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Electrically dependent engine - Dual Alternator Single Battery

johnbright

Well Known Member
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Update... I changed to Z101 as a template when Bob Nuckolls published it in 2020. This will affect the relevance of some of the responses to this thread. I edited my responses in this thread appropriately.

I wanted to have a detailed drawing of my electrical wiring, electrically dependent, EFI + EI, so I sketched it using Eagle and also did a wire by wire failure analysis, engine start check list, and other documents.

I'm sharing my documents via google drive for those who may be interested. Feel free to reach out with any questions or comments.

https://drive.google.com/folderview?id=1u6GeZo6pmBWsKykLNVQMvu4o1VEVyP4K
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Raise the bar!

Wow John, talk about raising the bar! Impressive set of documentation.
 
Dual Alternator

Looks good. Right out of the Bob Knuckles playbook. I have the 2 B&C alternators, V regs, and relays if you haven't all ready purchased.

Regards
Bruce
 
Very well done! What standby alternator are you planning on using? A practice of mine is to use a very good one with enough capacity to not really have to make any changes to aircraft operation if and when it is needed. The newer B&C standby alternators that fit on the vacuum pad are rated for 40 amps. With our glass panels and LED lighting, that is enough capacity to run everything, including the pitot heat, all day long. Even at night.
No sense getting stranded when on a long trip away from home. 😀

Vic
 
Alternators etc

Looks good. Right out of the Bob Knuckles playbook. I have the 2 B&C alternators, V regs, and relays if you haven't all ready purchased.

Regards
Bruce

I do not have those items yet. I sent you a PM.
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Very well done! What standby alternator are you planning on using? A practice of mine is to use a very good one with enough capacity to not really have to make any changes to aircraft operation if and when it is needed. The newer B&C standby alternators that fit on the vacuum pad are rated for 40 amps. With our glass panels and LED lighting, that is enough capacity to run everything, including the pitot heat, all day long. Even at night.
No sense getting stranded when on a long trip away from home. 😀

Vic

Which model of the b and c standby alternators will give you 40 amps? I have a 410h and at 2400 engine rpm, it would generate around 30 amps. According to b and c's chart, it would have to run at 8000 rpm or around 6100 engine rpm to get 40 amps. Using the Lycoming ratio of 1.3 of course. Dazed and confused as usual:D
 
32 and 35 A pad alternators

Which model of the b and c standby alternators will give you 40 amps

B and C has two pad alternators, one delivers 32 and the other 35 A at cruise on Lycoming. Main advantage of larger one is output at lower RPMs. I summarized the spec'd outputs at the top of the "Calculations Measurements and Specs" spreadsheet in my folder "Engineering basic stuff".
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Excuse the noob question.

I have a plane power alternator. Would I do better to go with their standby alternator, or with the B&C standby.

(I'm sure I'll hear I should replace the primary with B&C, which is of course an option since it's pretty low time)

Thanks.

And thanks to the OP for the detailed drawings.
 
....With our glass panels and LED lighting, that is enough capacity to run everything, including the pitot heat, all day long. Even at night.
No sense getting stranded when on a long trip away from home. ��...

Dont mean to try pick a fight here Vic, but are you suggesting that in an "electrically dependent" airplane, you would launch with a failed primary alternator to get home? I've read many of your posts and you seem to be very conservative. The above appears to run contrary to this premise.
 
Come on gents....John asked for a review. Let's stay focused on that.

John, my compliments sir. I've downloaded your work for study, and as a nice reference. So far the only detail I'd note is the wire #55 connection to the battery positive post. I'd move the connection point to the battery bus. It would work the same, but make the backup alternator connection immune to battery terminal corrosion.
 
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Minor comment, but an aircraft voltage regulator instead of the 'generic' one would include the OV crowbar and remove one extra item from the system.

If you buy the B&C alternator they usually have a deal going that throws in the VR really cheap.
 
Diode Alternative

Looks pretty reasonable.

What is the normal operating position for the "inj bus alt feed"?

I like when failures don't require any pilot interaction because I know who is flying... so I'd probably make it SOP to fly with that on.

Food for thought, I used a different diode for isolating the critical bus and critical battery:

https://www.digikey.com/products/en?keywords=STPS24045TV

It is two diodes in one package, with screws pretty close to #6 size. The diodes are isolated from the base so you can bolt this to any beefy AL part of your plane and use the airframe as a heat sink. Not much heat will come out though, this diode is rated for 120 Amps, so the ~15 or so amps you're probably running it will not yield a lot heat, so you're not going to roast your airframe. There is a 0.062 cross member behind the panel I bolted to and then covered the entire diode array with some 1/2 inch vinyl tubing held in place with zip ties.

