Please Critique my SDS RV14 Electrical System

[TASEsq]

Hi Brains Trust,

I am working on planning out my RV14 electrical system, with the following design goals / features:
- IFR
- Garmin G3X with IFR Nav
- SDS EFI Fuel Injection / Ignition
- Single Battery
- B&C 60amp Main alternator
- Monkworkx 30amp alternator in Backup mode.
- The system needs to be passive - no failure can require pilot action to keep the engine going
- The system needs to handle failures such that i can be IFR/IMC at night and can still have what i need to safety fly a GPS based approach.
- It needs to be bog simple and intuitive. Turn all the switches on, leave them there and go flying.

To this end, I have been working on planning out the system using KiCAD as my planning software, and an Excel based workbook with each wire/group listed as equipment to equipment / pin to pin etc. It's here i am doing load analysis etc, and will eventually do a failure analysis. It is helping me to calculated what supplies i need to order.

Attached is a basic block diagram of the system, with a few KiCAD excerpts showing the main electrical generation schematic, as well as the SDS EFI wiring schematic.

The design is based on the great work by John Bright - thank you for the inspiration!

The design has the following notes:

• - A single dedicated engine bus fuse block located behind the panel feeds the engine electronics. This will live close by the SDS ECU / Relay box, and will be visible for pre-flight checks of the fuses. I will likely used a Bussman or Bluesea fuse block - i am happy that these are robust enough not to be a SPOF. (plus the injectors only have a single source of power anyway).
  
• - This bus is fed by the battery through a relay, or fed from the monkworkx generator, or via a diode from the main bus (and main alternator). (3 sources of power).
  
• - The relay is needed between the battery and the engine bus because i believe the Monkworkx will have a parasitic drain otherwise.
  
• - An Essential bus, is fed from the engine bus at one end, and the main but at the other end. This gives power to the basic IFR instruments (GAD27 - which feeds keep alive power to PFD, GEA24 etc, as well as G5, NAV, 1 x com, cockpit lights, Autopilot etc). This is a CB fed bus located where the RV14 circuit breaker panel is.
  
• - On the main bus / main fuse block is everything else in the aircraft. The main fuse block will only be installed if i run out of CB's on the main bus. This is also located on the RV14 Circuit Breaker panel area.
  
• - The idea is that any of the 3 DC source switches can be on in flight, and i will have the engine bus and minimum IFR instruments.
  
• - In normal ops, all 3 DC source switches are on at all times.

The normal use scenario is as follows:

• - Normal ops will see the ENG/ESS BUS - BATT and GEN switches turned on. This will power the engine ECU's, and basic instruments from the battery. One fuel pump is turned on. Both ignitions are turned on and engine is started.
  
• - The monkworkx should then take over supply of the eng/ess bus and show it is working. The ENG/ESS BUS - BATT switch could be turned off as a test to check the Monkworkx can handle all engine/ess loads.
  
• - The MAIN BUS BATT/ALTN switch is then turned on and the main alternator takes over, and the monkworks goes to sleep, checking the main alternator is working.
  
• - Runups would include swapping fuel pumps (and leaving them swaped to even out the duty cycles on the pumps), checking both ignitions. Swapping ECU switch to backup then back to primary. At this point no switches need be touched on the left switch panel until shutdown. Runup is simple really.
  
• - For normal ops, the main alternator would be feeding the main bus through a 70amp MIDI and a shunt, and through a diode the essential bus. Since the engine bus is connected to the essential bus, the main alternator would also supply the engine bus. As an additional pathway, the main alternator also supplies the engine bus through both battery contactors (aka dual feed pathways to the engine bus).
  
• - In a cockpit smoke scenario, or the main bus faults, the MAIN BUSS BATT/ALTN switch is turned off. This kills power to the main bus and everything except the essential items. I can then pull CB's on the essential bus, as required, if the smoke persists.
  
• - If the main alternator fails, the monkworkx passively takes over as the voltage drops to battery volts (with a CAS message etc).
  
• - If the battery catches fire, i can turn off both battery switches and the monkworks powers the engine and essential items.
  
• - If the essential bus faults - then i kill the main battery switch to turn off the main buss and it's supply to the essential bus. Do i need to place a relay inline between the engine bus and the essential bus, such that power flows through the normally closed contacts, and a "shed" switch to turn off this bus? Or just pull the 5-6 breakers along the bottom row? This will be fed with a fuseable link from the engine bus to protect it's feed wire, and from the main bus through a diode the wire will be short. But nothing really protects the essential bus itself from a short? (this will be CB's and a copper bar).
  
