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Avionics Master: Do you need a relay or a switch?

S

szicree

Well Known Member
I know some folks don't like em, but I've decided I want an avionics master. My question is to the folks who've wired one, do you need a relay or will a switch do the job? I'm calculating my total current to be around 15 amps. So can I get by with a switch?
 
I used a Potter & Brumfield circuit breaker with toggle, # W31X2M1G35. Look on page 428 of Aircraft Spruce. Thats what was in my Cherokee Six for a avionics master no relay required.
 
Yes

Hi Steve,
It's a wise move to protect your avionics from startup voltage surges. We used the same circuit-breaker switch and ran a wire over to the avionics circuit breakers which were on their own buss bar, plus a spare (VOR maybe). We placed the avionics circuit breakers in the same order as they were stacked vertically.

Regards,
 
I used a double throw switch and wired my starter switch through the back side so that the starter is inop when the av master is on. Some people don't like this because, if the engine quits in flight, you have to remember to turn off the avionics to restart. My answer to this is; if the engine quits in flight, the prop should continue to windmill and you don't need the starter. If the prop does not windmill, you have other problems and the starter won't help anyway.
 
DPST switch...

You can use a double pole switch and wire both contacts in parallel for a bit more reliability...

Costs very little more, and guards against contact failure.

Make sure a single contact can handle (is rated for) the current load, 15 Amps in your case.

gil in Tucson
 
Switches fail...

szicree said:
I know some folks don't like em, but I've decided I want an avionics master. My question is to the folks who've wired one, do you need a relay or will a switch do the job? I'm calculating my total current to be around 15 amps. So can I get by with a switch?
Click to expand...

If you are going to use a switch, you might think about wiring two in parallel so that when the first fails you can still continue with the second.

Kent
 
kentb said:
If you are going to use a switch, you might think about wiring two in parallel so that when the first fails you can still continue with the second.

Kent
Click to expand...

I'm with Kent. I'll take 2 flea market switches over just about any single switch, regardless of it's pedigree.
 
It turns out that most of the stuff I want to protect with an Avionics Master are also on my Essential Bus!! That means that if my Avionics switch fails, by Essential Bus switch will turn them on.
 
f1rocket said:
It turns out that most of the stuff I want to protect with an Avionics Master are also on my Essential Bus!! That means that if my Avionics switch fails, by Essential Bus switch will turn them on.
Click to expand...

Exactly my plan too :D
 
Baa Humm bug

Avionics switch a good idea. 15 amps! wow that is a panel. Rocker switches have more capacity. You may need a relay. I bought two 40 amp solid state relay's on eBay for cheap $15 each. They cost about $50-$60 normally. Sorry I'm going to use them.

As far as redundancy, DPST, two switches, Essential Buss backup..... really on a VFR single engine plane?

Well I guess with glass cockpits, which mine is, even a VFR plane with a dynon, GRT eis4000, gps does rely heavily on electricity. Back in the day..... GA planes had mechanical and VAC engine and flight instruments, so electrical redundancy was not as critical. Times have changed in just the last 10-15 years. No one had a PC 20 years ago. Now if you don't have a cable modem you are in cave technologically.

I have yet talk to any one who has had an avionics switch fail or who's day was saved by an essential buss.
 
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gmcjetpilot said:
Avionics switch a good idea. 15 amps! wow that is a panel. Rocker switches have more capacity. You may need a relay. I bought two 40 amp solid state relay's on eBay for cheap $15 each. They cost about $50-$60 normally. Sorry I'm going to use them.

As far as redundancy, DPST, two switches, Essential Buss backup..... really on a VFR single engine plane?

Well I guess with glass cockpits, which mine is, even a VFR plane with a dynon, GRT eis4000, gps does relay heavily on electricity. Back in the day..... GA planes had mechanical and VAC engine and flight instruments, so electrical redundancy was not as critical. Times have changed in just the last 10-15 years. No one had a PC 20 years ago. Now if you don't have a cable modem you are in cave technologically.

I have yet talk to any one who has had an avionics switch fail or who's day was saved by an essential buss.
Click to expand...
Yep, Been flying since 1967 and the only switch I've had fail was the landing light breaker/switch on a Mooney. Switch still worked, it would just get warm and click off. BTW, the reason I used the DPDT switch for my AV mstr..that's what I had!
 
gmcjetpilot said:
Well I guess with glass cockpits, which mine is, even a VFR plane with a dynon, GRT eis4000, gps does rely heavily on electricity. Back in the day..... GA planes had mechanical and VAC engine and flight instruments, so electrical redundancy was not as critical. Times have changed in just the last 10-15 years. No one had a PC 20 years ago. Now if you don't have a cable modem you are in cave technologically.
Click to expand...

That pretty much sums it up, George. I'm not sure I'll have any backup gauges at all in my plane. If I set it up for IFR, I'll probably have analog Airspeed, Altimeter and T&B. If not, you won't see one steam gauge. On the other hand, it makes the avionics a little more critical. Even for day VFR, while I wouldn't consider loosing the entire panel anywhere near an emergency condition, what a PITA it would be if you had no radios, no altimeter to determine TPA, etc. The real weak links are the switch, CB and main contactor. For the price of a switch and some wiring, the realibility of the whole panel basically becomes the reliability of the battery (or more to the point, the installed individual avionics). An outright failure of properly installed and maintained batteries or wires is practically unheard of.