I only have one alternator so the critical bus is fed from both a switch off a back up battery and through a diode from the main bus.

The backup battery is maintained off the main bus through the other diode.

If my one alternator dies the main bus stays alive for as long as the main battery will carry it, and the critical bus stays alive for as long as the backup battery and main can carry it, and no pilot interaction is required.

So far no issues at ~1340 hrs. I've been meaning to add a backup alternator but never got around to it.

Bill
N84WJ, RV-8, 1340 hrs.
rv-8.blogspot.com
 
Alternate feeds and checklist

Looks pretty reasonable.

What is the normal operating position for the "inj bus alt feed"?

I like when failures don't require any pilot interaction because I know who is flying... so I'd probably make it SOP to fly with that on.

Edited after I changed to Z101 as a template when Bob Nuckolls published it in 2020.

The injector bus is now called engine bus and it's "alternate feed" is now "engine bus master". I list the engine master as on from engine start to engine shut-down. The Engine Start and Runup Checklist is in folder "Top level schematic" https://drive.google.com/folderview?id=1u6GeZo6pmBWsKykLNVQMvu4o1VEVyP4K

The preflight checklist does indeed have the engine master on during flight so that the engine bus is powered both by the battery and by the diode feed from the main bus. A failure of either of these paths is benign in flight and discovered at preflight (the pilot may notice the engine bus voltage fall by about 1/2 volts if its feed from the battery fails). In the event of engine trouble in flight the pilot:
  • Moves injector ECU switch to backup.
  • Turns on backup fuel pump.
  • Checks fuel valve.
  • Opens alternate air.

Also, both alternators are normally on. Main alternator failure is benign in flight and discoverable at preflight. Pilot may notice voltage fall by about 0.6 volts if main alternator fails, or more if load needs to be reduced below 32 or 35 A (at cruise RPM on a Lycoming) depending on vacuum pad alternator size.

SOP includes:
  • Fly periodically with engine bus feed from battery off to stress test its diode feed from the main bus. (battery feed to engine bus is normally stress tested due to voltage drop in diode feed from main bus)
  • Fly periodically with main alternator off to stress test battery alternator. (main alternator is normally stress tested because it is set to a higher voltage than the battery alternator)
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Schottky diode

Food for thought, I used a different diode for isolating the critical bus and critical battery:

https://www.digikey.com/products/en?keywords=STPS24045TV

It is two diodes in one package, with screws pretty close to #6 size. The diodes are isolated from the base so you can bolt this to any beefy AL part of your plane and use the airframe as a heat sink. Not much heat will come out though, this diode is rated for 120 Amps, so the ~15 or so amps you're probably running it will not yield a lot heat, so you're not going to roast your airframe. There is a 0.062 cross member behind the panel I bolted to and then covered the entire diode array with some 1/2 inch vinyl tubing held in place with zip ties.

That's a Schottky diode so it has half or less the voltage drop of silicon. From what I can tell reading the spec sheets, higher current diodes have slightly less voltage drop and more reverse leakage. I don't think reverse leakage matters in this application.
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voltage regulators

Minor comment, but an aircraft voltage regulator instead of the 'generic' one would include the OV crowbar and remove one extra item from the system.

If you buy the B&C alternator they usually have a deal going that throws in the VR really cheap.

I called B and C a couple months ago and they would not give me a discount. They have standard bundles but would not make a custom bundle for my needs.

In the mean time I am purchasing L60 (1.4 lb heavier than BC460-H), SD20 (identical to BC410-H), LR3C-14, and SB1B-14 used from someone on VAF.
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I was thinking about the choice between 2 batteries or alternators as well. Boeing and Airbus have a single ship's battery (separate APU start battery that cannot be connected to the ship). But obviously multiple charging sources, including a RAT, except for 737's.

The vast majority of alternator failures I have seen have been voltage regulator failures. Maybe all but one, and I have seen dozens (not aircraft). The voltage regulators that come in "rebuilt" alternators are junk. Over half I have used were failures within a few hours, if not straight out of the box.

Has anybody tried single battery, single alternator, dual external voltage regulators?
 
John.
Any wire connected to the PC680, if more than a few inches long, needs to be protected. You have 10 and 12 gauge wires, plus a #6 downstream of the master contactor that can all be smoked (i.e catch fire) by a short circuit. The # 4 starter wire may be OK.

ANLs or breakers work well or fusible links. I don't know your physical implementation, so you may have this covered.
 
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Excuse the noob question.

