Avionics Master

[Big Ed]

My 78 Super D does not have an avionics master. I am considering installing one.

Physically, seems pretty simple. Install a toggle switch in the switch bay and jumper the #1 radio fuse holder to it. Install 4 fuse holders in the upper left corner of the switch panel and bus then with a short section of copper rod. Run a thick wire from the switch to the bus rod. Wires from there to avionics devices.

Devices on the new bus would be radio, transponder, USB, and engine monitor. Fuel pump and AHI would remain on the master.

So the big question: major or minor? I have seen cases for both on the interwebs.

 

[Bob Turner]

A switch? Is a swich a change to the basic design of the electrical system?
But tough to add, physically. I added an alternator switch up there, and found it quite difficult with my size 10 hands.

 

[Big Ed]

Obviously I would prefer that it be a minor. But since I can't sign off on the change, my IA has to prefer that it be a minor too.

Here is my existing master bus circuit.




And here is my planned circuit. Changes are in red. Thoughts?

 

[Tangogawd]

To me you aren't changing the basic design on the electrical system. looks to me you are simply using the current "radio1" circuit and piggybacking the new items off that, followed by relabeling the switch. Just need to ensure that the wire can carry the load of all the components, or replace it with something that can. Then include a circuit breaker/fuse for each new component.

For what it's worth, I believe that this is a minor, as well as many installations of radios and intercoms, and and. Fortunately, I only have myself to convince.

 

[Big Ed]

To me you aren't changing the basic design on the electrical system. looks to me you are simply using the current "radio1" circuit and piggybacking the new items off that, followed by relabeling the switch. Just need to ensure that the wire can carry the load of all the components, or replace it with something that can. Then include a circuit breaker/fuse for each new component.

For what it's worth, I believe that this is a minor, as well as many installations of radios and intercoms, and and. Fortunately, I only have myself to convince.

I think so too, but worth asking. From what I hear, there are many IA's whose default position is a 337 for everything.

Radio, Transponder, and USB each draw 5A. EMS draws 2A if I go with the JPI 730 or 830. So total load of 17A. Was thinking a 20A fuse before the Avionics switch would be needed to avoid nuisance blows. AC 43.13 table 11-3 says 12 gauge wire, but that is for bundles, so 14 gauge should be sufficient.

 

[Hiperbiper]

First off, never fuse a fuse. Split the main buss and the avionics buss with a switch. Instead of another copper buss bar consider "daisy chaining" power off the avionics master using 18-20 ga. wire and insulated terminals. Easy to do and safer considering you're tight on space up there.
Instead of fuse holders use Klixon 77xx series CB's. $20.00 each, same footprint and no need to carry spare fuses. As to the minor or major argument; I'm on the side of your personal IA.
There is no harm in generating a 337 detailing the changes. The FAA will simply add it to the aircraft file, your IA gets an 337 to log and you airframe logbook gets to read "...see 337 in aircraft paperwork for details...".
337's...
They're not just for major work and STC's.
JMPO
Chris

 

[Helo pilot]

SD serial numbers 860-2000 and up have an optional avionics switch. I have attached a copy of the latest service manual electrical diagrams.

 

[Hiperbiper]

Ed, don't get confused over fuse rating vs. what a radio needs to work.

 

[Big Ed]

First off, never fuse a fuse. Split the main buss and the avionics buss with a switch.

Can you elaborate a bit? Are you saying I don't need a fuse between the main bus and the avionics switch?

Re CB's, can CB's and fuses be mixed in a panel? I would love to do a new panel with all CB's.

 

[Big Ed]

SD serial numbers 860-2000 and up have an optional avionics switch. I have attached a copy of the latest service manual electrical diagrams.

Ah OK, that is even easier. Just break the bar and switch the gap! And since I am just bringing it into compliance with the current spec, pretty good case that it is a minor, no?

