REFLECTOR: Avionics bus switch

[Brian Michalk]

I have a pair of computers installed for data collection, and am 
supporting them with a separate "backup bus".  Now this bus does more 
than just back up the computers.  I have other essential electrics on 
this bus, like the electronic injection and ignition system.  When I 
start the engine, the computers and everything on the bus are stable.
This backup bus is a 20AH pair of motorcycle AGM batteries that I 
consider to be capable of running the engine at least as long as the 
sump tank, which is about one hour.  I have measured them under load, 
and they should last longer than that if I reduce loads.

So, my battery system is:
alternator
     -> battery combiner
         -> main battery
         -> backup battery

The backup battery runs:
fuel injection
ignition
avionics
electric gyros
small red cockpit light

I have contactor switches on the panel to enable/disable either battery, 
and a third switch to join the two busses together.
So, assume the flight over water, and I lose the alternator:  I would 
join the two busses and turn off the backup battery saving it for the 
bitter end.

Nuckolls likes simple switch configurations, and my design may be a 
little more complicated than he would endorse, but it is three 
switches.  Mine seems to work okay, and gives me a lot of options for 
various failure scenarios.  In my case, I should run out of fuel before 
I run out of battery life, and even then I can turn on the backup fuel 
injectors that are simply misting nozzles, and a magneto, which allow me 
to turn off all of the electrics.

One of these days, I hope very soon now, I will get some performance 
numbers on my backup fuel injection system.  It's quite simple, and the 
parts are very cheap.  The hoses and fittings cost more than the nozzles 
and needle valve.


On 3/18/2013 7:37 PM, Don Johnston wrote:
> Because of institutional memory, OWT, etc. I'm trying to decide how to 
> handle the avionics bus.
>
> Since the first day I sat in an airplane, I've always been told to 
> never turn on the avionics until the engine is running lest to fry all 
> the expensive, delicate, precious electronic devices.  Then again, I 
> was also told to never run LOP, land without flaps, or taxi faster 
> than an 80-year-old man can walk. :-)
>
> Since I'm going to have all the engine info on GRT displays, there is 
> no way to see the actual engine information unless the avionics are on 
> before engine start.
>
> So how are you guys getting engine info in a pure electronic panel 
> environment? Separate, clean, protected bus for avionics? Ignore it 
> since it's an OWT?  So other solution?
>
> -Don
>
>
>
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