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Avionics Master switch - is it needed?

MS19087

MS19087

Well Known Member
Yes, I have read Bob Nuckolls document and he claims that our 'modern avionics' are no longer susceptable to transients produced at start-up and therefore we no longer need to isolate the avionics during this period. While I endorse the idea of an E-bus and would prefer not having another single point of failure in the feed, I spoke to a few avionics companies and they cautioned me about the transients . . . what are you folks doing??? Essentially my simple design works out that the avionics bus and the e-bus are virtually the same (for my XC VFR mission) . . . so do I put a switch in between the main bus and e-bus to isolate it ?? My current design has the e-bus drawing between 12.2-17.6 Amp (depending on Comm TX, internal battery charging, fuel pump operation, flap operation) - my derated 18ah battery should handle this load for at least 30-45 minutes. (by the way - following the KIS principle - going single batt & single alt - may move to dual batt if things get marginal)

Comments . . . Thanks - Mark
 
I like the avionics master for several reasons.
1/ It protects the avionics from spikes.
2/ It allows me to leave the volume controls where I want them.
3/ It's one switch instead one on each "box."
4/ I use a double throw switch with the starter circuit wired through the "backside". This does not allow the starter work with the avionics master on.
 
My essential buss & avionics buss, is the same too. I have the avionics switch between the main buss/E buss, and the essential switch routed through the diode as in BoB N's schematics.

I even threw in a switch to get the full voltage reading while on the essential buss. It has a drop across the diode.

BTW, I don't remember which piece of equipment it was, but the manufacture preferred a switch to turn off the avionics buss during startup.

L.Adamson -- RV6A
 
Its your choice

After going through many of the same questions I ended up with the avionics master and the E-bus alt feed from the battery bus a la Aeroelectric Z-11 layout. With the Dual Dynon D-100 and D-120 setup I have the D-120 on the main bus for engine and system readouts during engine start (OK with Dynon) and the D-100 along with the rest of the avionics on the E-bus which I can bring in after the engine is running.
In the event of a problem I can feed the essential avionics from the E-bus alt feed and have battery backup in the D-100 and the 496. Also per Matt Burch's design I have the cigar lighter jack on the E-bus so I can plug in my handheld comms transceiver if required.
Enough to get you to the ground I think.
There are as many opinions on this subject as there are forum members so it becomes a question of your personal choices.
 
I went the way Mel described for the same reasons. I also used no E-bus at all. If the alt dies, I'll turn things off myself.
 
vote for avionics master

MS19087 said:
Bob Nuckolls . . . claims that our 'modern avionics' are no longer susceptable to transients produced at start-up
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That's a sweeping generalization and not 100% true. Some 'modern avionics' do require they be turned off during start (per the manual and a call to manufacture). One example comes to mind, the popular Icom A200 Com radio. The new A210 may be more tolerant, since it has a universal (13.8/27.5 V) pwr input.

Many modern avionic devices can handle 10-30 volts w/ some higher voltage spike protection, but there's no guarantee a unit is made to Mil specs, unless it says so. There can still be real limits to transients with popular 'modern avionics' devices. You have to check with each manufacture. Some are OK with it, some are not.

I'm with Mel, I like the avionics master switch, if nothing else for convenience. True, you can always turn individual items on/off at the device but that's no fun. :eek:

PS: The avionics/essential buss with schottky diode is OK, but as Steve said you don't really need an essential buss, unless you have a space shuttle panel for IFR flight. Just turn stuff off. BTW a schottky diode has an approx 0.19 volt drop @ 15 amps, verse a typical power diode, which has a 0.60 volt drop.
 
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Transients

Tru Trak and Dynon want to avoid voltage spikes. Mine are switched. Radios will be switched individually as well, just to avoid single point failure. What do you think? Bill
 
Specs...

gmcjetpilot said:
.....
Many modern avionic devices can handle 10-30 volts w/ some higher voltage spike protection, but there's no guarantee a unit is made to Mil specs, unless it says so. There can still be real limits to transients with popular 'modern avionics' devices. You have to check with each manufacture. Some are OK with it, some are not.
.....
Click to expand...

George, as you say, check the data sheets.

Garmin sort of says to use an avionics master switch, but calls their input line "Aircraft Power".

The common specification is RTCA DO-160D, and the Garmin GTX-327 specifies level A.