I have a plane power alternator. Would I do better to go with their standby alternator, or with the B&C standby. SNIP

That is what I?m running in the RV-10. Each Vital Buss (Avionics #1 and Avionics #2) is fed from the standby alternator via a bank of diodes to make sure a fault in one of the two avionics busses does not take down the other.

Carl
 
I am going to build a QB kit starting in about 5 months. Mostly I will just do what others have done, because it works. But I have been designing, building, fixing things since I was way into the single digits. I got an engineering degree because I wanted to be able to build better things.

The RV forums are the largest source of great info for the latest and greatest in aircraft. I personally have 82 hours in light aircraft, 80 of which were prior to 1982. The rest of my adult life I have flown military/airline. I don't design aircraft, although that is a regret. But I have a bunch of type ratings, and have had to memorize a bunch of systems for those aircraft. I also have fairly extensive experience in auto and marine worlds. But my lack of experience in this realm of aviation, I am looking for info, not providing it.

I have been reading the VAF forums on and off for several years. I finally have to build my own aircraft electrical system next summer. In my mind, I want 1 battery, and multiple charging sources. This is what every OEM manufacturer does.
But in my experience, and my reading of this, and other forums, the part of the alternator that fails catastrophically most of 90% is the regulator.

The most reliable part of any aircraft electrical system is the battery. The mechanical part of the alternator is probably #2. The ground wire, contacter, etc are all worse. The absolute worst part is the voltage regulator. If I was a systems engineer, that is the part I would want to back up.

So why not 1 battery, 1 alternator, and 2 voltage regulators?. To my mind it backs up the alternator, and saves 5-8 pounds, and the same amount of dollars.

This is a question, not a statement. Lots of experience here on this forum. I am looking for a reason why I am wrong. Why have an entire alternator and regulator when the regulator is what fails the vast majority of the time?
 
John,

You have done a nice job mitigating the inherent risks associated with a single battery power distribution system.

I offer however, that a more robust design that has two batteries and one or two alternators would be better suited for your very electrically dependent airplane.

While alternator failure is the most common problem (and your design addresses that well) it is not the only risk. We have incident reports of twin engine aircraft (two alternators, single battery) that have suffered total loss of electrical power from a single buss fault. I'm not saying your design has this exact same fault, just pointing out that two alternator do not resolve all design risks.

If nothing else, I recommend you consider a diode protected backup battery for one side of the engine. I'm not a fan of backup batteries so I go with two identical main batteries as this better addressed my other design criteria.

Not attempting to flame - just providing feedback as you requested.

Carl
 
Thanks Carl;
Your example of the light twin losing power is a good one, and part of my reason for me wanting simplicity. Design engineers look at the risk associated with each component. They try to come up with a system that mitigates the risk of the most failure prone components, and has the system degrade "softly".

Everything you connect to the battery is less reliable than the battery. Taking two batteries or two alternators, and either putting them on a A/B switch or coming up with a circuit that puts one in standby, might add more risk. If you are not careful, you system will be far less reliable than just a simple single battery single alternator.

I am probably going to run a full EFI/EI on my airplane, so it will be electrically dependent. This forum is such a vast wealth of knowledge, I have spent a considerable amount of time reading it the last month or so. The poll on alternator failures confirmed my auto/marine experience. At first I thought it was because a lot of RV's use auto alternators with internal regulators, but the PP and BC alternators that "failed", also mostly failed because of the VR.

In the marine world, the 2nd most unreliable part of the electrical system is the battery switch. They are big 100+ dollar switches. You would think they would last forever. I have seen them last less than a year. I never saw one start a fire, but a few were close. The worst part is the shore power connection. They rarely last 2 years, and are the #1 cause of boat fires.

With external regulators, it sure seems like it would be easy to wire up a backup VR and mitigate 90% of the risk of a failed alternator, all for 150 dollars and less than 1/2 a pound.

Boeing and Airbus use one generator per engine, isolated, but connected through an automatic bus tie in case one generator fails. If both fail, you can start the APU and have the APU power the airplane. If all AC power fails a RAT pops out, and provides minimal electric/hydraulics to be able to fly and land the plane. But they all have 1 ships' battery. Boeing went cheap on the 737 and didn't put a RAT on it. If you lose all AC, you load shed as much as you can, and you get 30 minutes to land. I can't think of the last time a 737 landed on battery power. It might be decades. Looks like a lot of jet airplanes are carrying around a RAT for no good reason. The FAA came to the same conclusion, and 737's are actually able to fly ETOPS and do CAT III approaches, even though they don't have the equipment that used to be required to do those. They looked at the data, and the airplane was so reliable, there was no reason to to allow them to do it.
 
