Need constructive criticism for electrical design

[kirkbauer]

OK, I've done a lot of reading (including AeroElectric) and looked at plenty of other designs and I think I'm ready for initial feedback on my tentative electrical system design for my future RV-10. Some notes:

• Primary mission: cross-country IFR
• Redundancy: high redundancy for both flight safety AND convenience (e.g. not as likely to get stuck somewhere)
• Design: two separate busses with ability to cross-connect (z14 design)
• Bus #1: "clearance bus" designed to operate on ground prior to starting engine
• Bus #2: "main bus" in-flight convenience features
• Fault tolerance: flight can continue with little or no side effects with failure of either bus (battery, alternator, and/or contactor)
• Ultimate failsafe: dual pMags plus G5 (w/ battery) plus Foreflight plus handheld backup radio
• Without alternator(s): Bus 1 battery should last 49-91min; Bus 2 battery should last 21-66min (without any load shedding)

Here are some rough procedures to explain how it will work:



Notes on my design drawing:

• Not comprehensive -- doesn't include many details such as most wire sizes, sensors, minor items, etc.
• Somewhat figurative -- some connections won't be wired exactly as shown
• Contactors -- these contactors appear to be very robust and don't need external diodes
• Bus 1 and Bus 2 switches -- may rename, but each DPST switch controls its bus by turning both the battery and alternator on/off
• Circuit breakers -- only for items I want to be able to disable during flight
• Runaway trim -- need to pull both the AP and Trim breakers to fully disable trim
• Crossfeed -- still researching the maximum voltage differential between the two batteries where it would be safe to engage with both batteries "on", but generally it shouldn't be necessary to engage the crossfeed with both busses enabled. On the other hand, the batteries will be next to each other with very short runs of large wire, so it may be able to handle quite a bit of voltage differential.

Feedback requested:

• Am I missing any major electrical items?
• Any logical or procedural errors?
• Any incorrect fuse/breaker/wire sizes (for the wires where size is shown)
• Any safety/failure things I am overlooking?

 

[Watzlavick]

Seems reasonable in general for Z-14. You don’t need fuses on the low side of relays if they’re going to ground, the coil resistance limits the current. This assumes there is a short run between the battery and relay. If you had one side grounded and were switching the high side, then they would be needed. Be aware the B&C 410 doesn’t have any output at idle RPM, that’s one reason I went with the MZ-30L. On the 10A A/P breaker, that implies 18 ga wire and the DSUB connectors will only take 20 ga max. So if you splice to 20 ga, there will still some undersized wire. I put my yaw servo on a separate CB for that reason. Putting the GMC 507 on both buses won’t do any good since the servos are on only one bus unless you still want to be able to use the Flight Director and use the knobs from it. Not a bad idea, actually. An 8 ga wire from the main alt may be a bit small, might be ok in free air. I’d think 2 ga should be plenty for the cross tie instead of 0, could even use 4 ga if they’re close and in free air. I’d recommend putting the GMA 245 on the other bus as it has a passthru for COM1 only if it loses power. As it is now, if you lose bus 1, you can’t use the GTR 205.
-Bob

 

[dmattmul]

Be aware the B&C 410 doesn’t have any output at idle RPM, that’s one reason I went with the MZ-30L.

MZ-30L is a nice lightweight alternator. I run a B&C410 and output starts at ~900 RPM and produces 6-8 amps at ~ 1,200 RPM (Amps 2). I'm ok with that.

 

[wjb]

Here are some thoughts that jumped out at me:

Be sure the devices jointly connected to Bus1 and Bus2 have isolation diodes either externally or internally (eg, the Garmin boxes with multiple power pins) or you'll back power the other bus through one (or more) of the boxes. I believe the Garmin boxes are set up this way, but not certain that it's universal.

Also, the 650 and audio panel are on Bus1. The audio panel has a fail safe mode that connects Com1 to at least the pilot's headset when unpowered. Be sure to wire the Bus2 radio to Com1 on the panel. If you hooked Com1 up to the 650 (which may seem the natural choice), and that bus went out, you'd lose the use of Com2 as well.

