Avionics Master

[Ed Haywood]

I am updating the electrical panel on my Decathlon from fuses to breakers. I am taking the opportunity to install an Avionics Master switch. Interested in thoughts on which devices to put on it.

I know the latest Garmin avionics don't need to be switched, but I like the ability to do so and the convenience of a single switch.

Here is my planned configuration:

Main bus: stall, fuel pump, alt field, AHI, engine gauges
Individually switched: strobes, nav, landing
Avionics bus: radio, transponder, engine monitor, USB

I am not sure about the USB charging port. It does not need voltage spike protection. But seems like it might be handy to isolate in a pinch. OTOH I could just unplug any devices from it.

Thoughts?

 

[Jesse Saint]

You want your engine monitor on the main bus so it is on when you start your engine.

 

[Wagondriver]

My engine monitor is not on the main bus, but then, It is not primary instrumentation.

everything needs protection!

 

[Bell206]

I am updating the electrical panel on my Decathlon from fuses to breakers.

I am not sure about the USB charging port. It does not need protection.

FYI: "updating" your electrical system gets into major alteration territory. I assume your APIA is on board with your changes? And any circuit requires protection to include USB port. The CB/fuse protects the wire and not the device.

 

[asicer]

What about toggle switch breakers (as used on C172SP lighting)?
https://www.aircraftspruce.com/catalog/pnpages/11-03935.php

 

[Ed Haywood]

FYI: "updating" your electrical system gets into major alteration territory. I assume your APIA is on board with your changes? And any circuit requires protection to include USB port. The CB/fuse protects the wire and not the device.

Yes, he is good with it. Am updating to meet current TCDS spec as part of bare frame restoration, so it's going to have a monster 337.

USB circuit has a breaker. The question is whether to mount that breaker behind or in front of the avionics switch.

 

[Ed Haywood]

What about toggle switch breakers (as used on C172SP lighting)?
https://www.aircraftspruce.com/catalog/pnpages/11-03935.php

Already bought breakers and switches as a kit from the aircraft factory. As posted above, I am updating to new TCDS spec, which does not use breaker switches.

 

[Bell206]

so it's going to have a monster 337.

FYI: a single "monster" 337 is not always the best option. Better to use multiple 337s and manage your alterations. While it does save paperwork, it can create issues for ICAs and restoration issues in the future.

Already bought breakers and switches as a kit from the aircraft factory.

Does this kit come with any OEM documentation? If so it maybe considered an aircraft specification and not require a 337?

 

[Ed Haywood]

Does this kit come with any OEM documentation? If so it maybe considered an aircraft specification and not require a 337?

Yes, have factory drawing. See attached.

 

[Albany Tom]

I like having a separate switch for avionics because it lets you have them all disconnected during startup or shutdown. Those are two times when there could be over or under voltages on the bus. Modern electronics should be protected against that, but...not knowing how close the tolerances are of the protective devices compared to the surges, and me being cautious, they're safer if off. Just me, and it's based on an understanding of how typical linear step-down regulators work, supplying 5v or 3v logic from 12v. No idea if radios still use linear regulators to power the computer parts. They aren't efficient, but they're simple, reliable, and inexpensive, so I'd bet maybe.

The other thought I'd have is how to turn off everything but that one thing I wanted in case of total electrical failure. Maybe everything but one radio? That may be possible without any other switches.

If it didn't cost much, I'd probably put the USB outlet on a breaker that could be pulled out. Just in case it goes nuts and starts making smoke. Those are switch mode regulators, dc-dc converters actually, at least every one I've seen, and kinda high power for their size. There's enough energy there to make smoke. Again, me being paranoid a bit.

 

[Ed Haywood]

I like having a separate switch for avionics because it lets you have them all disconnected during startup or shutdown. Those are two times when there could be over or under voltages on the bus. Modern electronics should be protected against that, but...not knowing how close the tolerances are of the protective devices compared to the surges, and me being cautious, they're safer if off. Just me, and it's based on an understanding of how typical linear step-down regulators work, supplying 5v or 3v logic from 12v. No idea if radios still use linear regulators to power the computer parts. They aren't efficient, but they're simple, reliable, and inexpensive, so I'd bet maybe.

The other thought I'd have is how to turn off everything but that one thing I wanted in case of total electrical failure. Maybe everything but one radio? That may be possible without any other switches.