• - If the engine bus faults - will then i have an issue. But i think this is a low risk event. I will use a sealed automotive fuse block. What could really go wrong with this?
  
• - For those who have installed the Monkworkx, do i need the 30amp MIDI on the feed line from this generator? Or is it fuse protected internally?
  
• - There are probably many failure points i have not considered!
  
• - I was thinking for the 2 battery 'contactors' that i would use some Gigavac relays instead. I don't really see why i need the old tin can versions? Why not just a relay?

I know asking people to review my electrical system is a big ask - i am totally open to criticism and will happily take on all comments. I have an open mind as this is my first attempt at something like this. My aim here is to flesh this all out and try and think of as many gotchas now before i even get close to running wires.

Thanks in advance to the brains trust for your help and assistance. If anyone wants any more detail etc just ask and i will make a follow up post.

[esco]

Trent: I recommend you reconsider the boost pump function.

It appears to be an (added) single point of failure, with limited value: the fuel system would be usable by other pilots without this function, and YOU don't need it, as you already well understand normal, emergency, and maintenance procedures of the fuel system.

(I have a limited perspective, as I'm building, not yet flying, a very similar RV14A: IFR, G3X, SDS EFII, single battery, B&C 60A main; Monkworks 30A backup alternator...)

Thanks for thinking this through and asking for reviewers; I'll be watching for other input!

[Carl Froehlich]

Trent,

Before digging too deep, some questions and comments:
- Your first diagram and second diagram do not agree. Follow the battery connections that bypass the two solenoids. You have the starter solenoid always connected to the battery as your copper bar bypass the other two solenoids. Here you have no way to isolate the aircraft from the ship battery.
- Most people start the engine via power from the battery on the output side of the master solenoid. This also provides a means to isolate the big wires in the event of an electrical fire.
- I would not think a single ship battery ship, regardless of the backup generator scheme to be adequate for an electrically dependent engine and IFR.
- Your dependence on the Monkworz generator is noted. I fly with one as well (two battery, split buss system, pMags and traditional fuel injection). I know it works as I test it. In my case I have additional backup modes to support continued IFR flight to fuel exhaustion. For you I suggest a rigorous test program that starts with opening the master solenoid (after you rewire it to really isolate the battery) to verify the system works as you think it will.
- While you note you have three DC power sources you may only have two (the battery and the Monkworz). The primary alternator may not work without a battery on line - I don’t know you will need to verify it.
- The last issue is about power distribution. Most issues I run across that result in a dark panel have a root issue of getting power where you need it, not a lack of a power source. After you get your final design locked down look at the power flow - and what can fail that will turn it off. This includes relays, diodes, ground connections, etc. Here is where a two battery system may provide advantage.

Carl

[Mike S]

Why only a single battery?

I have a similar setup, dual battery and dual alternator and full SDS EFI&I.

One battery and one alternator (B and C SD8) are dedicated to the engine and the panel.

Second battery and main alternator (B and C 60A) are dedicated to the rest of the plane----AND feed power to the first circuit through a Shottkey diode. All items not needed to keep the engine running and the EFIS's lit up run off this second circuit.

One benefit of this design is the ability to power up the EFIS first, then start the engine from the second battery and I never see a panel brown out due to starter current draw, and I have all engine parameter gauges up and working during the start.

[Freemasm]

The single battery becomes a SPF though many will argue to the contrary. Even extensive testing for your alt or gen to stay on-line with a bad battery doesn't necessarily mean anything. A "pass" means it stayed on line on that day, under those exact variables. A quick change in load from a comm xmit, flap actuation, pump actuation, etc./vice versa -> undamped load swing -> blackout.

Batteries are cheap; or not-so-cheap but very light weight. Gage your risk.

[vlittle]

I am surprised no one has commented on your alternator connection. You have it directly connected to the main bus relay, without protection. Yes, alternators fail sometimes.

Normally, the alternator connects to a shunt, then an ANL fuse or circuit breaker, then to your main (Master) bus. (You can reverse the two if you want).

See http://www.vx-aviation.com/sprocket/...s-2/MASTER.pdf

or reference Bob Nuckolls aeroelectric drawings http://www.aeroelectric.com/PPS/Adob...itecture_Pdfs/

[supik]

I do have a very similar setup. My TT is 25hrs so not much real world experience..