As you can see below, this is 50 separate 600 volt spikes in one minute... pretty severe....:)

The only case I know of blown avionics was due to an alternator field line failure - in this case an Over-Voltage protection device would have been more effective than an avionics master switch...:rolleyes:

SECTION 17: VOLTAGE SPIKE
This test determines whether the EUT can operate as required during and/
or after voltage spikes are applied to the AC and/or DC power input(s). Any method of generating the spike may be used, provided that the pulse produced has a duration of at least 10 μs, a rise-time of less than 2 μs, and a source impedance of 50 Ω. A minimum of 50 voltage spikes are applied within one minute. This test is very similar to MIL-STD-462 test method CS06, except for the source impedance of the generator, which is 5 Ω for CS06.5

EQUIPMENT CATEGORIES
There are two Equipment Categories.
The Category B test level is twice the AC (rms) and/or DC line voltage (or
200 V, whichever is less). The Category A test level is 600 V

Quotes from here....

http://www.interferencetechnology.com/ArchivedArticles/EMC_Regulations/i_02_23.pdf?regid=102

I for one am happy with no Avionics Master Switch for the limited avionics I am planning... and I will install an OV Protection device.

A previous posting mentioned Dynon, but most (including Dynon) instruments that monitor engine parameters are designed to be ON during cranking. Personally, I want to see that oil pressure come up as soon as possible after engine start....:)

This is Dynon's quote...

Provides primary power to the instrument. The FlightDEK-D180 will switch on upon application of power. Connect to a switched power source. Will not be adversely affected by engine cranking.

gil A
 
I went the way Mel described for the same reasons. I also used no E-bus at all. If the alt dies, I'll turn things off myself.
Click to expand...

But...if the master contactor fails, you won't have the choice as to what to turn off and what to leave on....it'll all be dead unless you have batteries in individual devices.

The E-bus gives you the option of working around a failed master contactor.
 
True but...

Sam Buchanan said:
But...if the master contactor fails.
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What if the prop falls off! :eek: True Sam but how many master contactors have failed, in flight, after activation? :rolleyes:

That is the problem with fail-tolerant and fail-safe designs, it never ends. But in the end we fly single pilot single prop planes. To really have true fail-safe you need two or three totally independent electrical systems, not one system with an extra battery or alternator. I think each to his own, but at some point you have to say stop the madness! :D Seriously a little wire, fuse, switch and diode is reasonable to get a parallel path to the battery, aka essential buss.

I think more complication in wiring architecture has utility, only each builder should really evaluate DO YOU REALL NEED IT. Obviously we have some very smart folks here who have evaluated their needs against complication.

You do add weight with wire, switches and diodes. One RV-8 I saw had 25 switches in it, some switches with covers. I had no idea how or what was what. There is something elegant about a single master switch, on, off.
 
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avionics master

i have avionics master(s). i actually have two of them in parallel -- for those who complain about a single point of failure. they are 30amp switch/breakers. my avionics current load was about 16 amps (using max values) unless transmitting, then either 20.5 or 19.75, depending upon which com. probably could have gone with 20 or 25 amp breaker/switches, but i wanted a little headroom. i have no e-bus. i have dual batteries, each with its own master contactor.

i like the avionics master for all the reasons listed above. when i talked to various equipment manufacturers about their devices being able to withstand transients during starting, most said something on the order of "it would probably be o.k., but it would be better if they didn't have power on during starting." i paid enough for my avionics that i didn't want to rely on "probably".

i'm actually wiring the panel now -- the other night i put power to the center stack (minus the xpndr). had no antennas connected (except a dummy load on the 430w com, just in case the ptt was stuck or something)*, and the duckie on com3. intercom worked, got white noise when i defeated the squelch on the coms and the 430w nav, hooked up my handheld to the com3 jack and listened to weather radio on that (vertex something-or-other). so, looks like my stack wiring is (at least mostly) good. btw, i don't think i'd want to wire the "high density" garmin connectors without the daniels crimper (or equiv.)

*(probably don't need a dummy load on modern transceivers, but i'm old school, and i had the cantenna lying around.)

mho, ymmv, all the usual disclaimers.
 
gmcjetpilot said:
That is the problem with fail-tolerant and fail-safe designs, it never ends. But in the end we fly single pilot single prop planes. To really have true fail-safe you need two or three totally independent electrical systems, not one system with an extra battery or alternator. I think each to his own, but at some point you have to say stop the madness!
Click to expand...

