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wsimpso1
Joined: 04 Nov 2018
Posts: 33
Location: Saline MI
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 Posted: Sun Apr 14, 2024 7:46 am Post subject: Switch Schemes for Reliability |
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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
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1rv4
Joined: 11 Sep 2006
Posts: 24
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| Posted: Mon Apr 15, 2024 6:14 am Post subject: Re: Switch Schemes for Reliability |
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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
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Ceengland
Joined: 11 Oct 2020
Posts: 391
Location: MS
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| Posted: Wed Apr 17, 2024 12:51 pm Post subject: Switch Schemes for Reliability |
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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
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wsimpso1
Joined: 04 Nov 2018
Posts: 33
Location: Saline MI
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| Posted: Sat Apr 20, 2024 3:16 pm Post subject: Re: Switch Schemes for Reliability |
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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
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alec(at)alecmyers.com
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| Posted: Sat Apr 20, 2024 3:40 pm Post subject: Switch Schemes for Reliability |
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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.
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Eric Page
Joined: 15 Feb 2017
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bob.verwey(at)gmail.com
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| Posted: Sun Apr 21, 2024 4:46 am Post subject: Switch Schemes for Reliability |
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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: 391
Location: MS
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| Posted: Sun Apr 21, 2024 6:30 am Post subject: Re: Switch Schemes for Reliability |
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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
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wsimpso1
Joined: 04 Nov 2018
Posts: 33
Location: Saline MI
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| Posted: Sun Apr 21, 2024 4:34 pm Post subject: Re: Switch Schemes for Reliability |
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[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
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wsimpso1
Joined: 04 Nov 2018
Posts: 33
Location: Saline MI
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| Posted: Sun Apr 21, 2024 4:38 pm Post subject: Re: Switch Schemes for Reliability |
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[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
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alec(at)alecmyers.com
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| Posted: Sun Apr 21, 2024 5:06 pm Post subject: Switch Schemes for Reliability |
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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
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wsimpso1
Joined: 04 Nov 2018
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Location: Saline MI
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bob.verwey(at)gmail.com
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| Posted: Sat Apr 27, 2024 6:33 am Post subject: Switch Schemes for Reliability |
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.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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-Matt Dralle, List Admin.
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bob.verwey(at)gmail.com
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| Posted: Sat Apr 27, 2024 7:21 am Post subject: Switch Schemes for Reliability |
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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
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1rv4
Joined: 11 Sep 2006
Posts: 24
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| Posted: Sat Apr 27, 2024 5:47 pm Post subject: Re: Switch Schemes for Reliability |
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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
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Eric Page
Joined: 15 Feb 2017
Posts: 245
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| Posted: Sun Apr 28, 2024 4:40 pm Post subject: Re: Switch Schemes for Reliability |
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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 +).
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1rv4
Joined: 11 Sep 2006
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| Posted: Sun Apr 28, 2024 5:58 pm Post subject: Re: Switch Schemes for Reliability |
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Thanks very much Eric, that's really helpful and much appreciated.
Chris
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nuckolls.bob(at)aeroelect
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| Posted: Tue Apr 30, 2024 4:07 pm Post subject: Switch Schemes for Reliability |
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At 06:16 PM 4/20/2024, you wrote:
| Quote: | --> AeroElectric-List message posted by: "wsimpso1" <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, |
Under what circumstances would your main battery
ever 'go cold'?
When maintained with century old practices
for system reliability, a battery can
be the most reliable source of energy in
the system. So finding yourself lacking in
battery supplied energy means you've lost
all engine driven sources, your battery
is inadequate to the task of meeting your
battery only endurance goals; -OR- your system
architecture/craftsmanship fails to eliminate
all single points of failure in energy
conduction pathways to accessories necessary
for comfortable completion of a flight.
Adding any sort of 'back up' battery to a
system only adds to the number of devices
that beg for $time$ to maintain airworthiness
for yet another commodity . . . a thing
that begins to degrade, be consumed the day you
installed it. Tires, drive belts, engine oil, propeller
blades, FUEL, etc are all things we fuss over
every flight as a matter of course . . . walk-around
during pre-flight is drilled into us from
day-one. But my flight instructor never said
a peep about batteries.
For as long as I can recall reading the
dark-n-stormy-night narratives in the aviation
rags, pilots who survived the dark-panel-syndrome
never once suggested that the battery in
their airplane was not properly evaluated,
maintained or utilized for comfortable
completion of flight under the circumstances
they experienced.
