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Switch Schemes for Reliability

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wsimpso1

Joined: 04 Nov 2018
Posts: 31
Location: Saline MI

Posted: Sun Apr 14, 2024 7:46 am Post subject: Switch Schemes for Reliability

We all know we need backups on any circuit needed for comfortable completion of a flight. I aimed for that. I have several circuits that fit that need, things like pumps and ECU's and Ignition Coils. I am Installing Z-14 (IFR and Electrically Dependant Engine). I would like your review and criticisms of the schemes offered, and any alternatives you can offer too. 



I have two of each important devices (pumps, coil power, ECU's, injector power) and two always hot buses. I suspect  we benefit from being able to connect either pump to either bus, and have two independant switches/circuits to do so too. For take-off and landing, many of us will want both pumps (or other paired devices) running, so I am looking to that as well.



The clever way seemed to be two 700-2-10's for each device pair, in parallel, with one connected to the Main Battery Bus, the other to the Aux Battery Bus. I KNEW that gave me an error state that we would not do deliberately but which was likely to occur: both buses running one pump. Upon first review with both buses hot, this is a "so what?"  with the only loss being only one pump on when we were trying for both. But when one bus has gone cold, we are then trying to power everything left On on that cold bus through the fuse and switch for the Hot bus. Sure, if we REMEMBER to close the Bus-Tie first and both buses do go hot, no problem, but if one bus is still cold, we lose access to pump. One more out of order switch throw and we are out both pumps or coils or .... BAAAD.



Yes, I have run Fault Tree Analysis and then FMEA for my base scheme the way I was taught by duPont using order of magnitude estimates with detectability, severity, and failure probability. I plan to run the exercises on the alternatives too.  Fault Tree Analysis and Failure Modes and Effects Analysis are very useful tools for estimating if one scheme is better or worse in total.



So, what are the alternatives? 



Two SPST switches in parallel for each function, each connecting one bus to one pump. That costs me half of the bus-pump possibilities, which sounds detrimental.



Series switch set that uses a 2-10 sequenced to a 2-1 that let's me connect either bus to either pump and to run both pumps, without running one pump on both buses simultaneously. This looks worse with two switches in series - instead of having a SPOF, I have doubled it - UGH.



The scheme I like is paired 700-2-1's for each function with one switch per pump. This allows either bus to power each pump. No cross bus connection is possible, each pump (or other device) is separately powered by either bus, and there is always the other pump. 



Thoughts please on all of these schemes and my judgment here... If you have a better scheme to be replicated several times in my panel to allow running both devices from either bus, I am all ears.



Bill

1rv4

Joined: 11 Sep 2006
Posts: 22

Posted: Mon Apr 15, 2024 6:14 am Post subject: Re: Switch Schemes for Reliability

Hi Bill

It's been along time since I have posted on aeroelectric . The way I think I read your issue is that you don't want to backfeed from one bus to the other in fear of blowing the protective fuse to (A) the pump itself and (B)the fuse you have protecting your complete Secondary bus.

If that is the case maybe use diodes so pumps remain isolated from each other? If all your circuits are fuse protected on your Secondary bus on the output side is there a need for a fuse to protect the whole bus?

Regards

 Chris

Ceengland

Joined: 11 Oct 2020
Posts: 381
Location: MS

Posted: Wed Apr 17, 2024 12:51 pm Post subject: Switch Schemes for Reliability

On Sun, Apr 14, 2024 at 10:50 AM wsimpso1 <wsimpso1(at)comcast.net> wrote:



[quote] 



 We all know we need backups on any circuit needed for comfortable

 completion of a flight. I aimed for that. I have several circuits that fit

 that need, things like pumps and ECU's and Ignition Coils. I am Installing

 Z-14 (IFR and Electrically Dependant Engine). I would like your review and

 criticisms of the schemes offered, and any alternatives you can offer too

_________________
Charlie

wsimpso1

Joined: 04 Nov 2018
Posts: 31
Location: Saline MI

Posted: Sat Apr 20, 2024 3:16 pm Post subject: Re: Switch Schemes for Reliability

Recently I spotted a document from a well known brand of EFII recommending an “Essential Buss” backed up with a Aux Battery and simple switch to connect it. While this is supposed to make powering the engine possible even if the Main Battery Bus goes cold, it also appears to make possible powering the cold bus with many things connected through a single or few fuses on the hot bus. Sounds like a recipe for blown fuses followed by forced landings and maybe even dark avionics to me. What do you guys think about how to power engine buses? 



Then I came across some videos (IIRC, Bob Nuckolls, but I can not find those videos now) where two LightSpeed ignition modules were powered through a common connection and caused them both to go cold followed by a deadstick landing. 



With my consciousness suitably raised and supported by Bob Nuckolls’, I pored through switch and wiring schemes in an attempt at avoiding back powering cold buses, and even thought about mechanically gating these switch pairs to prevent switch setting combos we should avoid.  