Well said George.
 
Sam Buchanan said:
But...if the master contactor fails, you won't have the choice as to what to turn off and what to leave on....it'll all be dead unless you have batteries in individual devices.

The E-bus gives you the option of working around a failed master contactor.
Click to expand...

True, but I think it'll be ok for my plane. I've got one mag and my LSE is wired direct to my battery. Steam gauges for ALT and ASI, and the gps has it's backup batteries. I also carry a handheld com just in case.
 
I have one but I did it for convenience. Flip one switch and it turns on all my radios. If that switch fails, my Endurance Bus switch will turn them on so no single point of failure. An added benefit is that I can power up the endurance bus on the ground without the engine running to listen to ATIS, get a clearance, etc avoiding the current draw of powering up the primary bus. Not a biggie, but it sure has been suprisingly helpful.
 
You don't shut off your LSE ignition during start if you use it when starting, or you have a dual LSE on your engine. It works with either 14V or 28V systems, and has transient protection. One thing I would recommend is that you put a 10,000 uF, 35V cap from the buss to the return, and parallel it with a 1.0 uF film capacitor of 50V, along with .01 uF disc. These will give a good AC ground on your buss, provide a current source for narrow transient loads, and help attenuate transient spikes.
 
Avionics Master

Here's my two cents on avionics master switches. It is a whole lot cheaper to replace that one switch than to replace any single internal switch in any of the radios you are switching on. It simply cuts down on the cycles each switch for each of the devices has to go through over their lifetimes.
 
Great post

I am in the initial stages of wiring my RV-7A so this post is very timely.

I have planned an avionics master switch (engraved my placard panel already). My reasoning was to help keep the transients out of the electronics when starting up. I am using a Dynon FlightDek 180 so if I have the avionics master off during start-up I won't see the engine data until after I turn on the avionics master and the 180 comes online. I don't think that is a big deal. Is it?

I do like the thought of having the avionics master switch circuit act as the e-bus as well so I am considering wiring it to work both functions. This will mean using the diode pack and wiring the avionics master switch to draw power directly from the battery. The only drawback is that if you forget to shut off the avionics master switch your avionics will be powered up until the battery drains completely.

Any input would be appreciated.

Paul
 
Don't wire the Dynon through the avionics master. Dynon approves all of there stuff to be on during engine start.
 
Pmerems said:
I am in the initial stages of wiring my RV-7A so this post is very timely.

I have planned an avionics master switch (engraved my placard panel already). My reasoning was to help keep the transients out of the electronics when starting up. I am using a Dynon FlightDek 180 so if I have the avionics master off during start-up I won't see the engine data until after I turn on the avionics master and the 180 comes online. I don't think that is a big deal. Is it?

I do like the thought of having the avionics master switch circuit act as the e-bus as well so I am considering wiring it to work both functions. This will mean using the diode pack and wiring the avionics master switch to draw power directly from the battery. The only drawback is that if you forget to shut off the avionics master switch your avionics will be powered up until the battery drains completely.

Any input would be appreciated.

Paul
Click to expand...

Paul, you may want to rethink this and have the D180 on a seperate circuit breaker off of the Master bus, along with a Transzorb to absorb any **** that gets thrown at the instrument. You need to have the D180 on, even if all of the other avionics are off. How do you know if you have fuel pressure to start?

You could run the D180 off of a backup battery for engine start, though.

Vern
 
KISS

gmcjetpilot said:
...You do add weight with wire, switches and diodes. One RV-8 I saw had 25 switches in it, some switches with covers. I had no idea how or what was what. There is something elegant about a single master switch, on, off.
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George,

Well said; I think some have seen Apollo 13 one too many times and design their systems as such. As I was doing my usual perusal of the NTSB reports, I cam across this accident:

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20060522X00601&key=1

......Examination of the cockpit schematic revealed that the auxiliary fuel pump "high boost/low boost" switch was positioned next to the "Nav" lighting switch. In the airplane's "Before Takeoff" checklist, the "Lights-as required" task was to be performed four tasks prior to the "Takeoff" checklist.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

