and EFIS on a separate bus. As soon as the Master Switch is turned on they all get power. Then I could have another switch called "Avionics Master", but what do I want on that bus? Do I want lights on an "avionics" bus?

I'm wondering if the Essential Bus as I call it should even run through the Master Relay? What if the relay fails? I can loose everything and still fly, but I need the ECU and pumps to keep the fan turning.

If I didn't run it through the master relay, then I'd have to have a huge switch to run all the current for fuel pumps and ECU through.

I'm trying to keep the system very simple, but also well thought out so that I can isolate possible electrical fires in flight, but keep the engine running.

Here is a basic sample of a system where it all does run through the master relay and then splits between essential items and an avionics bus with non-essential things.

All thoughts and ideas are welcome!

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Instead of an "essential bus" consider a "battery bus" that is connected directly to the battery and not affected by opening the master relay. There is a book called The Aerolectric Connection. You can download it here:


I do have that book, and it's somewhat helpful, but not showing exactly what I think I need.

The prob with the battery bus is that I still need to be able to turn it off. I don't want the pumps and ECU powered when the plane is just sitting in the hangar. So now I would need a switch for the battery bus.

Here's the current drawing I'm working on. It has everything going through the master relay (and master switch) and then to a Avionics Bus or Battery Bus (depending on what I'd call it...)

The UL Power wiring diagram has the ECU and pumps powered through a bus that's connected to the Master Relay. So I guess it's good enough for them and I think I'll just do it this way. I haven't heard of too many Master Relays failing so....

With this diagram, I can turn off the Avionics Master and cut power to everything except what's required to run the engine.

I'm posting an update because electrical questions don't seem to get many replies which tells me that perhaps most builders are as clueless as me!

So if it helps you in your design, here is something I found which is exactly what I needed. SteinAir sells a "Avionics Relay Assembly" for $75.00 which lets you use a small switch for your Avionics Master. If you go to the link below, you can see the relay and also a diagram of how it's wired. The way they have the Main Buss and the Avionics Buss is very similar to mine which tells me I have it correct (except of course they inserted the relay to control the Avionics Buss).


Here is my most current drawing which is the SteinAir drawing modified. As you can see, the Master Switch energizes the Main Contactor which send power to what I call the "Distribution Buss". That buss is the part that comes from SteinAir. I have it boxed in red.

From there, power goes to the ESS BUSS which powers all critical engine components. Also from the Distribution Buss the SteinAir relay (through a small switch) powers the Main Buss which really is more than just avionics-it has all other electrical devices like lights, USB ports, etc...

Hope this info helps someone.

Keep in mind I'm posting this for discussion. It's probably not a good idea for you to take it as is and start wiring your airplane. Please do your own research as I can not guarantee that this is correct as is. I'm still learning.

What is ECU? Once you engine is running that has mags, it will continue to run unless the fuel or mags are cut off. With that in mind, why aren't the fuel pumps ahead of the relays/Master Switch and off the battery directly, of coarse protected with  individual switch and fuse. There must be some reason for that.


I will be tackling the electrical soon with a Viking engine that also uses an ECU.  I would recommend 2 batteries and 2 master relays in parallel.  Both connected to the engine bus. An engine that does not have magnetos needs a redundant electrical system.  The EarthX batteries are lightweight and a good choice.  The 2 battery solution will provide you with 3 power sources: the 2 batteries and the alternator. Set them up so you can switch each on independently. If the alternator fails, you still have 2 batteries to get you home.  Master relays produce a large reverse current when switched off so don't forget to install a diode across the coil leads to protect your electrical system.


With an electrically dependent engine I would be nervous of trusting my life to a relay, even one as reliable as a master relay. If the master relay fails and depowers your electrical system you are a glider. If you like having a master switch the solution, to me, would be to put a single pole double throw swtich into the power line to your "essential buss". One leg of the switch would power the essential buss the normal way, from your master relay controlled electrical system. The other leg of the double throw switch would power your Essential Buss by hotwireing it to your battery. You want those fuel pumps and the ECU to be able to get power no matter what.

To avoid leaving the switch in the hotwire position by mistake, you could use a switch with a safety guard that forces the switch to be in the normal mode unless you lift the guard and put the switch into the hotwire position. When the switch is in the hotwire position the red guard will be standing up from the panel to remind you the switch is in an uncommon position.


In order to keep buss names appropriate to what the buss does, I would suggest naming what you call the "Essential Buss" as the "Engine Buss" since it only has engine systems on it. Let the buss name reflect what the buss does, keep it as intuitive as possible. Same thing with switch names and position labels, name that guarded switch something like "engine buss feed" or "engine buss source". Mark the normal power side of the switch "normal" and the guarded position "battery" or "hot battery" or something like that, for instance. 