Anyhow, my favorite kind of flying is still a Champ with no electrics at all, or a Citabria with 1 radio and a starter. Total electrical failure in that is, at most, a ho hum moment. :D
 
I also used a avionics master switch for the same reasons that you want one.

It is a Mil Spec switch rated for the currrent.

NO problems with it in 1,976 flying hours. The switch cost around $15 10-years ago from Newark Electronics. Recently I paid $28 for a 3 position double pole switch that was the same spec number for an ignition upgrade.
 
Nothing wrong with an avionics master

Buy quality. Buy MilSpec. Rate the switch properly and enjoy the convenience. IMHO, if your avionics bus doesn't power dual EVERYTHING then your can ignore the 'single point of failure' issue since any 'one' failure will bring on the pain.

There is no argument. If you put a switch in your avionics line you have a 'single point of failure'. But.. got your approach punched into that GNS480? Got TWO of em? On separate busses? What happens if THAT is the failure point?? Then consider MY 'total electrical failure' experience.

Over the CA central valley, night VFR in a brand new '78 R182 when *wink* the whole thing went dark. Fog in the Bay Area so we did the fly the runway-wag the wings-thing at Modesto, got a green, pumped down the gear and landed. No biggie. Next day, found out that the problem was that the primary wire to the master soleniod fell out of a bad crimp. One wire. Dark airplane. Go figure.

So.. Design your airplane with the intention that it will work. And it will, if you wire it right. When it doesn't, have a plan and backup.

My backup is an ICOM A22 and an old G196 plus a Sporty's/Castleberry ADI/bubble to keep me upright. That and knowing that I'll NEVER fly this thing out of reach of VFR should do it.

Short answer:
Build the airplane like you want it to work. Have a plan when it doesn't.

John
 
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So George, if John's Cessa had had an Essential bus, all he would have had to do was flick the switch and continue on. I guess there's your "story" you are looking for. Not "necessary" as he got down okay without it, but that's what it's there for. I don't have any added complexity since I had to wire the devices up to a bus anyway, just doing this way gives me options. Plus, it backs up my avionics master switch.

No more complicated, just a different way of doing it.
 
Second Thoughts

I've experienced two switch failures (and one battery failure) in aircraft. The first was a battery master (the famous red split switch) failure, the second was an avionics master switch failure. Both occurred (at different times) in my '79 182. When I built my RV, the failure of the avionics master switch was fresh on my mind, so I didn't include one. I do have an endurance bus.

My point is, on retroflection, that the avionics master switch failed after 26 years of use. I must admit, I don't enjoy turning everything off manually. I'm not sure I made the right decision, but thought I'd add a data point for you. Either way, there is always that chance...
 
gmcjetpilot said:
I have yet talk to any one who has had an avionics switch fail or who's day was saved by an essential buss.
Click to expand...
Agree with first, only somewhat agree with the second, but is true ONLY when the pilot is intimately familiar with the electrical system and can quickly determine which devices to power off or breakers to pull to "shed the load."

I envision the the the intent of the E (endurance or essential) Buss is to automatically isolates all of the critical devices so if there is a failure, they will continue to operate regardless of the on/off state of the other devices NOT on the E buss. For example if you have an alternator failure and you don't remember to turn of the landing light, it won't drain your E buss battery. These are obvious examples but there could be many devices consuming battery power during an alternator failure, that a pilot may not be aware of.

In my system, I will have a main buss, an E buss, a main avionics and a E avionics buss. I will have a main battery and an E buss battery. Initially I will have only one alternator that will charge both buss but it will be "one way" -power can flow from main to E, but not from E to main. Later I will add a second alternator on the E buss. In "all electric" airplanes, an E buss cuts the workload in an emergency.
 
Gary Bricker

I have a spam can Beach that has two separate switches in parallel on the ground side of the Avonics. These were installed by a Avonics shop years ago when I bought it. I don't know if this is good or bad.
 
avionics master

Interesting thread for me as I am just starting the wiring and have been reading the aeroelectric book trying to decide the appropriate circuit.
As I plan a dual Dynon (D100 and D120) and a straightforward SL30, GTX 327 and have a 496 it seems to me that I do not need an essential or backup bus as with the exception of the comms I have battery backup to get me home (I do have a hanheld comms). It does seem to me that I still need the avionics master to isolate everything during engine start.
I have been looking at circuits Z-11 and Z-12 as the most appropriate starting place and I would like to keep thing simple, are these the best drawings to start with? I assume I will still need a battery master switch but I confess I do not understand the alternator part of the circuit.
 
uk_figs said:
Interesting thread for me as I am just starting the wiring and have been reading the aeroelectric book trying to decide the appropriate circuit.
As I plan a dual Dynon (D100 and D120) and a straightforward SL30, GTX 327 and have a 496 it seems to me that I do not need an essential or backup bus as with the exception of the comms I have battery backup to get me home<SNIP>
Click to expand...

I'm with uk_figs...want it simple all electrical, no redundancy, VFR only. All the essentials D100,D120, GPS got a battery backup. That only leaves Comms. Where I fly, most of time, it is out of uncontrolled airfield, no tower, no atc. In the unlikely event I loose electric power, so be it, the plane is still flying right, your are still VFR right (VFR plane), you have a bit of battery in your GPS to get you to the nearest field yes... Decide on your mission, for your plane and kit out accordingly as simple is as possible.

I have not looked at the diagrams yet, but reading Aerolectric, so would like also to know what is best in uk_figs situation.

Regards
Rudi
 
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