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Multiple Buses - Needed in simple VFR RV?

Brian Vickers

Well Known Member
An excellent, recent thread debated the merits of various systems and functions on the two or three different buses that more advanced airplanes have. I am not building an advanced IFR ship. I am building a very simple, VFR RV4 and considering putting everything on one bus. My airplane is all manual, carb/mag. I have spent many hours reading Aeroelectric Bob's book and system design Z-11.

My electrical needs are:

1) Comm
2) Transponder
3) Enigma (all in one glass, includes GPS)
4) Intercom
5) Whalen combo strobe/nav lights
6) Landing light
7) Taxi light
8) Fuel boost pump
9) Van's alternator warning light
10) Power receptacle for handheld comm

The Enigma unit will have a dedicated backup battery that is completely isolated from the main system. I have only seven bat switches total, not using keyed mag switch.

My airplane is simple for my personal mission profile, not budgetary. I have all the parts to put in two small fuse blocks and have roughed in (cleco) a small tray to mount them on. The more I put my mind on this, the more I think my system only needs one bus.

Any comments appreciated. I am trying to learn and be safe.

Sincerely,
Brian Vickers, RV4 finishing
 
Multiple buses are NOT needed in a simple VFR airplane. Many airplanes have been flying for many years with one buss. Some have an additional buss for avionics. Keep it simple. Keep it light.
OK, flame boys, come ahead!
 
Make as simple as necessary - and no simpler....

Or, as complex as necessary, but no more complex!

I agree 100% with Mel - for a simple Day/VFR airplane, a single bus is fine! And this is coming from a guy with three busses, two batteries, and two alternators....but I have all those things because I fly IFR with a full glass cockpit, and I need my architecture to satisfy my personal fault tolerance requirements.

1) I have multiple SOURCES of voltage, because sources can fail

2) I have multiple BUSES, because a short on a bus will make you power it down, and then your source does you no good!

3) I can cross-strap the buses and sources in whatever combination I want to get voltage to my important "stuff" - the REALLY important stuff is powered from multiple busses through diodes normally, so there is no interruption if I take a bus down.

While this system sounds complex to someone looking at it for the first time, the management of it is actually terribly easy. And none of that complexity is needed in a day/VFR aircraft in my opinion. Of course, if you're an engineer who likes to play around with electrons, then you might want to...but it's not needed!

Of all the things I learned in engineering school, the maxim that has stuck with me the longest is this:

"Perfection in engineering does not come when there is nothing more to add....but when there is nothing left that can be taken away!"
 
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Brian Vickers said:
An excellent, recent thread debated the merits of various systems and functions on the two or three different buses that more advanced airplanes have.
What would the consequences be of a total electrical failure, given the way you intend to fly the aircraft? Are you prepared to finish the flight with no radios, no power to the GPS, no transponder, no electrically powered engine indications, no cockpit or landing lights, etc. If those consequences are acceptable to you, then there is no need to add complication by going with multiple busses.

If there is some function that you don't want to do without, then maybe you need two ways to provide power for that function. But you also should have other redundancy as there are many other ways for a function to die than just losing electrical power. E.g, if you fly at night, and absolutely want to be sure you don't lose the landing light, then you had better install two landing lights, to cover a blown bulb, shorted wire, etc.
 
Brian,

I started the thread you mentioned and I learned a lot from the various replies. The main thing I learned is that everyone has a different view on what is necessary for continued flight if something fails so it is very difficuot to decide what goes on each bus. For your mission profile and simplicity of systems, I don't think you need multiple busses. In fact, I am starting to come to the conclusion that even for my plane, which has very complex systems, (I just built my breaker panel and it has 35 breakers), that I don't need multiple buses. I can shed any load via switches or breakers and can pick and choose which loads to shed based on current operating requirements. Now if I was using fuses rather than accessable breakers then multiple buses might make more sense. I do have a small backup battery that will power my EFIS and ignition if I ever run into a situation so serious that I need to completely shut down my main electrical system. This backup battery will allow me to get the palne on the ground. This would cover me for a shorted bus or main contactor failure which is about the only thing that I could not deal with by selectively shutting down systems.

I have a tendency to try to design systems to account for any possible scenario, but that adds so much complexity that that in itself adds new risks that need to be accounted for, which leads to a never ending circle. Thers comes a point at which simplicty is better and I am staritng to come to that feeling about multiple buses.

The most important thing you can do is ask yourself what your mission profile is, which you have done, and design the simplest solution that meets those needs.
 
One main buss with a switched buss for avionics. A Lowrance 2000 GPS using ship's power. I'll also carry a handheld radio and some spare batteries for the GPS. If I lose power I'll be able to talk and navigate just fine.
 
I like it

szicree said:
One main buss with a switched buss for avionics. A Lowrance 2000 GPS using ship's power. I'll also carry a handheld radio and some spare batteries for the GPS. If I lose power I'll be able to talk and navigate just fine.

Very good approach.....both you and Brian. Here's my buddy's -4 panel.

Good luck guys.

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Have a good one,
Pierre
 
szicree said:
One main buss with a switched buss for avionics. A Lowrance 2000 GPS using ship's power. I'll also carry a handheld radio and some spare batteries for the GPS. If I lose power I'll be able to talk and navigate just fine.
This a good idea but you will need a third bus for your "keep alive" items, such as the internal clock in your EFIS.

My day/night VFR ship has three busses; Main, Avionics, and Always Hot.

Granted, the Always Hot buss has only three items on it; EFIS and EMS on one breaker, one power port (for a trickle charger and use w/o turning on the master), and a courtesy light (EL light strip) for getting in and out of the plane.

Every item on my ship has its own pullable breaker or breaker-switch, thus I can shed the load as needed.
 
N941WR said:
This a good idea but you will need a third bus for your "keep alive" items, such as the internal clock in your EFIS.

EFIS?! I ain't got no steenkin' EFIS! :p
 
Kevin Horton said:
If there is some function that you don't want to do without, then maybe you need two ways to provide power for that function. But you also should have other redundancy as there are many other ways for a function to die than just losing electrical power.

Kevin nailed the key point here. If you really need it, then multiple busses for redundancy makes sense....but then it also makes sense to have a backup for the item itself. For VFR flight I personally think the best backup is a good handheld with VOR reception and a cheap GPS.

One thing you may want to consider is adding a cheap analog airspeed and altimeter to your panel since you plan to fly at night. That's a lot cheaper than a dual glass panel and it gets you a pretty reasonable amount of redundancy (for the most important stuff) for small $$.
 
szicree said:
EFIS?! I ain't got no steenkin' EFIS! :p
Steve,

You da man!

I was going to add that to get an idea of what you need, go fly a J-3.

The way some people talk you would think a plane will fall out of the sky if they can't talk on the radio.
 
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