The pilot's failure to maintain sufficient airspeed after takeoff to preclude a stall, which resulted in a loss of control and an inadvertent stall. Factors associated with the accident are the inadvertent stall, the loss of engine power due to the pilot's inadvertent activation of the high pressure auxiliary fuel pump, and the pilot's failure to abort the takeoff after receiving abnormal engine and fuel flow warnings on the primary flight display.
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Transients

Has anyone here ever hooked up an oscilloscope to see what kind of transient voltages are actually present at the bus bar during engine start? I'm thinking that the battery is a pretty good shock absorber for voltage spikes and if Dynon says it is OK to start with their equipment on line, that is good enough for me. I like Paul's suggestion of putting capacitors across the B+ to ground, except that so far I haven't seen any documented need for it. Plus, it violates the maxim of "add lightness and simplicate". Just asking. Without data we are all just speculating.
 
avionics master switch

In the planes I have flown, there is an Avionics Master switch which provides power to the radios, transponder and intercom. I'm wondering why I should botehr with such a switch. The only reason for it that I can see is to provide a way to kill those items with one switch. But of course it's more wire and another switch on the panel. I'm not sure the pilot workload in swithing off 3 or 4 radios (instead of one switch) to shed current load if needed, is really that significant.
Have others gone without the avionics master switch, or is there a reason I'm not seeing why it's a must-have?
 
The avionics Master switch provides a way to conveniently remove power sensitive devices from the airplanes power buss when starting or shutting down your engine. If you can remember to turn off ALL of your electronic devices individually before start up or shut down, you can probably get by without a Master switch. There have been discussions whether modern electronic devices really need to be issolated from the power buss during engine start up and shut down but to be safe it is probably a good idea to keep those devices off during those times.
 
Another minor advantage is that the volume can be left where you want it on all radios. On some radios it's easy to punch the squelch and adjust the volume. Others like the Xcom use the same control for squelch and volume, so it's not so easy.
 
Hi Phil,

The majority of builders in our Chapter put in an avionics master as a convenient way to turn on and off all the avionics ... for the reasons stated above. Few believe it's really necessary for isolation from voltage transients as in the "old days", because state-of-the-art gear usually has internal protection built in.
 
prkaye said:
...But of course it's more wire and another switch on the panel...
Click to expand...
The bigger drawback is that if that switch fails you lose everything that it controls (single point failure). I don't have such a switch and several electronic things in my panel don't even have an on/off switch. I have had no problems because of this. I have had switch failures though.
 
I don't know all the details but a friend had his SL-30 ruined during start up recently. Maybe new avionics are not 100% spike resistant.
 
How does the avionics master switch get wired? I guess I use a seperate fuseblock wired to the main fuseblock through that switch?
 
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Phil,
Yep. The switch is wired in between the power feed (usually the main buss) and a separate avionics buss.
 
n5lp said:
The bigger drawback is that if that switch fails you lose everything that it controls (single point failure).
Click to expand...
Not if you wire up your essential bus and avionics bus to be the same thing.

Another benefit not mentioned is that it is convenient to turn on the radios only prior to contacting Ground to get clearances, listen to weather, etc. I just flip on my essential bus switch for that. That way, I'm conserving a little battery juice and it's easy and quick.

I'll tell you that having the Avionics Master switch was much more convenient that I thought it was going to be. I wouldn't wire it up any other way. The only "gotcha" I found was to wire the engine monitor (GRT) to the primary bus so I could monitor oil pressure on startup. I had mine on the avionics bus so I was blind during startup to any immediate oil pressure problems.
 
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I built my airplane mainly for pleasure. I get a kick out of playing around with the various switches, gauges, knobs and controls so the more of these I can have the better. I made a point of leaving out an avionics master switch so I would have to turn ON and OFF all the switches. Its neat to see each instrument come to life one at a time and I am sure it would look more impressive to your non-aviation passenger rather then just flicking one switch.:rolleyes: Of course you could put in the avionics master and still switch each instrument ON/OFF and this would give an extra switch to play with.:)

Fin
9A
 
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I'm confused again

Not if you wire up your essential bus and avionics bus to be the same thing.
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That's exactly what I did.... too.
Click to expand...

Guys, say more about how you did that. The "standard Nuckolls" way to wire an E buss is so it is powered through a diode whenever the primary master switch is on. The standard way to wire an avionics buss is so that it isn't powered unless the avionics master switch is on.