If it didn't cost much, I'd probably put the USB outlet on a breaker that could be pulled out. Just in case it goes nuts and starts making smoke. Those are switch mode regulators, dc-dc converters actually, at least every one I've seen, and kinda high power for their size. There's enough energy there to make smoke. Again, me being paranoid a bit.

Bought the breakers from the aircraft manufacturer to avoid PMA/TSO issues. They are non-pullable.

 

[Albany Tom]

Bought the breakers from the aircraft manufacturer to avoid PMA/TSO issues. They are non-pullable.

Ah. I'd put the USB on a switch by itself then, or with things you didn't really need. While I've never heard of an aircraft approved USB outlet catching fire, I have for other converters, and I don't think the breaker by itself will limit the energy enough to keep it safe. Again, maybe me being paranoid...they may have thermal overload or other protective circuitry in them to make them completely safe.

 

[brcase]

You want your engine monitor on the main bus so it is on when you start your engine.

If it works great, but have seen a few engine monitors that flake out from the voltage drop from starting and need to be rebooted. Fix was to put it on the avionics master, so it booted up after engine start.
Brian

 

[Bell206]

Yes, have factory drawing. See attached.

Does your aircraft serial number fit into the range listed on that diagram? If not will need 337 unless you have a different doc listing your S/N for this drawing. However, if you basically replacing CBs and adding a couple more elec consumers you might be able to stay in the gray area as a minor alteration. The trigger to a major alt is "a change to the basic electrical system design." And if going 337 route would seriously reconsider pull type CBs like Klixon.

Bought the breakers from the aircraft manufacturer to avoid PMA/TSO issues. They are non-pullable.

FYI: in general, you'll find most common electrical parts are considered Standard Parts and do not fall under PMA/TSO requirements. However, those parts must be manufactured to an industry standard. You'll find most aviation CBs are made under the Military Specification (MS).

 

[Jay Urish]

I think the main purpose of the avionics buss is to isolate the sensitive electronics during engine start which can cause massive voltage fluctuations in the electrical system until the engine is running and alternator/generator is charging.

 

[MooneyDriver78]

I think the main purpose of the avionics buss is to isolate the sensitive electronics during engine start which can cause massive voltage fluctuations in the electrical system until the engine is running and alternator/generator is charging.

Not really necessary with today’s avionics, they have voltage regulators, this is why they can support 14 or 28 volts.
Also modern voltage regulators for alternators don’t enable charging at startup, preventing fluctuations and also prevents a load being applied while engine is starting. Of course our old planes may still have old regulators.
And leaving avionics switch off at startup helps reduce power required at startup.

 

[Ed Haywood]

Does your aircraft serial number fit into the range listed on that diagram? If not will need 337 unless you have a different doc listing your S/N for this drawing. However, if you basically replacing CBs and adding a couple more elec consumers you might be able to stay in the gray area as a minor alteration. The trigger to a major alt is "a change to the basic electrical system design." And if going 337 route would seriously reconsider pull type CBs like Klixon.

FYI: in general, you'll find most common electrical parts are considered Standard Parts and do not fall under PMA/TSO requirements. However, those parts must be manufactured to an industry standard. You'll find most aviation CBs are made under the Military Specification (MS).

That's helpful, thanks. I believe I may have another non-SN-specific drawing as part of the electrical system Service Bulletin update I am also doing. Just not on digits. I got it as part of an SB compliance kit from the factory, specifically for aircraft of my era that did not originally have breaker protection from the battery to the bus.

There is no change to the basic design in the other stuff I am doing. Just replacing fuses with breakers and adding a few loads. It is a very common mod, done to hundreds of pre-ACA Decathlons, Citabrias, and Champs. Most importantly, it eliminates multiple inline fuses from behind the panel after 45 years of previous minor alterations.

As a general rule, I like to order parts from the factory, because it comes with either a PMA stamp or acceptance tag and a part number that corresponds to the factory drawing, all of which makes paperwork easier. But I will also order standard parts when the spec is clear, such as AN bolts and MS electrical components. In this case, the breakers from the factory were significantly cheaper than Klixons from Spruce. The factory does not use pullable breakers, so staying with their breakers keeps me closer to the friendly side of the gray zone. Money has been spent, so I'm committed now.