As far as I know, all modern cars have 1 battery, 1 starter and relay, one fuel pump and relay, one ignition and relay, and 1 ECU. The number one failure for a newer car to stop running, is for the knucklehead behind the wheel to crash into something. We, personally, are less reliable than the cars electrical system.

Same with single pilot airplanes I believe. Big airplanes always have a backup for us. LOL. But only 1 battery, and multiple charging sources.
 
John,

Some thoughts - inserted after your comments:

Everything you connect to the battery is less reliable than the battery. Taking two batteries or two alternators, and either putting them on a A/B switch or coming up with a circuit that puts one in standby, might add more risk. If you are not careful, you system will be far less reliable than just a simple single battery single alternator.
- Agreed, and I don't advocate that. The design must be able to sustain a single failure (component, wire, connection, buss unions, etc,) and still sustain IFR flight and the engine (if the engine is electrically dependent), without pilot action.
- I do not like backup or standby batteries as they are just dead weight - and a two main battery design provides for their function.

I am probably going to run a full EFI/EI on my airplane, so it will be electrically dependent. This forum is such a vast wealth of knowledge, I have spent a considerable amount of time reading it the last month or so. The poll on alternator failures confirmed my auto/marine experience. At first I thought it was because a lot of RV's use auto alternators with internal regulators, but the PP and BC alternators that "failed", also mostly failed because of the VR.
- How the alternator fails is academic, just assume it will and make sure your design is robust enough to continue operation.

In the marine world, the 2nd most unreliable part of the electrical system is the battery switch. SNIP
- One switch, one master solenoid, one avionics master.... All such single point failure design elements violate the criteria for electrically dependent flight.

With external regulators, it sure seems like it would be easy to wire up a backup VR and mitigate 90% of the risk of a failed alternator, all for 150 dollars and less than 1/2 a pound.
- I offer this is not a practical solution - and even if it is, it only addresses one risk.

Boeing and Airbus use one generator per engine...SNIP
- I offer that trying to scale large airliner design to our application is worth the effort if you want to compare design objectives. Beyond that however there are practical limitations.

Carl
 
A lot of big planes actually have more than one battery.

The A330 I fly has three batteries (of identical size). Also regarding the RATs (Ram Air Turbine), on both the B767 and the A330 the primary function of the RAT is to provide backup hydraulic power for the flight controls. A secondary consequence is that because many big jets have emergency hydraulic powered electrical generators, the RATs can power these too and provide some limited electrical power, but indirectly. The B737 can, I believe (disclaimer: not type qualified on it) be flown manually with unpowered flight controls, though I imagine with more difficulty.

The big jets have multiple electrical generators because a) risk management for flying 200-300 people all day every day dictates that they should, and b) they actually need all that electrical power to run everything (loss of just one generator results in certain load shedding, usually galley power first up, so no hot coffee for you! ;) ).

Anyway, some of these principles are not applicable, or should only be applied with careful consideration, to small experimentals. I would be aiming to keep the electrical system simple, but with a "get out of jail free" card up your sleeve to make it to VFR conditions or a diversion airport if you're IFR (or have totally electrically dependent ignition) and suffer a primary electrical failure. Just my thoughts. :)
 
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Carl;
I have seen a couple of alternators almost fail mechanically. But almost. They were still charging. Usually bad bearings. Once I had the large pivot bolt shear, the alternator was wobbling on the adjustment arm. The belt was alternately tight, and loose. But it was still charging.

I have seen dozens of failed voltage regulators. If you have two alternators, by definition you have 2 regulators. It is just more complicated, as you also have to wire that alternator into the electrical system. If it is always the regulator that fails, why not just have two of those? The other logic, taken to the extreme, would be to have a standby alternator, mounted on a standby ENGINE, ready to go.

I can't speak for the 330 as I never flew it, but I have flown every Boeing except Sparky and the 747, and have lots of time in the 320. The 320's RAT created hydraulic pressure, which powered flight controls, and generated a little electricity to power the Captains instruments, 1 com, and 1 nav radio.As far as I can remember, they all carried 2 identical batteries. (There are customer options I believe, I just checked) 1 was dedicated to start the APU, one was the ships battery. There were not switchable. The house battery, was the backup battery. I am current on the 737NG. Yep, it can fly manually, and has no RAT. I don't know how long it has been since one landed using battery power as the last remaining power source. I will ask.