-single battery
-dual alternator
-1x SureFly Mag (electr. dependent)
-1x classic Mag (if all elec fails, this should keep the fan spinning)
-G5 Standby EFIS with its own backup battery
-the Main Bus feeds the ESS Bus via 2 parallel Schottky Diodes any time the MASTER Switch is set to BAT
-During NORMAL Operations both the Master Switch and ESS Switch are set ON (the Master Contactor and ESS Contactor are closed with both Alternators armed (the Main Alternator is set for higher voltage / has priority)


I'm wondering if there would be any disadvantage if the MASTER Switch would power both the MASTER Contactor and the ESS Contactor at the same time.
- This would avoid the diode voltage drop on the ESS bus as long as the ESS bus is not powered by its own Contactor.
- Should the ESS Contactor fail, the Diodes will take over.
- I could still preflight-check the Diodes operation if I power the ESS Contactor only (no reverse flow to the Main Bus).

[Watzlavick]

You might want to use a fuse upstream of the diode. If you get a short on the essential bus, the 10 ga wire (or the diode) will become the fuse, opening before the 70 A MIDI fuse. Going to a larger gauge wire won’t help because if the 70 A MIDI blows due to an ess bus short, both buses would go down.

The MZ-30L has a 30 A output fuse so in theory you don’t need an extra fuse on the output.

-Bob

[TASEsq]

Firstly, thank you all for the initial look at the system. I will try and give some more info below, but again open to discussion! I truly appreciate all the assistance.

The 'block diagram' was a quick and dirty representation i made (in word!) of the system to help explain how it works and assist this discussion - i messed up a few things on it, and it's been corrected and V2 attached.

Single Battery: This is a great discussion. I went back and forth, around and around about this. Initially i had a B&C backup alternator in the system - that for sure would mean a second battery is required, as (same as the primary) it needs a battery in the system to work properly. But the Monkworkz has changed my thinking totally - it is like having a second battery in the system, however it does not run out of juice. It is able to run by itself, with no battery present. I am able to totally isolate the 'main bus' side of the system, then disconnect the battery from the ENG/ESS bus side of the system, and the Monkworkz can power all of the essential loads without a battery present.

The thinking on not going with the 2 batteries was mostly that the batteries (EarthX) are very expensive. They are also consumable, and impossible to get when away from base. I have worries that something would happen and the BMS would switch both of them off. Hard to buy in Australia. Also, on the RV14 i haven't been able to find anyone who has a good location for the battery - Andrew Kilroy has used 2, and placed his high on the left hand side, so that is a possible location, but i would need to really have the engine hung to know.

I know the Monkworz is a new player - but it really seems to change the game for electrically dependent engines. Have i got too much love for the unit? In my scenario, to end up on the single battery only i would have to have both a main alternator and the Monkworkz fail (and both sources get tested every flight). If the battery itself failed, i would be on the Monkworkz alone - but this would be enough to fly until the fuel ran out.

Keen to continue the discussion on this, as i am not wedded to my system design by any means.

Originally Posted by esco

Trent: I recommend you reconsider the boost pump function.

It appears to be an (added) single point of failure, with limited value:

If you take a look at the SDS design page, there is an explanation of how the 'boost pump' works. The SDS has 2 fuel pumps, and one runs all the time. The other is able to be used for take-off and landing as a backup should the main fail. In my system, both the pumps are run through relays - on the normally closed contacts. Similar to how the injector relays work in the SDS design. Aka, they run all the time unless you ground the relay. The 'boost pump' switch is where this ground comes from. With it selected 'off' there is a ground available to both of the fuel pump switches. Meaning, when i select one or both of the boost pump switches to off, they connect to the ground which comes through the 'boost pump' switch. Turning the boost pump switch 'on' simply removes the ground from both the pumps, so they turn on. This is a passive failure system. The failure of any fuel boost pump switch, the electrical failure of a relay, failure of the boost pump switch, or loss of both the grounds off the boost pump switch, all result in the fuel pump running. A total physical destruction of the fuel pump relay might render the pump US. I was planning on placing them in different locations in the airframe (10" apart or something). I liked this design as it was similar to most other commercial aeroplanes - turn the boost pump on for take-off or if something happens. Yes - i could also just turn on the other fuel pump switch to achieve the same thing - but i like the idea of never having to touch a switch on the left hand engine switch panel in flight. Less likely of turning off an ignition after takeoff!

Originally Posted by Carl Froehlich

- Your first diagram and second diagram do not agree. Follow the battery connections that bypass the two solenoids. You have the starter solenoid always connected to the battery as your copper bar bypass the other two solenoids. Here you have no way to isolate the aircraft from the ship battery.
- Most people start the engine via power from the battery on the output side of the master solenoid. This also provides a means to isolate the big wires in the event of an electrical fire.

Thank you - this is corrected in the block diagram V2. The KiCAD diagrams show it properly. The power for the starter comes from the 'downstream' side of the ENG/ESS bus relay.