 

[Watzlavick]

MZ-30L is a nice lightweight alternator. I run a B&C410 and output starts at ~900 RPM and produces 6-8 amps at ~ 1,200 RPM (Amps 2). I'm ok with that.

That's interesting - when I asked B&C about it, they said I shouldn't count on anything below 1000 rpm. Good to know though in case the MZ-30L doesn't work out.

 

[kirkbauer]

Putting the GMC 507 on both buses won’t do any good since the servos are on only one bus unless you still want to be able to use the Flight Director and use the knobs from it. Not a bad idea, actually.

Yeah, that was my plan -- convenient even when AP isn't working. Plus it already has dual power inputs.

I’d recommend putting the GMA 245 on the other bus as it has a passthru for COM1 only if it loses power. As it is now, if you lose bus 1, you can’t use the GTR 205.

Hmm, yes, that's a very good point. Originally I had it on Bus 2 but then realized perhaps my co-pilot would like to be able to hear the clearance delivery process, for example. And passengers could test and get their headsets situated. But you are right, now you have no radio on only Bus 2.

Perhaps I should put the GMA 245 on Bus 1 (as drawn) but make the GTN650 COM2. Then I can make the GTR205 COM1 connected to Bus 2. Now, during startup, the audio panel is up and working. If I lose bus 1, then the pilot's headset connects to COM1 which is running on Bus 2. Does that make sense?

Thanks for the info regarding the wire sizes, fuses, and backup alternator. I will dig into all of it.

 

[kirkbauer]

I have a question about the GSA28 autopilot servos. Based on the information above about them only being able to accept 20 AWG wires, I'm having trouble figuring out how to properly wire them. I see that they can pull a maximum of 2.8 amps. It would seem that a 3A fuse would be pretty risky, so you could use a 5A fuse for just one servo. But the wiring runs are pretty long -- I'm estimating about 15ft from the panel to each servo. 5A for 15ft comes up as 18AWG per the official FAA chart. It doesn't use 2.8amps continuously, but I have to match the wire size to the fuse, right?

 

[Halleffect]

I have a question about the GSA28 autopilot servos. Based on the information above about them only being able to accept 20 AWG wires, I'm having trouble figuring out how to properly wire them. I see that they can pull a maximum of 2.8 amps. It would seem that a 3A fuse would be pretty risky, so you could use a 5A fuse for just one servo. But the wiring runs are pretty long -- I'm estimating about 15ft from the panel to each servo. 5A for 15ft comes up as 18AWG per the official FAA chart. It doesn't use 2.8amps continuously, but I have to match the wire size to the fuse, right?

Kirk, on the -14 WH-00125 wiring schematic & harness, Van's is using 18AWG for the AP servos.

 

[dmattmul]

Kirk, on the -14 WH-00125 wiring schematic & harness, Van's is using 18AWG for the AP servos.

Here is the wiring diagram for the GSA 28 as presented by Vans. For the trim wires it does switch to 22 awg at the wing root but the main power stays 18 awg.

 

[kirkbauer]

Well, now I'm confused -- I concur that 18AWG is an appropriate size, and what Van's specifies in the -14. But I double-checked the G3X Touch Installation Manual (Rev AU, page 1-8) and it says 20-24AWG for the GSA 28 connector. So I wonder how people are connecting that 18AWG wire to the GSA 28? Perhaps @g3xpert can help?

 

[dmattmul]

My interconnect drawing {from Stein) shows 20 awg wiring. Note: Stein also produces the WH00125 harness. Probably all about voltage drop and allowable risk tolerances.

 

[Walt]

My 2c;
Reliability is MUCH more dependent on quality of materials/installation, than complexity.
You should see some of sh** I pull out of RV's, not pretty,

 

[Watzlavick]

I’m thankful for this discussion. I used 22 ga for the A/P wiring, looks like I need to upsize to 20, missed that. The Ray Allen servos say 800 mA max so assuming 2 GSA 28 servos plus 2 trim actuators is 5.6 A max/peak. AC 43.13-1B Fig 11-3 says 20 ga is good for 20 ft intermittent at 5A. If you want to use 18 ga for most of the run that’s fine but I would leave the CB at 5 A like Garmin recommends. You can use Raychem environmental splices or PIDG butt splices to get back down to 20 at the DSUB connectors.
-Bob

 

[johnbright]

… If you want to use 18 ga for most of the run ... You can use…. splices to get back down to 20 at the DSUB connectors.

