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Electrical Single Point of Failure?


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Ever since I did a deep dive into my ‘68F electrical system to find the cause of low voltage, I’ve been thinking through some kind of scary failure modes.  I guess I should have known this a long time ago, but the old POH didn’t really spell it out and/or maybe my airplane has been modified a bit through the years with new avionics and circuit breakers.

Basically, my new fear is the connection to the Aux circuit breaker connection at the Power Bus.  The Battery connection from the battery master solenoid (through the starter solenoid) is directly onto the CB with a very thick gage wire.  There is also a jumper on the CB over to the Main Bus which is also thick gage.  The connection from the alternator is to the Main Bus.  So, from this one Aux Bus CB connection on the right side panel, it’s wired over to the avionics master on the left, then back over to the avionics bus CBs back on the right.  I see why there’s a rats nest back there.

My fear is that I’m IFR and this one aux bus CB breaks or the tiny little screw holding the battery and jumper wire to it comes out or whatever.  That one CB kills all my radios, navigation, and transpoder.  I’d be left with my pitot static instruments, 2xG5s (no nav info), autopilot (which strangely isn’t on avionics bus), and engine instruments.  Obviously the best plan would be to get VMC somewhere, anywhere.  If no VMC, I think I’d have to use FF on my phone (my iPad gets gps from the transponder) to navigate to an instrument approach and use the phone to actually fly down the course while descending iaw the approach.  All without atc comms.  This doesn’t sound good at all.  

I did practice turning on the internal G5 gps which then provide gps track info since the GMU-11 is out as well.  The autopilot will only do altitude hold and “straight” which isn’t much but still helps.  I have a 5w handheld but don’t trust it for comms outside about 5nm from a reception antenna.  Maybe it’ll work in the pattern...

At least an alternator failure gives you a chance to “gracefully degrade”.

Are your planes wired differently?  Have you guys thought through a failure like this?  Thoughts?

Edited by Ragsf15e
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On the M20J (I haven't checked other models) the Avionics master switch controls a normally closed relay. The switch operates in reverse of what you would expect. When you turn the Avionics master OFF it energizes the relay and that removes power from the Avionics bus. Turning the Avionics master ON de-energizes the relay which then closes powering the Avionics bus. As the Avionics master gets power from the Aux bus, you can demonstrate this by turning on the Master with the Avionics master turned OFF and then pulling the Aux breaker which will cause the radios to power up.

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11 minutes ago, PT20J said:

On the M20J (I haven't checked other models) the Avionics master switch controls a normally closed relay. The switch operates in reverse of what you would expect. When you turn the Avionics master OFF it energizes the relay and that removes power from the Avionics bus. Turning the Avionics master ON de-energizes the relay which then closes powering the Avionics bus. As the Avionics master gets power from the Aux bus, you can demonstrate this by turning on the Master with the Avionics master turned OFF and then pulling the Aux breaker which will cause the radios to power up.

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I had heard this and I’ll try your exact steps to see what happens.  However I don’t think mine is set up that way.  On my last scenic flight I pulled the Aux Bus CB with everything functioning normally and did indeed lose all my avionics.  Either the older models didn’t have that “safety measure” or someone rewired mine.

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Everything should have a plan B...

All MSers think this way...

Their Plans B become more elaborate when IMC is involved...

Their power outages become more complex...

their is a great thread around here recently on supplying E-power to the avionics bus... done differently in Europe...

In the US one of the remote failure modes isn’t covered as well...

what happens in flight when. The avionics switch fails...  AND the avionics relay fails to close properly...

The discussion was.... what does this switch do?

Take a look at what you have... compare to what gets built today...

There are three busses for a good reason...

A bunch of wires to a relay is kinda like a bus...

By the time you are done thinking about your situation...  you probably will be thinking about adding a bus or two, a second battery or two... and a spare alternator...

And upgrading your portable radio to include gps and ILS capability...

When it comes to cost....

Don’t get confused with false economics....

We’re not upgrading a 1960s plane with expensive electronics....

We are protecting the people inside the 1960s plane with expensive electronics...

The people are probably worth it...

PP thoughts only, not a mechanic...

Best regards,

-a-

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You can strive to reach true redundancy but in the end there is always a single point of failure.   We have one engine and usually one pilot.  I guess the goal is to reduce the risk as much as possible.   