Originally Posted by Carl Froehlich

- Your dependence on the Monkworz generator is noted. I fly with one as well (two battery, split buss system, pMags and traditional fuel injection). I know it works as I test it. In my case I have additional backup modes to support continued IFR flight to fuel exhaustion. For you I suggest a rigorous test program that starts with opening the master solenoid (after you rewire it to really isolate the battery) to verify the system works as you think it will.

This would be normal ops i think. I would start the engine with the MAIN BUS totally off. After the engine runs, I should see the volts increase and the Monkworkz power the system. I can turn off the ENG/ESS bus battery switch to truly test this. This is the reason there are 2 switches for the ENG/ESS bus (as the monkworkz can run by itself without a battery), but a progressive switch for the MAIN BUS side (as the B&C alt needs the battery to run).

Originally Posted by Carl Froehlich

- While you note you have three DC power sources you may only have two (the battery and the Monkworz). The primary alternator may not work without a battery on line - I don’t know you will need to verify it.

You are probably right here. Both the battery and the Monkworkz can be used standalone, but the alternator needs the battery.

Originally Posted by Carl Froehlich

- The last issue is about power distribution. Most issues I run across that result in a dark panel have a root issue of getting power where you need it, not a lack of a power source. After you get your final design locked down look at the power flow - and what can fail that will turn it off. This includes relays, diodes, ground connections, etc. Here is where a two battery system may provide advantage.

Thank you - this is my plan. Once i flesh out a design, i will make up a table where i fail one thing at a time and log what would happen. It may required changing the system design.

Originally Posted by vlittle

I am surprised no one has commented on your alternator connection. You have it directly connected to the main bus relay, without protection. Yes, alternators fail sometimes.

Thank you - this was an error in the block diagram. On the proper diagram i have a #10 fuseable link on the B lead feed to the hot side of the the MAIN BUS contactor. My thinking was that this then protects the B lead from shorts to the main contactor. From there i need to protect the #6 though the firewall - this was the job of the MIDI. I had based this on the Z101 - this has a #12 FL on the B lead to the starter contactor (at the contactor end). He then had an unprotected #4 between the starter contactor and battery contactor (this is ok because it is short), then (strangely??) no protection on the #6 feed from the hot side of the battery contactor to the main bus. I didn't think this was a great idea, so added the 70a midi.

I may be totally misunderstanding how these protections are supposed to work of course!!!

By swapping them, do you mean i could have the shunt / midi in either order?

[Edit - text too long. See second post]

[TASEsq]

Originally Posted by Watzlavick

You might want to use a fuse upstream of the diode. If you get a short on the essential bus, the 10 ga wire (or the diode) will become the fuse, opening before the 70 A MIDI fuse. Going to a larger gauge wire won’t help because if the 70 A MIDI blows due to an ess bus short, both buses would go down.

This is interesting. So if the essential bus shorts, the #10 feed from the main bus to the essential bus would go down before the 70amp midi? I think this is what i would want to happen? This would mean the main bus would remain powered. The essential bus is also fed from the engine bus, but there is a fuseable link on this feed line, so this would also blow, isolating the shorted essential bus.

Perhaps a #14 fuseable link on the short #10 feed from the main bus, through the diode to the essential bus is a good idea? If the essential bus shorts, then this fuse link goes to stop flow from the main bus, and the fuselink on the feed from the engine bus also goes - isolating the shorted bus automatically? Am i understanding this ok?

If the main bus shorted, then the 70amp midi would blow, isolating this bus (as the diode is there to stop current flow from the essential bus back to the main bus).

Originally Posted by Watzlavick

The MZ-30L has a 30 A output fuse so in theory you don’t need an extra fuse on the output.

I was not totally sure about the 30amp midi to be honest. I will be flying with both battery contactors open. This means that the essential bus will be powered by the main alternator most of the time. There are 2 feed paths - one is via the diode from the main bus, the other is through the 2 contactors to the engine bus, then the essential bus.

Should the power feed from the Monkworkz the the ENG/ESS bus contactor short - the fuse in the Monkworkz will protect the line. However, since the feed line to the engine bus is connected to the hot side of the ENG/ESS bus contactor, and this is getting fed from the alternator - i thought that if this line was to short, it would not be protected by the Monkworkz fuse. It needs the 30amp midi to protect this line from a short?

Does this thinking made sense?


Thank you all for your feedback so far. Please see amended V2 of the block diagram. The KiCAD source diagrams are unchanged at this point, however will consider the fuseable link between the MAIN BUS / ESSENTIAL bus feed as discussed above.