How about simply remove some strands from the awg 18 until it fits into the size 20 D-Sub contact?
.

 

[Walt]

GSA28 specs. Below.
Two seat RV controls are very light, low torque = low current. (Torque = 30% typically)
Even the “heavies” use less than 50% torque so current draw is certainly closer to the ‘‘typical’ value.
18ga is overkill, 20ga good, 22ga should also be fine.

 

[rocketman1988]

GSA28 specs. Below.
Two seat RV controls are very light, low torque = low current.
Even the “heavies” use less than 50% torque so current draw is certainly closer to the ‘‘typical’ value.
18ga is overkill.

Also, no real reason to not use 20 AWG instead of 22 AWG. 20 AWG makes things simpler and it is easier to work with than 22 AWG.

 

[Richard Connell]

Hi Kirk
I’m running a stock z-14 as well with dual B&C alternators and PC680s. My mission is describe as being the same as yours.
It’s performed fine for around 250hrs.
Same as you, the idea is that I can run both busses on one alternator with minimal if any load shedding. In a worst case scenario each bus has enough equipment on it to complete a flight safely.
I personally don’t see any benefit in the “clearance” bus idea. I tend to just turn both masters on, do my stuff, then start it up. Even if that takes 10minutes it has no noticeable affect on cranking and I’ve never had any brownouts. As a result both of my batteries always seem to maintain the same voltage.

I recently started using flysto and one of the many interesting data points is voltage drop at startup. Its never dropped below 10v and has a mean of 11v so for my setup it just doesn’t seem like an issue.

The low idle output of the B&C 410 isn’t a problem as others have said. I’d maybe look at a MW just to save weight if nothing else.

20AWG is fine for the servos. 2.8A is max servo load plus max trim motor load simultaneously. Which would be extremely unlikely and very short duration anyway. Even the max servo load of 1.8A as Walt says would be highly unusual in an RV.

Nice wiring diagram

 

[Freemasm]

OK. I'm way outside of my briar patch so please consider the following as questions, nothing more.

A clearance bus makes no sense to me. Treat each as independent avionics buses. The cross tie would only (possibly) be utilized for starting.
Only one ADAHRS but it is labeled #2. No real worries as your G5 is the defacto "other" ADAHRS. Sorry, didn.t see #1ADAHRS. Second one is well into diminished returns with a G5 installed
What is your reasoning the ADAHRS IS NOT powered from each bus?
Ditto G5?
Ditto GAD 29?
Will the dedicated G5 BU battery bring real value with four other wattage sources (assuming dual feed from each bus. See above

Reiteration = these are questions, not critiques. I would like to understand your thoughts and reasoning. Thx.

Edit = I'll add things as I see them. Easier to see when on a real screen. some comments center around the premise/concept of independent avionics buses versus a clearance bus.

- Your Pmags (or any EI or EFI) should be connected directly to the battery. You want as few appurtenances between the wattage source and vital equipment as possible. Having them on the same bus makes little sense to me. Reference previous assumption.
- The DPST switches for main bus relay control make no sense to me. Without independent control of the battery and "related" alt, there's no way to utilize the "other" alt e.g. an alt drops off-line and/or you need to isolate it, you've lost that battery wattage as well which defeats that redundancy..
- I'd consider all "non-vital" equipment to have a dedicated bus fed from hot (alt) side of battery relays. If alt anomalies create need to isolate them, this load is automatically shed.
- Your logic on the starter relay is backward. You want control on the positive side for fail-safe reasons.
- Five current ducers probably isn't going to help you, in fact it could lead to confusion IMO. If you can't diagnose an anomaly from the two alt current ducers and the corresponding bus voltages, it's probably something that should be left until safely on the ground.