Interestingly, the factory has wide latitude in the components they use. The ACA chief engineer has said he could legally install a toaster if he wanted to, and putting it on the specs makes it an aircraft part. I'm sure that is an oversimplification of the process, but still interesting. That explains the OEM gauges on my 78 Decathlon, which are Stewart Warner 2 inch auto parts.

 

[Bell206]

the breakers from the factory were significantly cheaper than Klixons from Spruce

Thats the reason OEMs like those type of CBs, they're cheap. Real OEMs use Klixons or equivalent.

The ACA chief engineer has said he could legally install a toaster if he wanted to, and putting it on the specs makes it an aircraft part. I'm sure that is an oversimplification of the process, but still interesting.

It's not an oversimplification. TC/PC holders have a wide latitude when it comes to building and equipping their own aircraft. Its the reason the official definition of a major alteration excludes OEM aircraft specifications. Anytime you can follow an appropriate factory method with documentation is definitely the route to go as it falls under a simple logbook entry.

 

[Ed Haywood]

Anytime you can follow an appropriate factory method with documentation is definitely the route to go as it falls under a simple logbook entry.

I leverage the heck out of that. ACA does a great job of supporting people like me too. Very responsive parts department, and always willing to discuss. Can't say enough about what good people they are.

 

[Ed Haywood]

You want your engine monitor on the main bus so it is on when you start your engine.

To clarify, I have round gauges for primary engine instrumentation. MP and RPM are mechanical. OP, OT, and FP are mechanical but getting converted to electrical and will be on main bus. I may add a JPI 730 for CHT/EGT. If so, it will go on avionics bus, per installation instructions.

 

[Ed Haywood]

FYI: "updating" your electrical system gets into major alteration territory. I assume your APIA is on board with your changes? And any circuit requires protection to include USB port. The CB/fuse protects the wire and not the device.

I edited my post to clarify what I meant, which was that the USB port does not require voltage spike protection, AFAIK.

That is my understanding of the main purpose of an avionics bus: to isolate sensitive and expensive electronics from fluctuating current during starting, correct?

Garmin says their stuff does not need a separate bus, and I have a new Plane Power voltage regulator that presumably does a better job regulating spikes. So I get that I don't NEED a separate bus.

But, it is handy for shutdown, and a trivial addition: just a switch, 2 jumpers, and cut the bus bar. Since I am building a new bus from scratch, might as well do it now, to give me more configuration options. Can be easily reversed later by replacing the bus bar.

I tend to leave my iPad and portable ADS-B box connected to USB. Putting the USB on the avionics bus is a convenient way to reduce current draw for starting, so I think I'll do that.

Engine monitor install instructions seem to vary by whether they are primary or not. Right now my plan is to have one breaker on the main bus for the primary engine gauge cluster (OT/OP/FP), and another on the avionics bus for a CHT/EGT monitor such as the JPI 730. If I later decide to convert to a monitor for primary, the gauge cluster circuit on the main bus provides a convenient upgrade path.

So now I have radio, txp, USB, and GEM on the avionics bus. Everything else is either separately switched, needed during startup, or critical in-flight instruments (stall horn, AHI). All of this is consistent with the TCDS schematic, with the exception of the USB port, which was not a thing in 1990.

Anyone think this is a bad plan? I start cutting and crimping this weekend, so let me know now!

 

[Bell206]

my understanding of the main purpose of an avionics bus: to isolate sensitive and expensive electronics from fluctuating current during starting, correct?

In general yes. But separate power busses are also used for power management and design aspects which can assist in matching the power distribution system to the available space. However, you'll find most modern electronic avionics have built in voltage regulators which prevent spike damage.

Since I am building a new bus from scratch, might as well do it now, to give me more configuration options.

As long as you're following the OEM diagrams for the "basic system design" then you should keep things on the minor side. One thing to look at is if you ever want to power up certain systems with a ground power unit. If your aircraft doesn't have an EPU connection may look to see if Champion has an upgrade to install one.

 

[gismo]

If there's any way you can use pullable CBs, I strongly recommend going that way. Besides the obvious advantages (electrical fire, load shedding, diagnosing) of being able to take any circuit off the bus, all CBs suffer from internal contact surface oxidation and occasionally pulling a CG wipes that off.