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Life also used to be simpler. Systems were straightforward with a minimum of interactions between instruments and avionics. Now, everything is interconnected in ways that are not always obvious. I tried doing a failure modes and effects analysis on my bird and soon gave up -- the failure modes are not all obvious and neither are the effects. I couldn't even get good information from the manufacturers. Case in point: I asked Garmin what the effect on the GTX345 AHRS would be if the GPS signal from the GNC430W was lost. Tech Support didn't know and asked Engineering. The answer came back that it should still work.

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54 minutes ago, PT20J said:

Life also used to be simpler. Systems were straightforward with a minimum of interactions between instruments and avionics. Now, everything is interconnected in ways that are not always obvious. I tried doing a failure modes and effects analysis on my bird and soon gave up -- the failure modes are not all obvious and neither are the effects. I couldn't even get good information from the manufacturers. Case in point: I asked Garmin what the effect on the GTX345 AHRS would be if the GPS signal from the GNC430W was lost. Tech Support didn't know and asked Engineering. The answer came back that it should still work.

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Ha!  I think this is basically what dawned on me.  I didn’t understand this particular failure mode until I dug into it.  By itself it’s a little scary to me, but it also got me wondering what else I don’t know.  A bit of ignorance is bliss, but too much might be an NTSB finding.

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4 minutes ago, chriscalandro said:

Anything glass and primary should have a backup battery. There are no redundant power systems in these old airplanes, unless you have new avionics - in which case, backup battery. 

I guess in my particular case it’s not losing “glass” that worries me.  I do have 2xG5s but they have battery backup.  It’s the loss of my 430W, SL30, and transponder which takes out all my nav, approach, and communications.  If it was 2 old nav/coms and an old transponder the result would be the same.  

I guess I’m just not sure I like all that stuff wired through one avionics bus.  Even then, I’m sure there’s a way to cause a total electrical failure.

Edited by Ragsf15e
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21 hours ago, PT20J said:

On the M20J (I haven't checked other models) the Avionics master switch controls a normally closed relay. The switch operates in reverse of what you would expect. When you turn the Avionics master OFF it energizes the relay and that removes power from the Avionics bus. Turning the Avionics master ON de-energizes the relay which then closes powering the Avionics bus. As the Avionics master gets power from the Aux bus, you can demonstrate this by turning on the Master with the Avionics master turned OFF and then pulling the Aux breaker which will cause the radios to power up.

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As a data point, I tried this on my 252/Encore today and it did NOT behave this way. Hard to say if it was like this from the start or years of avionics changes have altered its behavior.

Despite the redundancies elsewhere (dual alt, dual vacuum), the master, avionics master, and each respective solenoid are single points of failures in a series line. Somewhat chilling to think about.

Edited by smwash02
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16 minutes ago, smwash02 said:

As a data point, I tried this on my 252/Encore today and it did NOT behave this way. Hard to say if it was like this from the start or years of avionics changes have altered its behavior.

Despite the redundancies elsewhere (dual alt, dual vacuum), the master, avionics master, and each respective solenoid are single points of failures in a series line. Somewhat chilling to think about.

Well, if you really want to lose sleep, start thinking about the single points of failure in the engine ;)

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I can't speak for the  electrical schematic of a 68 F model but on the '67 C (at least mine anyway) the aux  CB controls every switch above the pilot's  left knee (fuel pump,pitot heat, lights etc) the original power relay is grounded through the master switch. I thought about adding avionics master switch  but   just adding a separate bus  for avionics bus in my mind would be adding another point electrical of failure. Unless I wanted to  get  into  the  great expense and weight gain to do it properly.  it probably wouldn't be worth it just my opinion. Electrical failure could happen regardless of number of backups that you have at any given time.  I can't think of any accident where the pilot(s) knew too much their aircraft system(s) (electrical, hydraulic, pneumatic, etc)  caused the accident, but  I have heard of several incidents/accidents where not knowing the aircraft systems cause them to give up trying to fly the aircraft or at least try something that might get you down to ground safely. it sounds to me like you have been going through the the "what if" scenarios which probably the best that you can do.

for what it is worth,

James '67C

Edited by jamesm
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If I had an all electric panel with modern avionics, I would ditch the avionics bus. The only reason for it is to supposedly protect electronics from voltage spikes during engine start up and shut down. I seriously doubt it was really ever necessary but it certainly isn’t necessary with modern electronics. As the Surefly team found out, there are voltage spikes on the power bus of many airplanes during normal operation.