 

[Halleffect]

Well, now I'm confused -- I concur that 18AWG is an appropriate size, and what Van's specifies in the -14. But I double-checked the G3X Touch Installation Manual (Rev AU, page 1-8) and it says 20-24AWG for the GSA 28 connector. So I wonder how people are connecting that 18AWG wire to the GSA 28? Perhaps @g3xpert can help?

The 18AWG wire carries two or three servos if you wire all three on the same circuit.

 

[dmattmul]

The 18AWG wire carries two or three servos if you wire all three on the same circuit.

Good point Stein used 20 awg for the 3 servo's on mine and Van's used 18 awg for 2 with an option to add a yaw servo. Stein had a 5-amp fuse protecting the circuit.

 

[N49ex]

Am I missing something? Bus 1 sees alternator 1 output but no connection to Bat 1. Bat 1 can cross feed Bus 2, but no connection to Bus 1. Can’t run stuff with just the alternator.

 

[jclark]

OK, I've done a lot of reading (including AeroElectric) and looked at plenty of other designs and I think I'm ready for initial feedback on my tentative electrical system design for my future RV-10. Some notes:

• Primary mission: cross-country IFR
• Redundancy: high redundancy for both flight safety AND convenience (e.g. not as likely to get stuck somewhere)
• Design: two separate busses with ability to cross-connect (z14 design)
• Bus #1: "clearance bus" designed to operate on ground prior to starting engine
• Bus #2: "main bus" in-flight convenience features
• Fault tolerance: flight can continue with little or no side effects with failure of either bus (battery, alternator, and/or contactor)
• Ultimate failsafe: dual pMags plus G5 (w/ battery) plus Foreflight plus handheld backup radio
• Without alternator(s): Bus 1 battery should last 49-91min; Bus 2 battery should last 21-66min (without any load shedding)

Here are some rough procedures to explain how it will work:

View attachment 61732

Notes on my design drawing:

• Not comprehensive -- doesn't include many details such as most wire sizes, sensors, minor items, etc.
• Somewhat figurative -- some connections won't be wired exactly as shown
• Contactors -- these contactors appear to be very robust and don't need external diodes
• Bus 1 and Bus 2 switches -- may rename, but each DPST switch controls its bus by turning both the battery and alternator on/off
• Circuit breakers -- only for items I want to be able to disable during flight
• Runaway trim -- need to pull both the AP and Trim breakers to fully disable trim
• Crossfeed -- still researching the maximum voltage differential between the two batteries where it would be safe to engage with both batteries "on", but generally it shouldn't be necessary to engage the crossfeed with both busses enabled. On the other hand, the batteries will be next to each other with very short runs of large wire, so it may be able to handle quite a bit of voltage differential.

Feedback requested:

• Am I missing any major electrical items?
• Any logical or procedural errors?
• Any incorrect fuse/breaker/wire sizes (for the wires where size is shown)
• Any safety/failure things I am overlooking?

View attachment 61733

Are you open to sharing your drawio file(s)/symbol library?

 

[Halleffect]

OK, I've done a lot of reading (including AeroElectric) and looked at plenty of other designs and I think I'm ready for initial feedback on my tentative electrical system design for my future RV-10. Some notes:

• Primary mission: cross-country IFR
• Redundancy: high redundancy for both flight safety AND convenience (e.g. not as likely to get stuck somewhere)
• Design: two separate busses with ability to cross-connect (z14 design)
• Bus #1: "clearance bus" designed to operate on ground prior to starting engine
• Bus #2: "main bus" in-flight convenience features
• Fault tolerance: flight can continue with little or no side effects with failure of either bus (battery, alternator, and/or contactor)
• Ultimate failsafe: dual pMags plus G5 (w/ battery) plus Foreflight plus handheld backup radio
• Without alternator(s): Bus 1 battery should last 49-91min; Bus 2 battery should last 21-66min (without any load shedding)

Here are some rough procedures to explain how it will work:

View attachment 61732

Notes on my design drawing:

• Not comprehensive -- doesn't include many details such as most wire sizes, sensors, minor items, etc.
• Somewhat figurative -- some connections won't be wired exactly as shown
• Contactors -- these contactors appear to be very robust and don't need external diodes
• Bus 1 and Bus 2 switches -- may rename, but each DPST switch controls its bus by turning both the battery and alternator on/off
• Circuit breakers -- only for items I want to be able to disable during flight
• Runaway trim -- need to pull both the AP and Trim breakers to fully disable trim
• Crossfeed -- still researching the maximum voltage differential between the two batteries where it would be safe to engage with both batteries "on", but generally it shouldn't be necessary to engage the crossfeed with both busses enabled. On the other hand, the batteries will be next to each other with very short runs of large wire, so it may be able to handle quite a bit of voltage differential.

Feedback requested:

• Am I missing any major electrical items?
• Any logical or procedural errors?
• Any incorrect fuse/breaker/wire sizes (for the wires where size is shown)
• Any safety/failure things I am overlooking?

View attachment 61733

If you think that you might add an oxygen system at some point, you might consider using Aithre Shield EX 3.0 instead of the CO Guardian.

 

[Carl Froehlich]

Kirk,

I suggest you tell us how your POH will be incorporated in this design. Here is the acid test:
- IFR at night and you smell smoke in the cabin. What are your immediate actions? What will you have left for avionics after you take those actions?

Carl

 

[rocketman1988]

Kirk,

I suggest you tell us how your POH will be incorporated in this design. Here is the acid test:
- IFR at night and you smell smoke in the cabin. What are your immediate actions? What will you have left for avionics after you take those actions?

Carl

A little vague, don’t you think?

There is no “magic bullet” that works in every situation.

It is, however, important to think through what might realistically happen, during this stage of the build. Things like an alternator failure, voltage regulator failure, and possibly a battery failure.

 

[Carl Froehlich]

A little vague, don’t you think?

There is no “magic bullet” that works in every situation.

It is, however, important to think through what might realistically happen, during this stage of the build. Things like an alternator failure, voltage regulator failure, and possibly a battery failure.

Hardly vague at all. I was very specific.

While there is nothing to mitigate every situation, this still leaves us the task of examining the results for probable failures to determine if we accept those outcomes. If not then we design in a risk mitigation.

For example loss of an alternator(s) is a probable risk, but the outcome is very manageable if the battery(s) are of known capacity. Immediate actions for smoke in the cabin from an electrical fault should be covered in everyone’s POH. While this is not a likely as losing an alternator, the outcome can be so bad we should know what to do If it happens, and what is left working on the panel so we can safely fly the plane to the nearest airport.

Carl

 

[rocketman1988]

Hardly vague at all. I was very specific.

While there is nothing to mitigate every situation, this still leaves us the task of examining the results for probable failures to determine if we accept those outcomes. If not then we design in a risk mitigation.

For example loss of an alternator(s) is a probable risk, but the outcome is very manageable if the battery(s) are of known capacity. Immediate actions for smoke in the cabin from an electrical fault should be covered in everyone’s POH. While this is not a likely as losing an alternator, the outcome can be so bad we should know what to do If it happens, and what is left working on the panel so we can safely fly the plane to the nearest airport.

Carl

So what do you recommend for “smoke in the cabin”? There is no one task that will mitigate the myriad of potential causes.

I am not saying you shouldn’t think about it but I am saying that you cannot predict every possible failure, nor write a manual to solve every possible failure.

At work, this scenario has two immediate action items, that’s it. Everything else is a procedure for troubleshooting the issue and hopefully stopping the smoke. Unfortunately, this wouldn’t apply to our aircraft as most of them do not have a full face oxygen mask.

 

[Walt]

So what do you recommend for “smoke in the cabin”? There is no one task that will mitigate the myriad of potential causes.

I design so if that happens master goes off, that kills everything except what's running on backup batteries.
(keep alive instruments keep running, and being that I'm not a fan of electrically dependent engines it keeps running as well)

 

[Carl Froehlich]

So what do you recommend for “smoke in the cabin”? There is no one task that will mitigate the myriad of potential causes.