 

[Bartman]

First off, never fuse a fuse. Split the main buss and the avionics buss with a switch. Instead of another copper buss bar consider "daisy chaining" power off the avionics master using 18-20 ga. wire and insulated terminals. Easy to do and safer considering you're tight on space up there.
Instead of fuse holders use Klixon 77xx series CB's. $20.00 each, same footprint and no need to carry spare fuses. As to the minor or major argument; I'm on the side of your personal IA.
There is no harm in generating a 337 detailing the changes. The FAA will simply add it to the aircraft file, your IA gets an 337 to log and you airframe logbook gets to read "...see 337 in aircraft paperwork for details...".
337's...
They're not just for major work and STC's.
JMPO
Chris

Chris,

I have one of ACA's panel wiring drawings and they use a switched CB as an avionics master
W31X2M1G25

Tyco Circuit Breaker W31 Series | Aircraft Spruce ®

Tyco Circuit Breaker W31 Series These bi-metallic single pole breakers weigh less than 2 oz. and are especially suitable for homebuilt aircraft. Used as original equipment in Beech, Cessna and Piper aircraft.

www.aircraftspruce.com

and then run power to two standard CB's before it goes to the radio and transponder units.

They use a 25 for the switch and then a 5 for the transponder (about 1.5A max) and a 10 for the comm (about 7.5A max). I'm planning a 15 with a 10 and a 3.

 

[Big Ed]

The main obstacle to following the factory method is that I don't have enough fuse holders installed on the existing bus rod. I only have 6 fuse bays, and I need 10 to accommodate all of my devices.



Adding more panel fuse holders to the existing rod bus looks like a giant PITA. The rod is soldered in place through the fuse holder eyelets. Maybe doable on a bench, but hanging by 2 inches of slack in the upper corner of my cockpit, sounds like a bear. Maybe cut through the side of the eyelet, bend it back, slip it on the rod, then bend closed and solder to the rod?



There is actually more room to work with above the existing bus, adjacent to the rear audio jacks. You can see that 2 fuse holders have already been added there, using a stripped wire as an expedient bus. It would not easy to remove those, add 4 in a row, then bus them with a section of solid copper wire. Then connect that to the master bus with a switch and I'm in business.

 

[Hiperbiper]

Chris,

I have one of ACA's panel wiring drawings and they use a switched CB as an avionics master
W31X2M1G25

Tyco Circuit Breaker W31 Series | Aircraft Spruce ®

Tyco Circuit Breaker W31 Series These bi-metallic single pole breakers weigh less than 2 oz. and are especially suitable for homebuilt aircraft. Used as original equipment in Beech, Cessna and Piper aircraft.

www.aircraftspruce.com

and then run power to two standard CB's before it goes to the radio and transponder units.

They use a 25 for the switch and then a 5 for the transponder (about 1.5A max) and a 10 for the comm (about 7.5A max). I'm planning a 15 with a 10 and a 3.

It serves no purpose to have a split buss fused going to fuses...
ACA and I will agree to disagree. Definitely not the first time.
Chris

 

[Hiperbiper]

Can you elaborate a bit? Are you saying I don't need a fuse between the main bus and the avionics switch?

Re CB's, can CB's and fuses be mixed in a panel? I would love to do a new panel with all CB's.

Yes, in the interest of safety you and your IA can install CB's in place of your current (no pun intended) fuses.
JMPO

Chris

 

[Bartman]

It serves no purpose to have a split buss fused going to fuses...
ACA and I will agree to disagree. Definitely not the first time.
Chris

I wasn't planning to have CB;'s for individual avionics but each install manual specifies a dedicated, resettable CB that is within reach of the pilot.

 

[Bartman]

@Big Ed

do you have room on your panel to put the Av Master and individual component breakers?

Ed, it might be easier to make the changes in a second overhead panel and then swap the existing stuff into it. I have an extra that I think is in good condition, it's yours if you want it.

 

[Big Ed]

@Big Ed

do you have room on your panel to put the Av Master and individual component breakers?

Ed, it might be easier to make the changes in a second overhead panel and then swap the existing stuff into it. I have an extra that I think is in good condition, it's yours if you want it.

Probably not enough room anywhere on the instrument panel, not unless I went with a primary EMS that allowed removal of my existing gauges.

Redoing the whole overhead panel is an option. But like always, this idea is snowballing.

 

[Hiperbiper]

BTW, as I'm not really going to be doing any more work on other people's planes I was wondering if anyone out there needs some of these:

20 ga. Bleed Leads.
You guys doing installs might find a use for 'em...Free of charge.
Just like my damn flashlight batteries .
PM me your address and I'll send you some...

Chris

 

[Bartman]

I just bought a bunch of those so I'll let someone else have all of the fun and a big thank you for putting them out there to the forums free of charge!

 

[Big Ed]

What do they do?