The danger discussed by Bob (and reinforcing my recently raised consciousness) is that you can have one element connected to both buses. Let’s just think of two switches wired to power two pumps from two buses by having one buss on one switch with both pumps, the other bus to the other switch also for both pumps. With two 700-2-10’s we have several switch states, two of which are powering the same pump from both buses. (Main A/Aux A  or Main B/Aux B). Yeah, the pilot should remember not to double connect elements, but pilots ARE human. 



Look it over and it looks like “no big deal” to have both buses connected to one pump while both buses are hot. But we want to be failure tolerant...



Imagine one bus goes cold – all the elements on the cold bus are connected through one fuse on the hot bus – fuse goes pop! Among all the other fuss with a cold bus, the engine is windmilling. Once the pilot gets the light back on, the attitude and navigation gadgets rebooted, our pilot sees the fuel pressure alarm and wants to restore pressure to the injectors. Yes, the switches for the EFI pump will get exercised, looking for a setting that restores fuel pressure. Trouble is one state is off, another state will blow the other fuse, more states are now unintentional ‘off”s, and only one state will run the airplane. Remember to fly the airplane while getting all of this sorted. This difficult human factors situation has high likelhood for becoming a “Greek Tragedy”. Nope, let’s avoid all that. 



First thought was “Can I build some sort of mechanical gate that prevents the back power combinations?” Best scheme I came up with is kind of  inelegant with a little plate with notches for switch handles on a pivot attached between two switch handles. Try to move one handle to a prohibited combo and it drives the other switch to an allowed position. Not sure I want to do that five places in my airplane, but it could work.   



Another scheme is to run only one element (like one pump) per switch and select from buses on each switch. No back powering, but we do have some reduced redundancy at lower probability orders. So, how do we make our decision on just how to build? 



And are there other schemes that could work for this scenario, access both buses and both pumps or other elements, and keep us from having a windmilling prop?



More in the next post.



Billski

alec(at)alecmyers.com
Guest

Posted: Sat Apr 20, 2024 3:40 pm Post subject: Switch Schemes for Reliability

Isn’t that what Schottky diodes are for?



On Apr 20, 2024, at 19:18, wsimpso1 <wsimpso1(at)comcast.net> wrote:

With my consciousness suitably raised and supported by Bob Nuckolls’, I pored through switch and wiring schemes in an attempt at avoiding back powering cold buses, and even thought about mechanically gating these switch pairs to prevent switch setting combos we should avoid.

Eric Page

Joined: 15 Feb 2017
Posts: 244

Posted: Sat Apr 20, 2024 5:39 pm Post subject: Re: Switch Schemes for Reliability

Have you looked at Bob's newest electrical architecture, Z-101B?



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z101B.pdf



This is his current state-of-the-art for an electrically dependent engine.



Eric

bob.verwey(at)gmail.com
Guest

Posted: Sun Apr 21, 2024 4:46 am Post subject: Switch Schemes for Reliability

Thought provoking post..looking forward to the responses from the brains trust



On Sun, 21 Apr 2024, 01:20 wsimpso1, <wsimpso1(at)comcast.net (wsimpso1(at)comcast.net)> wrote:



 	  Quote: 	
		  --> AeroElectric-List message posted by: "wsimpso1" <wsimpso1(at)comcast.net (wsimpso1(at)comcast.net)>

 

 Recently I spotted a document from a well known brand of EFII recommending an “Essential Buss” backed up with a Aux Battery and simple switch to connect it. While this is supposed to make powering the engine possible even if the Main Battery Bus goes cold, it also appears to make possible powering the cold bus with many things connected through a single or few fuses on the hot bus. Sounds like a recipe for blown fuses followed by forced landings and maybe even dark avionics to me. What do you guys think about how to power engine buses? 

 

 Then I came across some videos (IIRC, Bob Nuckolls, but I can not find those videos now) where two LightSpeed ignition modules were powered through a common connection and caused them both to go cold followed by a deadstick landing. 

 

 With my consciousness suitably raised and supported by Bob Nuckolls’, I pored through switch and wiring schemes in an attempt at avoiding back powering cold buses, and even thought about mechanically gating these switch pairs to prevent switch setting combos we should avoid.  

 

 The danger discussed by Bob (and reinforcing my recently raised consciousness) is that you can have one element connected to both buses. Let’s just think of two switches wired to power two pumps from two buses by having one buss on one switch with both pumps, the other bus to the other switch also for both pumps. With two 700-2-10’s we have several switch states, two of which are powering the same pump from both buses. (Main A/Aux A  or Main B/Aux B). Yeah, the pilot should remember not to double connect elements, but pilots ARE human. 