It's easy to debate 'reliability' at length
and with great enthusiasm. It's tempting
to pile on 'back-ups' at the risk of increasing
weight, cost of ownership and complexity of operation
. . . complexity that only work against 'aviate, navigate,
communicate' when things are not going well in the
cockpit.
I'll suggest a FOURTH feature in the famous
axiom for emergency management: 'First aviate,
then navigate, then communicate, but last and
certainly least, FIDDLE WITH STUFF ON THE PANEL'.
Having a battery 'go cold' is the final chapter
in a litany of failures that PRECEDE loss of the
the battery. Things over which we have absolute
control.
I've not forgotten about you guys. The new
crowbar OVM project is sitting on the bench
waiting for a window of opportunity amongst other
matters. Additionally, we need to spool up
the discussion on Figure Z-101. I'm increasingly
of the opinion that Z-101 should be the ONLY
z-figure in any 'upgrade' to the connection.
It has the opportunity of being the legos/
tinkertoy/modular plug-n-play system with options
that adress EVERY electrical system configuration
requirement from ultra-light to LA-IVP with all
the 'goodies'.
But we need a cogent, convincing narrative to
accompany Z-101 . . . a option/by/option
account of when, why and how any particular
feature should be considered.
>Recently I spotted a document from a well known brand of EFII
>recommending an Essential Bus backed up with a Aux Battery
>and simple switch to connect it.
Yeah, Klaus was of similar opinion when I met
him back in '86 . . . and in all years since. So
to have MOST producers of really slick electro-whizzies
in years since. It's really easy to suggest that
"adding back up to keep MY miracle product
powered and happy" is a good thing to do . . .
but I'll wager that few if any have brought
the forces of experience and thoughtful Failure
Mode Effects Analysis to bear on their recommendations.
The goal: craft and maintain a system with an
extremely high probability of bringing
you home without breaking a sweat. Every
failure is a maintenance even, not an
emergency.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
for the-best-we-know-how-to-do based
on physics and repeatable experiment.
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kenryan
Joined: 20 Oct 2009
Posts: 426
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| Posted: Wed May 01, 2024 11:52 am Post subject: Switch Schemes for Reliability |
|
|
Bob,
I think your idea for a single expandable drawing makes a lot of sense. I envision an edition of the Connnection that uses the simplest possible version of the drawing to explain basic concepts, and then expands it to include more features and teach more concepts.
On Tue, Apr 30, 2024 at 4:11 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | At 06:16 PM 4/20/2024, you 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, |
Under what circumstances would your main battery
ever 'go cold'?
When maintained with century old practices
for system reliability, a battery can
be the most reliable source of energy in
the system. So finding yourself lacking in
battery supplied energy means you've lost
all engine driven sources, your battery
is inadequate to the task of meeting your
battery only endurance goals; -OR- your system
architecture/craftsmanship fails to eliminate
all single points of failure in energy
conduction pathways to accessories necessary
for comfortable completion of a flight.
Adding any sort of 'back up' battery to a
system only adds to the number of devices
that beg for $time$ to maintain airworthiness
for yet another commodity . . . a thing
that begins to degrade, be consumed the day you
installed it. Tires, drive belts, engine oil, propeller
blades, FUEL, etc are all things we fuss over
every flight as a matter of course . . . walk-around
during pre-flight is drilled into us from
day-one. But my flight instructor never said
a peep about batteries.
For as long as I can recall reading the
dark-n-stormy-night narratives in the aviation
rags, pilots who survived the dark-panel-syndrome
never once suggested that the battery in
their airplane was not properly evaluated,
maintained or utilized for comfortable
completion of flight under the circumstances
they experienced.
It's easy to debate 'reliability' at length
and with great enthusiasm. It's tempting
to pile on 'back-ups' at the risk of increasing
weight, cost of ownership and complexity of operation
. . . complexity that only work against 'aviate, navigate,
communicate' when things are not going well in the
cockpit.
I'll suggest a FOURTH feature in the famous
axiom for emergency management: 'First aviate,
then navigate, then communicate, but last and
certainly least, FIDDLE WITH STUFF ON THE PANEL'.
Having a battery 'go cold' is the final chapter
in a litany of failures that PRECEDE loss of the
the battery. Things over which we have absolute
control.