Keep in mind most people are flying with a very fat wire going to a battery. If this wire should short to ground the only protection is to open the master solenoid.

In my POH the immediate action for all such electrical faults is to open both master solenoids (one on each battery). This removes power from the ”fat cables”. The panel remains powered as it fed via separate avionics busses, each buss powering half the panel (EFIS #1, COMM#1, etc.). If the electrical fault continues (now on much smaller wires and with current limiter) then isolate one side at a time.

My assumption is an electrical fault will happen, not what will happen. The POH immediate action is generic to protect against the more severe outcomes.

Carl

 

[Walt]

Keep in mind most people are flying with a very fat wire going to a battery. If this wire should short to ground the only protection is to open the master solenoid.

In my POH the immediate action for all such electrical faults is to open both master solenoids (one on each battery). This removes power from the ”fat cables”. The panel remains powered as it fed via separate avionics busses, each buss powering half the panel (EFIS #1, COMM#1, etc.). If the electrical fault continues (now on much smaller wires and with current limiter) then isolate one side at a time.

My assumption is an electrical fault will happen, not what will happen. The POH immediate action is generic to protect against the more severe outcomes.

Carl

You should do an experiment, try shorting a 10ft (or just a piece scrap) 20ga wire across a battery and watch what happens, if you want to really appreciate it do it in a small enclosed area the size of your cabin. Tefzel won't catch fire, but the smoke is something else.
Bottom line, unless you have smoke hood you won't have time to "isolate" anything.

 

[rocketman1988]

Keep in mind most people are flying with a very fat wire going to a battery. If this wire should short to ground the only protection is to open the master solenoid.

In my POH the immediate action for all such electrical faults is to open both master solenoids (one on each battery). This removes power from the ”fat cables”. The panel remains powered as it fed via separate avionics busses, each buss powering half the panel (EFIS #1, COMM#1, etc.). If the electrical fault continues (now on much smaller wires and with current limiter) then isolate one side at a time.

My assumption is an electrical fault will happen, not what will happen. The POH immediate action is generic to protect against the more severe outcomes.

Carl

You should do an experiment, try shorting a 10ft (or just a piece scrap) 20ga wire across a battery and watch what happens, if you want to really appreciate it do it in a small enclosed area the size of your cabin. Tefzel won't catch fire, but the smoke is something else.
Bottom line, unless you have smoke hood you won't have time to "isolate" anything.

...and so it goes. Everyone has their own levels of risk aversion and there really isn't a "right" answer that fits everyone.

Personally, I can completely kill both electrical systems by switching the Masters off. Then the IBBS system carries what it can and the G5 has an independent battery built in. I also have the option of troubleshooting to isolate the issue, if time and conditions allow.

 

[kirkbauer]

Sorry for the delay and thank you for all of the feedback!

Am I missing something? Bus 1 sees alternator 1 output but no connection to Bat 1. Bat 1 can cross feed Bus 2, but no connection to Bus 1. Can’t run stuff with just the alternator.

Yes, I realized that mistake after I posted it -- there is now a connection from Bat 1 to Bus 1.

Are you open to sharing your drawio file(s)/symbol library?

I just used the standard library in draw.io but had to hand-craft a few things. Message me your email and I'm happy to share.

A clearance bus makes no sense to me. Treat each as independent avionics buses.

I have had some pretty long clearance delays -- by having the equipment needed to get clearance all on one bus, I can leave the other bus (and the engine) off while waiting. For example, I was recently in the NYC area and I was running for almost 30min waiting on clearance before I moved the plane. My Bus 2 has 20-60min of runtime without the engine running. In that scenario I don't like the idea that I could be down to less than half of my battery before starting the engine. My Bus 1 has 30-80min of runtime.

The cross tie would only (possibly) be utilized for starting.
Only one ADAHRS but it is labeled #2. No real worries as your G5 is the defacto "other" ADAHRS. Sorry, didn.t see #1ADAHRS. Second one is well into diminished returns with a G5 installed
What is your reasoning the ADAHRS IS NOT powered from each bus?