 

 Look it over and it looks like “no big deal” to have both buses connected to one pump while both buses are hot. But we want to be failure tolerant...

 

 Imagine one bus goes cold – all the elements on the cold bus are connected through one fuse on the hot bus – fuse goes pop! Among all the other fuss with a cold bus, the engine is windmilling. Once the pilot gets the light back on, the attitude and navigation gadgets rebooted, our pilot sees the fuel pressure alarm and wants to restore pressure to the injectors. Yes, the switches for the EFI pump will get exercised, looking for a setting that restores fuel pressure. Trouble is one state is off, another state will blow the other fuse, more states are now unintentional ‘off”s, and only one state will run the airplane. Remember to fly the airplane while getting all of this sorted. This difficult human factors situation has high likelhood for becoming a “Greek Tragedy”. Nope, let’s avoid all that. 

 

 First thought was “Can I build some sort of mechanical gate that prevents the back power combinations?” Best scheme I came up with is kind of  inelegant with a little plate with notches for switch handles on a pivot attached between two switch handles. Try to move one handle to a prohibited combo and it drives the other switch to an allowed position. Not sure I want to do that five places in my airplane, but it could work.   

 

 Another scheme is to run only one element (like one pump) per switch and select from buses on each switch. No back powering, but we do have some reduced redundancy at lower probability orders. So, how do we make our decision on just how to build? 

 

 And are there other schemes that could work for this scenario, access both buses and both pumps or other elements, and keep us from having a windmilling prop?

 

 More in the next post.

 

 Billski

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=513480#513480

 

 

 

 

 

 

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Ceengland

Joined: 11 Oct 2020
Posts: 381
Location: MS

Posted: Sun Apr 21, 2024 6:30 am Post subject: Re: Switch Schemes for Reliability

I don't know what happened to my previous post in this thread, but what showed up in this forum version had nothing of what I sent in the email. Here's what I wrote earlier:



I'll take a short swing at it, by simply describing my thought process & resulting decisions. I decided that *buses* don't fail. As long as they're installed properly and inspected regularly, they should be given the same confidence as wing spars. Power supplied to one could fail, in which case a backup source should be available. With that starting point, I planned two buses; one with *everything* for the electrically dependent engine, and the other for airframe power. Crossfeed ability between the two. Individual circuits protected by individual fuses, protecting the bus. I have redundant fuel pumps, etc, but both are controlled from the engine bus.  Even then, switchology was still more complicated than I wanted, but it was stripped down as clean as I could reasonably get it with the 'unconventional' engine. 



Trying to power each item (if you're doing the pumps, the ECUs need it too, etc etc) from multiple source buses seems to me like planning for multiple failures in the same function-path on the same flight, a possibility so remote that the mitigation attempts actually increase the risk, and it's also a strong magnet drawing one into  pilot overload if something does go south in flight. We manage to mismanage stuff in stressful situations even with all the control motions being hardwired by decades of flying. A radical departure in 'switchology' beyond that absolutely required,  and/or multiple added layers, seems to me to make it more likely that something minor will become something major very easily.



Just my thought path and results; yours will likely vary.



Charlie

_________________
Charlie

wsimpso1

Joined: 04 Nov 2018
Posts: 31
Location: Saline MI

Posted: Sun Apr 21, 2024 4:34 pm Post subject: Re: Switch Schemes for Reliability

[quote="alec(at)alecmyers.com"]Isn’t that what Schottky diodes are for?



This and another post suggested diode isolation.



Trouble with adding elements to our systems is they too have failure modes, downstream effects, etc. Diodes can and do fail open and fail closed. So, we have to put them in the Fault Tree and Failure Modes and Effects work, estimate failure rates, and see if they help or hurt for forced landings and dark avionics suites. Have not done it yet, but might yet.



Billski

wsimpso1

Joined: 04 Nov 2018
Posts: 31
Location: Saline MI

Posted: Sun Apr 21, 2024 4:38 pm Post subject: Re: Switch Schemes for Reliability

[quote="Eric Page"]Have you looked at Bob's newest electrical architecture, Z-101B?



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z101B.pdf



Yes. I have been keeping up. A friend is using it in his RV-7. Same issue comes up there - how do we run our engine and such without reducing function when stuff breaks?



Billski

alec(at)alecmyers.com
Guest

Posted: Sun Apr 21, 2024 5:06 pm Post subject: Switch Schemes for Reliability

I suspect the failure rate of a single correctly specified diode is many many orders of magnitude lower than a mechanical switch. And you can parallel two, three or four such diodes if you wish.



On Apr 21, 2024, at 20:35, wsimpso1 <wsimpso1(at)comcast.net> wrote:







[quote="alec(at)alecmyers.com"]Isn’t that what Schottky diodes are for?



This and another post suggested diode isolation.