I've not forgotten about you guys. The new
crowbar OVM project is sitting on the bench
waiting for a window of opportunity amongst other
matters. Additionally, we need to spool up
the discussion on Figure Z-101. I'm increasingly
of the opinion that Z-101 should be the ONLY
z-figure in any 'upgrade' to the connection.
It has the opportunity of being the legos/
tinkertoy/modular plug-n-play system with options
that adress EVERY electrical system configuration
requirement from ultra-light to LA-IVP with all
the 'goodies'.
But we need a cogent, convincing narrative to
accompany Z-101 . . . a option/by/option
account of when, why and how any particular
feature should be considered.
>Recently I spotted a document from a well known brand of EFII
>recommending an Essential Bus backed up with a Aux Battery
>and simple switch to connect it.
Yeah, Klaus was of similar opinion when I met
him back in '86 . . . and in all years since. So
to have MOST producers of really slick electro-whizzies
in years since. It's really easy to suggest that
"adding back up to keep MY miracle product
powered and happy" is a good thing to do . . .
but I'll wager that few if any have brought
the forces of experience and thoughtful Failure
Mode Effects Analysis to bear on their recommendations.
The goal: craft and maintain a system with an
extremely high probability of bringing
you home without breaking a sweat. Every
failure is a maintenance even, not an
emergency.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
for the-best-we-know-how-to-do based
on physics and repeatable experiment.
|
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wsimpso1
Joined: 04 Nov 2018
Posts: 33
Location: Saline MI
|
| Posted: Wed May 01, 2024 5:24 pm Post subject: Re: Switch Schemes for Reliability |
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|
In using the direction we all read and seriously buy into from the AeroElectric Connection, I have asked what happens if this piece misbehaves? Found a bunch around the battery. How can a main bus go "cold"?
First step is any of these failures occur:
Battery + cable comes loose at battery;
Battery + cable comes loose at contactor;
Battery - cable comes loose at battery end;
Battery - cable comes loose at ground bus;
Battery internal terminal connection fails open;
Contactor fails open;
Contactor wiring comes loose;
Contactor Master switch fails open.
I ignored the case of discharged battery, jump start and go flying immediately. We already KNOW this is how three airplanes flying SDS and one Diamond twin diesel have found themselves dead stick. Let's not try to add in too much stupidity.
Once any events off the list above occurs, we arguably still have the alternator associated with that bus running, so we are not cold bus yet... But if the pilot sees unusual charging system indications, falls back on primary training, and "reset" the alternator, then this bus is cold.
Carrying this through, we can engage the Bus-Tie (Cross Feed) and get the switched bus back, but several failure modes still leave the "always hot bus" out of reach, so its dead stick and dark avionics functions had better be covered elsewhere.
When I polled folks a while back, both here and on homebuiltairplanes.com, I did get a couple batteries that failed completely in flight, so battery failure is not made up. The cables coming off are unlikely, but possible, so they stay.
Three of the eight failure modes cited can quiet the engine and/or avionics completely unless we have another bus and alternator that also runs these things. Z-14 has this covered pretty well, and so dead stick landings can be largely prevented to pretty low orders of probability.
I am acutely aware that some "solutions" can reduce reliability - there are more things to break. I ran all of the single point failures, all two-part failure modes, and deliberately searched for nasty combinations to make sure I have included them. If modes got down to low enough likelihoods, I usually stopped chasing that direction. I have chased through some 455 modes in my system, and the other pilot/engineer in the house gave me some new things to chase here too.
We can assign likelihoods and severities on each of the eight modes and then 10% likelihood of a pilot trying to "fix" an issue and depriving the airplane of that side by cycling power to the regulator. Likelihoods of deadstick landings this way were higher than I like with single switches.
One of the startling things about all of this was how often a single switch for a function limits how low we can drive deadstick landings. This has caused me to pursue extending having two of each thing to include switches. That bumped the likelyhood of deadstick landings and dark cockpits down into the e-6 range, an order of magnitude better than the analysis showed in my current IFR steed.
So, let's not go suggesting an engineer looking for a more reliable way to connect redundant electrical power to redundant elements with redundant switches has failed some big important task. This really experienced engineer is trying really hard to make sure the bad results are really rare. Maybe having a 700-2-10 (or 700-1-1) for each element in a redundant pair is the best scheme. It is looking pretty good to me...
Billski
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