Wiring the second VR? I believe most have 4 wires. I think at least 2 can just be run parallel to the other VR. 1 or 2 could be on a switch. Or really simple, have them mounted next to each other, and if one fails, just pull the connector, and connect it to the other VR. The battery can take the load for the 5-20 seconds it would take to swap the connecter. The whole setup adds a couple of ounces the aircraft, and mitigates almost all the risk of losing the alternator's ability to power the aircraft.

I am trying to find a reason to not just carry a backup VR. I am definitely looking for a reason why it won't work, and provide way over 90% of the functionality of dual alternators. And my house battery will be big enough to run the engine for 1, and probably 2 hours. I believe the ECU, 1 fuel pump, and 1 ignition will use <=7 amps.
 
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I started reading this thread because I have to make the same choice for an airplane. I was leaning towards 1 alternator/1 battery, or maybe add a second alternator. Unless someone can convince me otherwise, I think I will be installing a backup VR, and call it good.

I am building a mostly day VFR plane, but it will have EFI/EI. So I do need the juice to keep flowing.
 
Carl;
SNIP
I am trying to find a reason to not just carry a backup VR. I am definitely looking for a reason why it won't work, and provide way over 90% of the functionality of dual alternators. And my house battery will be big enough to run the engine for 1, and probably 2 hours. I believe the ECU, 1 fuel pump, and 1 ignition will use <=7 amps.

John,

I looks like your mind is made up on some set up to replace the voltage regulator in flight - which I again consider inpracital. So I recommend you do a lot of testing on whatever design you come up with. Some other thoughts:
- Of the four failed aircraft alternators I?ve seen, only one was a bad voltage regulator. One was failed output rectifiers and two were broken off output leads. How would your two voltage regulator scheme address those failiures?
- Do you know how your system will react to a failed battery connection? The battery itself migh be fine but either the positive or negative terminals, or a common buss connection can develop high resistance contacts - which leads to metal melting (the bad buss connection was the single point failure mode for the twin engine plane I previously mentioned). Can your system operate without a battery? If you think it will, again I recommend extensive testing.
- As you have an electrically dependent engine, I again strongly recommend at least adding a diode protected backup battery for one side of the engine - and VFR only ops.

A straight forward two battery, single alternator design can address the most likely single point risks of our aircraft power distribution systems (this assumes the builder is careful to not add single point failure elements - like one master solenoid, both nav/comms on the same battery, both batteries hard wired together, etc.). How you size the batteries determines your battery reserve to power your plane. It is not hard to acheive two hours of fully supported IFR flight with a modern EFIS based panel. For an electrically dependent engine I would find such a design criterial a requirement (along with testing) before the plane leaves the ground.

There are other ways to achieve this design objective but the schemes I?ve seen tend to be overly complicated and/or require pilot action for restoration when something goes wrong. However you do it, make sure you test it.

Carl
 
Why have an avionics diode, and a second avionics switch? It will still be subject to starter brown-out because you can't isolate avionics on start. If you trust the avionics for it I suppose its okay but I don't.... There is also a low voltage drop across the diode and heat from it. Switch or relay is personal choice I suppose but a relay for main connect and a backup switch might be preferable to a diode.
 
John,

Carl;
first off I greatly appreciate your response. People with your experience are what I am looking for to decide what to do for an electrical system.
I haven't made my mind up yet except for one part, and that is only one battery. It will be 1 size bigger than I need, so my "backup battery" will be built into the main battery. To my mind, the battery is the most reliable part of the electrical system, I don't see the requirement for 2. I do see the requirement for more capacity, if the alternator fails. I will replace it every 2 or 3 years, and check, and clean, the contacts religiously. Especially the ground.

The aviation and automotive world went after reliability differently. Aviation assumed some parts would fail, and designed backups and multiple load paths so the overall system would fail "softly". Two of the terms used are "Fail-operational" and "Fail-passive". 50 years ago, that was not just a smart choice, but required. Electronics were unreliable, gyros,magnetos were and are still, unreliable, everything was unreliable. The dispatch reliability of the best airliners is over 99%. They have so many backups, and associated wiring, that little things break all the time.

The automotive world is different, because if your car stops, you pull over and try to fix it. They went for no backups, but over the decades made every part as perfect as they could. In the end, even though a Honda Accord is 100X as complicated as a Model T, the Accord is 100X or more reliable, because every part is darn near perfect, and doesn't fail or wear out for 10+ years. What is the dispatch reliability of a Honda Accord or Toyota Corolla? If it were 99%, you would trade it in for a better car. It is as close to 100% as you can get. For the first 10 years, usually the only time it won't start and run is if you ran down the battery, or didn't replace it like you should have and it died after 7 or 8 years.