Trouble with adding elements to our systems is they too have failure modes, downstream effects, etc. Diodes can and do fail open and fail closed. So, we have to put them in the Fault Tree and Failure Modes and Effects work, estimate failure rates, and see if they help or hurt for forced landings and dark avionics suites. Have not done it yet, but might yet.



Billski





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=513492#513492

wsimpso1

Joined: 04 Nov 2018
Posts: 31
Location: Saline MI

Posted: Fri Apr 26, 2024 7:43 am Post subject: Re: Switch Schemes for Reliability

We have had a rather lively discussion over at: https://www.homebuiltairplanes.com/forums/threads/switching-options-of-essential-equipment.49627



No one had a better switch setup allow either element to be powered by either bus, including a backup to the switch. Sounds like simple really is good...



Billski

bob.verwey(at)gmail.com
Guest

Posted: Sat Apr 27, 2024 6:33 am Post subject: Switch Schemes for Reliability

.when in doubt..build in simplicity and lightness



On Fri, 26 Apr 2024, 17:48 wsimpso1, <wsimpso1(at)comcast.net (wsimpso1(at)comcast.net)> wrote:



 	  Quote: 	
		  --> AeroElectric-List message posted by: "wsimpso1" <wsimpso1(at)comcast.net (wsimpso1(at)comcast.net)>

 

 We have had a rather lively discussion over at: https://www.homebuiltairplanes.com/forums/threads/switching-options-of-essential-equipment.49627

 

 No one had a better switch setup allow either element to be powered by either bus, including a backup to the switch. Sounds like simple really is good..

 

 Billski

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=513518#513518

 

 

 

 

 

 

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 eferrer noreferrer" target="_blank">http://forums.matronics.com

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bob.verwey(at)gmail.com
Guest

Posted: Sat Apr 27, 2024 7:21 am Post subject: Switch Schemes for Reliability

Can you post the schematic



On Fri, 26 Apr 2024, 17:48 wsimpso1, <wsimpso1(at)comcast.net (wsimpso1(at)comcast.net)> wrote:



 	  Quote: 	
		  --> AeroElectric-List message posted by: "wsimpso1" <wsimpso1(at)comcast.net (wsimpso1(at)comcast.net)>

 

 We have had a rather lively discussion over at: https://www.homebuiltairplanes.com/forums/threads/switching-options-of-essential-equipment.49627

 

 No one had a better switch setup allow either element to be powered by either bus, including a backup to the switch. Sounds like simple really is good..

 

 Billski

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=513518#513518

 

 

 

 

 

 

 ===========

  -

 Electric-List" rel="noreferrer noreferrer" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List

 ===========

  FORUMS -

 eferrer noreferrer" target="_blank">http://forums.matronics.com

 ===========

 WIKI -

 errer noreferrer" target="_blank">http://wiki.matronics.com

 ===========

 b Site -

           -Matt Dralle, List Admin.

 ="noreferrer noreferrer" target="_blank">https://matronics.com/contribution

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1rv4

Joined: 11 Sep 2006
Posts: 22

Posted: Sat Apr 27, 2024 5:47 pm Post subject: Re: Switch Schemes for Reliability

Any recommendations for p/n that would be a suitable diode to use for isolating ECU's, or ignition coils?

 Was there a wiring diagram available Bill, I would be very interested to see if if there is.

  Thank you 

    Kit

Eric Page

Joined: 15 Feb 2017
Posts: 244

Posted: Sun Apr 28, 2024 4:40 pm Post subject: Re: Switch Schemes for Reliability

That depends on what you're feeding through the diode (how much current it has to carry), and your tolerance for installation hassle.  Generally speaking, you want something in a physically robust package that has leads that won't flex/bend easily.  For low to medium current applications, look for packages like DO-201, R-6 or P-600.  For higher current applications or for ease of installation, you can use one diode in a bridge rectifier package.



Examples:



3A diode: https://www.digikey.com/short/j9j2r3cf



10A diode: https://www.digikey.com/short/mvh87bdn



20A diode: https://www.digikey.com/short/v59dmvmf



Bridge Rectifiers: https://www.digikey.com/short/fqb5t704



The first three can be crimped in-line with the supply wire for the appliance that you're feeding, then covered with heat shrink.  A bridge rectifier would be bolted to a surface (preferably metal, for heat dissipation) and connected with 0.25" PIDG quick-connect terminals.



Pay attention to polarity.  The supply connects to the diode's anode and the consumer connects to its cathode (on the first three, the cathode is marked with a white stripe).  For the bridge rectifier, connect the supply to either AC terminal (marked ~) and the consumer to the positive terminal (marked +).

1rv4

Joined: 11 Sep 2006
Posts: 22

Posted: Sun Apr 28, 2024 5:58 pm Post subject: Re: Switch Schemes for Reliability

Thanks very much Eric, that's really helpful and much appreciated.

    Chris

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