I would not bother with the extra layer of relays your diagram uses. You essentially have two master relays, one after the other. If either one fails you have problems. Keep it simple.

Why an avionics buss?  It's just another switch.  If you smell smoke and turn off the avionics switch, you have no way of determining what is actually smoking until you turn the avionics switch back on and start switching the equipment individually.  I would try to eliminate as many parts as possible.

In addition to Mark's point (some avionics don't have an off switch anymore) the other reason for an avionics master is to protect all those expensive black boxes from power surges during engine start and rarely during engine shutdown. Modern electronics are better able to withstand surges but they can still be an expensive problem. An avionics master lets you leave all the individual electronic devices switches turned on and disable them all with one switch to protect them during engine start up and engine shutdown. Unless you are using a lot of high amperage electronics a plain old switch circuit breaker makes a fine avionics master switch. If you want to use a tiny switch as the avionics master Cessna makes a nice avionics master relay that uses tiny amounts of power for the control switch so you can use one of those mini switches. Also, the Cessna relay fails to the operating condition so if your relay fails you will still have avionics working.

Don- the idea of an Avionics Master is to be able to turn everything off quickly at once. If you smell electrical smoke while flying, how are you going to turn off your transponder? Your ADSB receiver, or any of the other electrical boxes that don't have an on/off switch?

From my research, the question to have an avionics switch or not is a debate going back 30 years. It's sort of like asking, "should I prime or not". There are pros and cons to having one and not having one.

I'm not going to repeat everything here, but Google "Should I have an Avionics Switch".

In my case, there is no "con" what so ever to having one. If the switch fails and I lose every single one of my avionics or lights, it would have zero affect on my flight (safety wise)

You should have Circuit Breakers on each of the avionics so they can be pulled individually. An avionics master is a good idea to protect from voltage spikes when turning the master switch on and off or starting, make sure to use a diode to protect the ECU.  In an electronic airplane a second battery is always a good idea, even if it only gives you an hour of engine time with no avionics. You should have a mechanical airspeed and altimeter as a minimum for backup and a paper chart. If it all goes blank you can switch to the backup battery to power the ECU and one pump then land at the nearest opportunity. If the Alternator goes out you might not notice for half an hour and the battery voltage at that point might be dangerously close to shutting down the ECU if you are running radios, efis, lights, transponder, ADS-B and whatever else. You can put in a master switch with a guard that needs to be lifted for emergency mode and has a normal position for regular operation. Think what do I NEED. I would not put the EFIS on the essential bus as if it has a problem it could drag everything else down. Put the ECU and one pump or both, selectable, on the essential bus. If the ECU goes out you are screwed anyway. Maybe have a backup oil pressure gauge and temp gauge, even if they are tiny car gauges, so you can figure out, once the EFIS is gone, if the Alternator failed because it is covered in engine oil or coolant and with a Airspeed, Altimeter, compass and chart you should have no problem getting to an airport. Good Luck.

If that ECU or fuel pump loses power you become a glider - right? For a system so critical, I would have 9and other's recommend) a second power source.  I have done 2 full electronic IFR "glass" panels and the situation is nearly the same if you lose your EFIS in IFR - a bad ending.  So in both of my designs I looked at weight and complexity and decided on a backup battery instead of alternator.  You can buy a 20 A-Hr AGM battery such as used in handicap scooters for about $75 that is compact, deep cycles well, and are very robust. 

In both cases I set up a dual feed priority bus bar that used diodes to isolate the circuits so that a fault on one won't  cause the good bus to backfeed the fault and smoke your reserve power.  The backup battery charges and discharges through a diode set to isolate it,  and feeds a split bus to supply your essential devices from either source. 

The backup battery charges automatically any time the main battery master is on and the engine running. Loss of one power source seamlessly transfers to the other with no moving parts or interruptions. You don't need an engine hiccup when the lights go out too. Diodes are so cheap that I double them up so a single diode failure can't kill the backup power.

For about $30 I found a 1.25" dia LED combination volt and amp meter that I have in my backup circuit. When the circuit is active you can monitor your backup bus voltage and current draw.  Pretty good info to know when you are running on the backup. Also for the backup battery I run it through its own breaker($20 for a Klixon)  and a heavy duty double pole switch ($12) with the poles wired in parallel , so even one contact failure can't cause loss of power. It is also cheaper lighter, more reliable, and simpler than a second master relay. Since currents for your essential item bus is very low compared the load on the master relay, the switch will last forever.


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