I have been reading about electrical systems on small aircraft, and they seem to me to be relics of the past. On a certified 1958 Cessna 185 or Bonanza, they probably still need to be. They assume that a lot will fail. Building a homebuilt in 2017, I believe very little will fail if done properly, and simply. Technology has made a lot of what we need better, and for us, simpler. I might want to have a backup for the least reliable component. In my experience, which is not light aircraft, that part is the VR. My experience with external VR's in the marine world, I only saw a couple fail, but a couple of them, did fail. Internal VR failures in the automotive world, I have seed dozens. I have seen a few alternators in really bad shape mechanically, but they were all still charging.

I am going for the Honda Accord model for reliability, but the experienced pilot in me wants a little redundancy.

So I haven't decided yet, and don't have to. I will be looking into it, mostly from people like yourself with experience in this part of the aviation world. I will try to select the most reliable components I can find, and install them with great care. Thanks to forums like this, it is easier now than ever before.

Thanks again Carl, and for anyone else that chimes in.
 
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Frankly, the backup regulator is compelling at first blush. I'm going to have to think on that one for a bit....
 
I would say the least reliable major component of jets are the generators. Usually not the generator itself, but most are driven by a Constant Speed Drive (CSD). They fail with more than anything I can think of. Boeing couldn't take it anymore, and they made the generators direct drive on the 787, unfortunately it had a couple of other issues, and is now called "Sparky".

Massive redundancy/load paths is absolutely required in this case. I don't know what the stats are on light aircraft alternators/VR's, so I don't know how much redundancy I want to carry around as weight.
 
I just went through all the posts on the alternator failure thread and added up hours and failures. I am guessing the numbers are skewed to the negative - meaning there are probably lots of people who don't think about their alternators, because they work.

The auto alternators had a little under 21,000 hours report, and 21 failures, which includes both VR and mechanical. The hours were all over the map. Some failed in a few dozen hours, some lasted a couple thousand. If I need electricity, I would definitely need 2 of these.

The PP alternators had a little under 11000 hours reported, and 17 failures. So a failure for every 660 hours. Surprisingly these were mostly mechanical kills. I would definitely want 2 of these.

The B&C had a little under 15,000 hours and 4 bad ones. 3 were VR's. Only 1 mechanical. That is a total failure rate of 1 every 3700 hours. That is a mechanical failure rate of 1 every 15,000. I trust the numbers on this much less as their were only about 20 people that reported their hours and failures for B&C. One guy had 4000 hours and no failures.

I believe the real stats are probably better than these, but the companies will never publicize their actual numbers. It makes me want a B&C alternator, and maybe flip a coin on the extra voltage regulator.

I you really, really need electricity for your plane, mission, or peace of mind, 2 cheap ones have almost no chance of both failing simultaneously, unless you kill them simultaneously with the install.
 
John,



In the end, even though a Honda Accord is 100X as complicated as a Model T, the Accord is 100X or more reliable, because every part is darn near perfect, and doesn't fail or wear out for 10+ years. What is the dispatch reliability of a Honda Accord or Toyota Corolla? If it were 99%, you would trade it in for a better car. It is as close to 100% as you can get.

My experience with external VR's in the marine world, I only saw a couple fail, but a couple of them, did fail. Internal VR failures in the automotive world, I have seed dozens. I have seen a few alternators in really bad shape mechanically, but they were all still charging.

I am going for the Honda Accord model for reliability, but the experienced pilot in me wants a little redundancy.

I'm interested to know what brands of IR VRs you've seen fail by the dozens? Domestic brands? ND?
 
Switch comment to OP question for review

John,

Thank you for posting your well thought out system design ideas for others to share.

Have you considered using high quality DPDT switches for your ECU/Coil 1 & ECU/Coil 2 switching? This would save you 2 switch locations and still allow the coil switching "mag check" to occur at run up. I used this set up on my -7 running the SDS ECUs and coils for ignition only and it worked well. Stein sells nice high quality locking switches (MS27408-4A) that are very solid.

I can't think of a scenario where you would have to take an ECU offline to negate the DPDT approach for ECU/coil select if you wanted to save some space.
 
Switches of any kind become a failure point. Use the highest quality mil spec devices you can find. Honeywell is better than Carling, but no guarantees. I still have a bunch of faulty Carlings that overheated and failed in my 9A.

A bad alternator switch can lead to overvoltages and cause OVP devices to trigger.

A bad strobe or landing light switch can lead to smoke in the cockpit.

This switch problem is so bad that I eliminated almost all load-carring switches in my Rocket and went with a combination of relays and electronic drivers, such as most automobiles use today. Yes, relays can fail as well, but they are reliable and easy to change out.