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If I had an all electric panel with modern avionics, I would ditch the avionics bus. The only reason for it is to supposedly protect electronics from voltage spikes during engine start up and shut down. I seriously doubt it was really ever necessary but it certainly isn’t necessary with modern electronics. As the Surefly team found out, there are voltage spikes on the power bus of many airplanes during normal operation.
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It probably was necessary when voltage regulators were mechanical, today’s electronic regulators it’s not a problem.
I suspect there are voltage variations as the demand changes, the voltage regulator boosts the charging because of greater demand, demand then drops and voltage momentarily rises until VR adjusts. My first suspect would be the strobes, especially if they’re synced. All modern avionics are designed for 14-28v systems, so they have built in voltage regulators.
The one nice thing about the avionics bus is it lowers the demand on the battery during startup, not sure I would ditch it.
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A cell phone provides backup GPS.   If paired with an iPad you get moving maps.    I also carry a $90 "Dual" brand WAAS GPS in my flight bag as another backup.  All that stuff will work even if ships' electrics go T.U.

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14 hours ago, chriscalandro said:

Anything glass and primary should have a backup battery. There are no redundant power systems in these old airplanes, unless you have new avionics - in which case, backup battery. 

I do think it's a little odd that the JPI primary displays don't have battery backup.   

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12 hours ago, PT20J said:

If I had an all electric panel with modern avionics, I would ditch the avionics bus. The only reason for it is to supposedly protect electronics from voltage spikes during engine start up and shut down. I seriously doubt it was really ever necessary but it certainly isn’t necessary with modern electronics. As the Surefly team found out, there are voltage spikes on the power bus of many airplanes during normal operation.

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Back when I designed airliner avionics for a living, aircraft power on transport category aircraft was notoriously horrible, especially on Airbus.  Everything had to be designed to handle complete momentary dropouts, because they'd routinely occur during normal operation as systems got switched around.  Naturally when the dropouts occurred, there'd be a spike on restart.   Anything with digital logic, RAM or a processor had to be able to survive these without losing state.   Lightning protection was easier to get sorted out than the power quality.

 

Edited by EricJ
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Lots of things can make the power go out.  This is why we have 2 magnetos for example.  This is why I never understand folks who chest thump after throwing their vacuum system overboard.  

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Lots of things can make the power go out.  This is why we have 2 magnetos for example.  This is why I never understand folks who chest thump after throwing their vacuum system overboard.  

Because vacuum pumps fail with regularity (I average a failure every 700 hours). And attitude indicators almost as often. So if you have a battery as backup the failure rate should be lower.
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1 hour ago, ArtVandelay said:


Because vacuum pumps fail with regularity (I average a failure every 700 hours). And attitude indicators almost as often. So if you have a battery as backup the failure rate should be lower.

I agree with this.  Actually I’m much less worried about having attitude information fail than I was before I had the 2xG5s installed.  Both have attitude and battery backup.  It can be subtle and misleading as a vacuum fails too.  The electrical ADIs generally red X and are constantly comparing solutions.  It’s the rest of the avionics bus going dark that worries me.

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19 hours ago, PT20J said:

If I had an all electric panel with modern avionics, I would ditch the avionics bus. The only reason for it is to supposedly protect electronics from voltage spikes during engine start up and shut down. I seriously doubt it was really ever necessary but it certainly isn’t necessary with modern electronics. As the Surefly team found out, there are voltage spikes on the power bus of many airplanes during normal operation.

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I may do this at some point.  I turn everything off before shutdown and on after start anyway.  Audio panel and GTX345 would be the two things that will come on by themselves as power is applied.  I’ll definitely talk to someone about this, but maybe eventually just putting a jumper between the main bus and the avionics bus to skip the avionics master and aux bus cb would take care of it.

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3 hours ago, ArtVandelay said:


Because vacuum pumps fail with regularity (I average a failure every 700 hours). And attitude indicators almost as often. So if you have a battery as backup the failure rate should be lower.

But why remove the vacuum attitude? Who ever complained they had too many attitude attitude indicators ? Mine sits to the side at the ready if I lose all 3 interconnected AHRS units. 

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7 minutes ago, RobertGary1 said:

But why remove the vacuum attitude? Who ever complained they had too many attitude attitude indicators ? Mine sits to the side at the ready if I lose all 3 interconnected AHRS units. 

If you don't want to maintain it, as in keep buying pumps and AI overhauls,  or if you want the pad space back on the accessory case, or if you want the weight back, or if you want the panel space back, or if you want to declutter the associated hoses, etc., etc.

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