I guess I don't see how can you have a buss that is both of those at the same time...?

--Paul
 
Garage Guy said:
Guys, say more about how you did that. The "standard Nuckolls" way to wire an E buss is so it is powered through a diode whenever the primary master switch is on. The standard way to wire an avionics buss is so that it isn't powered unless the avionics master switch is on.

I guess I don't see how can you have a buss that is both of those at the same time...?
Click to expand...

My avionics/essential buss is a seperate fuse buss. It gets power from the main buss & avionics switch. If the master switch is off, then no power flows through the avionics switch.

The essential switch is wired to the E buss using the Nuckolls method of going through the diode.

L.Adamson
 
L.Adamson said:
The essential switch is wired to the E buss using the Nuckolls method of going through the diode.
Click to expand...

In a Nuckolls E buss, the E buss switch doesn't go through the diode, it powers the E buss directly from the battery (well, through a fuse)... is that what you meant?

If so I think I get it... you have both an E buss switch and an avionics master switch? The avionics master powers the E/Avionics buss from the main buss (and I would guess maybe going through the diode), and the E buss switch powers the E/Avionics buss from the battery? So no single point of failure, because you can power the E/Avionics buss either way?

--Paul
 
Garage Guy said:
If so I think I get it... you have both an E buss switch and an avionics master switch? The avionics master powers the E/Avionics buss from the main buss (and I would guess maybe going through the diode), and the E buss switch powers the E/Avionics buss from the battery? So no single point of failure, because you can power the E/Avionics buss either way?
Click to expand...

That's correct.
 
I liked the advantages of a master avionics switch but I didn't want to negate all the redundancy I built into my system with a single point of failure.

I have a 30 amp breaker switch for a master and I have a 30 amp pull type breaker wired in parallel to the switch. Breaker stays pulled except for when I have a switch failure.

Ken
 
DanLandry said:
Me too. I turn on the Av Master immediately to check OP.
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Just to add to the possibilities... if your EFIS/EMS has a backup battery, oil pressure can be checked throughout startup without worrying at all about any spikes through the main or av busses. Once your happily running, the av master switch can come on and start powering the EFIS/EMS and recharging the internal battery.
 
Garage Guy said:
Guys, say more about how you did that. The "standard Nuckolls" way to wire an E buss is so it is powered through a diode whenever the primary master switch is on. The standard way to wire an avionics buss is so that it isn't powered unless the avionics master switch is on.

I guess I don't see how can you have a buss that is both of those at the same time...?

--Paul
Click to expand...
I have the wiring diagrams posted on my website. I can't insert the direct link right now as the firewall at work blocks me from doing so. A picture is worth a thousand words. Click on the link in my signature.
 
Yep, some hacker hit my site. I'll fix it tonight.

UPDATE: Fixed as of 1:00 PM today. Let me know if you are still having a problem.
 
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Lets just put it this way, it's A LOT cheaper to buy one avionics master switch than replacing worn out switches on the components trust me.
 
f1rocket said:
Let me know if you are still having a problem.
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OK, much better now that the Russians aren't redirecting me to their PHP scripts :).

I like the redundancy you get with your setup.

On our RV-6, we also have the avionics buss doing double duty as the E-buss. Not exactly the same equipment on it, but similar. But the main difference is that I eliminated the feed through the diode from the main buss entirely; it is just powered from the battery through a single locking toggle switch labeled Avionics Master. So while I like the redundancy you have, I elected not to have it. My thinking is that a toggle switch is very unlikely to fail unless it is being moved, and I never move the avionics master unless I'm on the ground. So I can live with that risk. And this setup is simpler, for sure.

--Paul
 
That's fine. Different strokes for different folks. I see your point and agree that it makes sense. I assume you have a breaker/fuse on this line feeding your avionics switch? If so, then you also risk losing all your avionics if the fuse blows, whick is probably more likely than the switch failure.
 
ArizonaAero said:
Lets just put it this way, it's A LOT cheaper to buy one avionics master switch than replacing worn out switches on the components trust me.
Click to expand...

Charlie has a good point. For those avionics that have the off/on switch on the volume control, the pots DO wear out. I've replaced several on com radios for customers who don't have avionics master switches.
 
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