Adding switches to a design will lower reliability. Diodes are better, but must be carefully chosen. For example, high power Schottkys seem ideal until you factor in high leakage currents. This can discharge back-up batteries.
 
Switches of any kind become a failure point. Use the highest quality mil spec devices you can find. Honeywell is better than Carling, but no guarantees. I still have a bunch of faulty Carlings that overheated and failed in my 9A.

A bad alternator switch can lead to overvoltages and cause OVP devices to trigger.

A bad strobe or landing light switch can lead to smoke in the cockpit.

This switch problem is so bad that I eliminated almost all load-carring switches in my Rocket and went with a combination of relays and electronic drivers, such as most automobiles use today. Yes, relays can fail as well, but they are reliable and easy to change out.

Adding switches to a design will lower reliability. Diodes are better, but must be carefully chosen. For example, high power Schottkys seem ideal until you factor in high leakage currents. This can discharge back-up batteries.

I couldn't agree more. The "race to China" has made everything suspect. You never know what you are buying, or where it came from. I may go to the biggest junkyard I can find, and source a bunch of identical switches and relays from late model cars. Toyota and Honda seem to have figured out how to source quality switchgear. As individuals, we don't have the same resources available to us.

Ross;
Probably 80% of the alternators were for Subaru engines, with 10% VW,and 10% others. I am sure the Subaru's were the same internally as most Japanese brands. The half that were good, were out of or nearly out of spec for charging voltage. I tried all the big name auto parts stores. Same result. A couple came from a local alternator rebuild shop. They said "that is all we can get".

Out of another couple of dozen I used from junkyards, either all were good, and spot on for charging voltage, Or maybe I got one bad one. I can't remember, it has been a while.

If you could take the ND alternators, and rebuild them with quality parts, I am sure they would be great alternators. Maybe that is what B&C does. They were great alternators in the cars they originally came mounted in.

I read the term "Krylon-rebuild" on the alternator failure thread. Sounds about right.
 
John,

Thank you for posting your well thought out system design ideas for others to share.

Have you considered using high quality DPDT switches for your ECU/Coil 1 & ECU/Coil 2 switching? This would save you 2 switch locations and still allow the coil switching "mag check" to occur at run up. I used this set up on my -7 running the SDS ECUs and coils for ignition only and it worked well. Stein sells nice high quality locking switches (MS27408-4A) that are very solid.

I can't think of a scenario where you would have to take an ECU offline to negate the DPDT approach for ECU/coil select if you wanted to save some space.

No I was going to use STSP switches for everything that has a dual, and they will probably control relays for the fuel pumps. I am not sure for the ignition. They don't draw much current. I don't want one switch to take out both ignitions/fuel pumps, etc. I will use fuses for everything that I can get away with.

I am just going to use a single EFIS, which looks like it is going to do an amazing job of reducing electrical switches/wiring, and massively reduce the weight of the instrument panel. An IPAD will be my navigation backup. Plus my smartphone in my pocket. Day VFR only.
 
Ross;
Probably 80% of the alternators were for Subaru engines, with 10% VW,and 10% others. I am sure the Subaru's were the same internally as most Japanese brands. The half that were good, were out of or nearly out of spec for charging voltage. I tried all the big name auto parts stores. Same result. A couple came from a local alternator rebuild shop. They said "that is all we can get".

Out of another couple of dozen I used from junkyards, either all were good, and spot on for charging voltage, Or maybe I got one bad one. I can't remember, it has been a while.

If you could take the ND alternators, and rebuild them with quality parts, I am sure they would be great alternators. Maybe that is what B&C does. They were great alternators in the cars they originally came mounted in.

I read the term "Krylon-rebuild" on the alternator failure thread. Sounds about right.

Ok, just clarifying. The OEM stuff was great, rebuilds really bad. Same as my experience then.
 
Thanks Blusky,

You say DPDT but do you mean DPST?

I looked thru my checklist and load analysis and I see anytime an ECU is powered its coilpack is also powered so DPST switches make sense to save panel space.

My wire by wire failure analysis would not be affected by using DPST switches for ECUs / coilpack.

Is it fair to say a single point of failure would be added in that the DPST switch could mechanically fail and take out an ECU and coilpack simultaneously so if the injectors were on that ECU the engine would stop? I guess not since such a failure would be equally likely to affect an ECU SPST.