With two ADAHRS I figured it is better to separate them just in case there was some sort of crazy electrical event on only one bus. The redundant power isn't necessary and adds just a tiny bit more risk in my opinion. Also it saves me two fuses to not have both ADAHRS on both busses.

I'm not dead set on having two ADAHRS, but the cost is also (relatively) low. I like the fact that the avionics can alert me on disagreement between the ADAHRS systems, and then having the G5 as the tie breaker.

Ditto G5?
Ditto GAD 29?

G5: Kind of like the ADAHRS, with the 4-hour internal battery the redundant power connection seems completely unnecessary, and introduces a tiny bit of risk if either of the busses have a very high voltage event, for example. I guess also it will add a bit of drain to my Bus 1 battery during clearance delivery.

GAD 29: figured I don't need dual power since it doesn't appear to have any function if the GTN 650Xi is offline. So I figured I would want both the GAD 29 and the GTN on the same bus (and only that one bus).

Will the dedicated G5 BU battery bring real value with four other wattage sources (assuming dual feed from each bus. See above

If there is ever a fire situation, electrical smell, etc, and I'm not in heavy IMC, I will turn off both busses (which includes the batteries and alternators) and land with my G5, handheld radio, and/or Foreflight.

- Your Pmags (or any EI or EFI) should be connected directly to the battery. You want as few appurtenances between the wattage source and vital equipment as possible. Having them on the same bus makes little sense to me. Reference previous assumption.

I have pMag 1 on Bus 1 and pMag 2 on Bus 2. Considering the pMags can also generate their internal power, I would need both electrical busses and both pMags to fail before I lose my engine. That's why I don't have them directly on the batteries but I could be convinced otherwise. I figured it was safest to be able to fully turn off all power with the main Bus 1 and Bus 2 switches.

- The DPST switches for main bus relay control make no sense to me. Without independent control of the battery and "related" alt, there's no way to utilize the "other" alt e.g. an alt drops off-line and/or you need to isolate it, you've lost that battery wattage as well which defeats that redundancy..

This one I have gone back and forth on (in my head of course). What may not be obvious is I have a pullable breaker for each alternator. So you can turn the battery ON and the alternator OFF. You can't turn the battery OFF and the alternator ON. This was designed to avoid somebody trying to run an alternator without the battery also enabled, and overall to simply the in-flight procedures.

With the current setup, I can do any of these scenarios:

• Both batteries, both alternators (standard configuration)
• Only BAT1+Alt1
• Only BAT2+Alt2
• BAT1+BAT2+Alt1 (w/ crossfeed and Alt2 breaker pulled)
• BAT1+BAT2+Alt2 (w/ crossfeed and ALt1 breaker pulled)

My intention is that only the first three options are going to be used in flight except in some sort of unexpected and extreme scenario (stuck in heavy IMC over the Rocky mountains?).

- Your logic on the starter relay is backward. You want control on the positive side for fail-safe reasons.

I'm not following this one.

- Five current ducers probably isn't going to help you, in fact it could lead to confusion IMO. If you can't diagnose an anomaly from the two alt current ducers and the corresponding bus voltages, it's probably something that should be left until safely on the ground.

Agreed -- it's only because I'm a techie and I like information. I'm not sure if I'll actually put in that many. I need to figure out if I even have the inputs for them.

 

[kirkbauer]

Kirk,

I suggest you tell us how your POH will be incorporated in this design. Here is the acid test:
- IFR at night and you smell smoke in the cabin. What are your immediate actions? What will you have left for avionics after you take those actions?

Carl

Turn off Bus1, turn off Bus2, land with G5, handheld radio, and/or Foreflight. I have practiced full instrument approaches with only G5+Foreflight and it is very doable.

If I don't think I can safely complete the instrument approach without the GTN650, then in this dual-bus system I can turn on only Bus 1 at some point in the approach to bring the 650 back online. In this scenario, I was at least able to remove all power while setting up for the approach, and I'd be able to land with Bus 2 completely off. It's possible the fire/smell was only coming from Bus 2.