Ross Farnham / SDS published a drawing showing both ECUs unswitched. (Find it in my google file space; "SDS wiring ECUDual4 REV 1.0.jpg". Note the injector relay wiring is simplified; I show it in more detail on sheet 2 of my electrical drawing.) I need to switch the main ECU because it is on my battery bus and I switched the backup ECU for consistency.

I haven't bought the system yet so I haven't seen the wiring diagrams. I was hoping to have the two ignitions switched separately so I could check them, and shut one off if I lose the alternator. I was originally thinking just fuzing the ECU, and having it come on with the master. But again I haven't seen the wiring. The ignitions don't take that much juice, but I would like to check them.
 
I was hoping to have the two ignitions switched separately so I could check them, and shut one off if I lose the alternator. I was originally thinking just fuzing the ECU, and having it come on with the master. But again I haven't seen the wiring. The ignitions don't take that much juice, but I would like to check them.

Many ways to skin this cat, but consider the following random thoughts:

- The SDS ignition pulls very little power, so shutting one down is only going to save you a little over an amp. Weigh that against the (admittedly remote) risk of having the good one fail while in limp home mode.

- How about pullable CB's for the coils? If you really want to check them, you can. However, that does bring up a possible paradigm shift... Is there really a need to do an EOR ignition check with EFI and EI? I'd argue that an abnormal or errant EGT is going to tell you just as much as the traditional "sound and feel" method.

- Also consider the consequences of tying the ECU to the master. We have been trained through decades of muscle memory that you can shut down the master on a little airplane and it will continue to run. There are human factors to contemplate.
 
twin batteries?

John,

You have done a nice job mitigating the inherent risks associated with a single battery power distribution system.

I offer however, that a more robust design that has two batteries and one or two alternators would be better suited for your very electrically dependent airplane.

While alternator failure is the most common problem (and your design addresses that well) it is not the only risk. We have incident reports of twin engine aircraft (two alternators, single battery) that have suffered total loss of electrical power from a single buss fault. I'm not saying your design has this exact same fault, just pointing out that two alternator do not resolve all design risks.

If nothing else, I recommend you consider a diode protected backup battery for one side of the engine. I'm not a fan of backup batteries so I go with two identical main batteries as this better addressed my other design criteria.

Not attempting to flame - just providing feedback as you requested.

Carl

Thanks Carl,

I wish I had details of twin engine, or single engine, single battery dual alternator total electrical failures. Perhaps these had a single point of failure my Z101-based design does not have.
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avionics diode

Why have an avionics diode, and a second avionics switch? It will still be subject to starter brown-out because you can't isolate avionics on start. If you trust the avionics for it I suppose its okay but I don't.... There is also a low voltage drop across the diode and heat from it. Switch or relay is personal choice I suppose but a relay for main connect and a backup switch might be preferable to a diode.

My understanding is modern avionics are not damaged by being connected at engine crank.

I no longer have an avionics bus with my 2020 update to Z101 template.
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My understanding is modern avionics are not damaged by being connected at engine crank. Are you concerned about voltage spikes during crank?
SNIP

As I understand it if I use the Dynon Skyview backup battery the EFIS will not reboot during crank. The other Dynon devices will reboot so quickly it doesn't matter.

I talked to Dynon on this a couple of times. They firmly state the SkyView system will not be damaged by buss voltage droop on engine start. That said I don?t have the same confidence on the very expensive GTN-650.

As a precaution, I start the engine with the panel powered down or just one side powered up (the side with the right display that is typically set up for EMS half screen display, and no GTN-650). After engine start I bring up both sides of the panel.

If you install the SkyView backup battery on one of your panels, and can power it up by itself, you sidestep the issue.

Carl
 
Many ways to skin this cat, but consider the following random thoughts:

- The SDS ignition pulls very little power, so shutting one down is only going to save you a little over an amp. Weigh that against the (admittedly remote) risk of having the good one fail while in limp home mode.

- How about pullable CB's for the coils? If you really want to check them, you can. However, that does bring up a possible paradigm shift... Is there really a need to do an EOR ignition check with EFI and EI? I'd argue that an abnormal or errant EGT is going to tell you just as much as the traditional "sound and feel" method.

- Also consider the consequences of tying the ECU to the master. We have been trained through decades of muscle memory that you can shut down the master on a little airplane and it will continue to run. There are human factors to contemplate.

Yeah I was thinking about not needing to check the EI's.
If I put the ECU on the master, I would have a fuze on it that I could pull.

I think I would like to check both fuel pumps. I totally trust the pumps, but you can here them strain if the filter starts getting clogged. Interestingly, a lot of boats and heavy equipment have 1 fuel pump, and dual, switchable fuel filters. I won't go that route.
 
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