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Redundancy for Electrics

kens_cockpit

kens_cockpit

Well Known Member
I want to do IFR flight

I have a Dynon EFIS D10A as the principal flight instrument. It contains all the required flight instruments for IFR flight within it. I also has the internal battery option for backup.

I also have for backup an ordinary ASI and Altimiter, the Trutrak Pictorial T&B and the Trutrak Digiflight II VS Autopilot which has turn, altitude hold and vertical speed.

How should I organise the electrical redundancy? Is having the Dynon internal backup battery enough to cover the failure of the main system? If the Dynon and the alternator fail I can power the backup instruments from the battery alone. But if the alternator fails and the battery discharges that may not be enough. Should I have a second alternator or a second backup battery for the backup instruments?

Should I run the Dynon and the backup instruments off separate power buses?

I'ld appreciate your wisdom on this.

Ken Stanton
 
I was looking at a Lancair IVT with all glass. Only steam gauges were cabin altitude and pressure differential. No backup battery, no Dynon. Scary, scary. With an alternator failure, the glass panels would have the battery below 9 volts in 15-20 minutes with other IFR loads on. How would you like that panel to go dark in the clag at night?

The Dynon with its internal backup battery is a really good idea but it's nice to have another backup for anything else electric that is important. I'd recommend a separate battery wired to the main bus independent of the master contactor or to a backup essential bus. Use a diode or switch to isolate the backup battery from the main battery and alternator.
 
You should do a load analysis of your plane. Figure out how much power each item in the plane needs. Then pick a few scenarios (day VFR, night IFR, etc.) and figure out what needs to be powered up in each scenario. Now figure out what you could give up in each scenario if the alternator fails and you have to rely on the battery alone. Calculate how much time you'd have left running on the battery with those items turned on.

There are a few other variables you need to consider. Is your engine dependent on electricity from the bus to keep running (e.g. electronic ignition vs. mags)? Are you planning some long cross country flights where you don't want to be stuck in the middle of nowhere? In which case a back-up alternator would be a good idea.
 
When calculating battery life with load, don't use the amp hour rating. Most battery manufacturers will publish a current rating to discharge to 9 volts which is where most avionics will go down. This is substantially less time than a full discharge takes. Also note that batteries lose some capacity with cycles and age and should be replaced every 2-3 years to be safe.

Based on my experience with alternator and subsequent battery discharge, I'd highly recommend an aural low voltage warning as lights may not be noticed for some time if you are busy or on the dials in actual IFR conditions. It's important to shed non-essential load as soon as the alternator fails to give you the most time to get down safely.
 
I have a similar panel.

I used the Dynon switched power output as a backup source for my Rocky Mountain engine monitor.

Along with a GPSMap 296 with it's own battery, and a handheld comm mounted on the side panel, I have all critical elements backed up.

In the event of an alternator failure and if the main battery runs down, I will have primary flight instruments, primary nav, engine instruments and comm functioning off of secondary battery power.

I think this is fine for night VFR flight.

FMI: http://www3.telus.net/aviation/flying/RV-9A/rv-9a
Vern Little
RV-9A
 
GRANT ED said:
I plan of using a full GRT glass cockpit with a Dynon as a backup.
I have not decided of my buss setup yet but I did consider using a second battery as a back up. That was until I found this
http://www.aircraftspruce.com/catalog/eppages/sdaltreg.php
Lighter than a battery and will last as long as the engine is running.
Something to consider..
Click to expand...

This is a Model SD spline driven 14 volt alternator system that fits in to the vacuum pump drive pad. it only weighs 3.7 pounds. and includes and adjustable solid state regulator.

I think this might be just what I'm looking for.

IMHO this is an excellent backup subject to the 8 amps output being sufficient. I think I wold run this to the backup instrument bus (for which I will have to do a load analysis) and then possibly to the Garmin 430.

I won't waste more time with my ponderings on this thread.

thanks all for your input
 
Not to pick nits, but you might still be left with a single point of failure in your system, namely, the battery itself.

The new RG batteries used for aircraft are incredibly reliable, but a broken terminal is still conceivable. The SD alternator NEEDS a battery to excite it, and if your battery goes offline, the SD won't wake up.

This may or may not be an issue for your installation, but you should be aware of that scenario.
If you can't accept even the possibility of a broken battery terminal, you could always add a second, really small battery to serve primarily as the "starter" for the SD backup alternator.

The likelyhood of losing your main battery is almost nil, so simply adding the SD alternator takes you scads beyond the reliability you get in 99% of GA aircraft.
 
It is more difficult than you would think to totally eliminate all single point failures. That is one failure that would reduce the capability of the aircraft. To do this you need redundant EVERYTHING and would need to do analysis of the system to eliminate cascading failures. Most aircraft manufactures get around this by having things such as two radios fed of two avionics busses. One relay could fail and take out the whole avionics bus but you should have enough capability to get on the ground. I think the idea here is to give yourself enough time to get on the ground should something happen.
The internal battery of the dynon gives I believe 2 hours of run time. If I had a total electrical failure I'd be looking for a place to land now rather than want to keep flying for more than two hours.
I guess what I'm trying to say here is don't get too carried away with redundancy trying to make your plane invincible. Think carefully about your setup. Having the backup battery for the dynon and the smaller alternator as Highflight says is better than 99% of GA aircraft in terms of redundancy.

"The new RG batteries used for aircraft are incredibly reliable, but a broken terminal is still conceivable. The SD alternator NEEDS a battery to excite it, and if your battery goes offline, the SD won't wake up."
In this situation the SD alternator would be ok because it would be fed power from the main alternator through the battery buss. If you were then to loose the alternator, I'm not 100% familiar with this alternator and it would depend a little on the setup but provided it was on at the time it should be self sustaining. Supplying itself with the power needed to operate.
 
kens_cockpit said:
I want to do IFR flight

I also have for backup an ordinary ASI and Altimiter, the Trutrak Pictorial T&B and the Trutrak Digiflight II VS Autopilot which has turn, altitude hold and vertical speed.

Ken Stanton
Click to expand...
There's your backup for the Dynon.
 
Other alternator failure modes?

Does anyone worry about failure modes other than an alternator dying? Having a 2nd alternator (and regulator, battery, contactor, etc.) keeps you from losing all avionics if an alternator dies, granted.

Now, if you put all the essential stuff on one bus, and a generator overvoltages, it can zap everything you need to land in IFR. I understand that most of us have faith in the overvoltage relay...its job is to prevent just such an occurence.

My paranoia is based on the inability to know if the OV relay is working before I take off. Yeah, I know it's a very improbably set of failures: first the OV relay has to fail, and then the regulator has to fail, causing an overvoltage. But it happened to me, in IFR at night, in a Cessna, so I'm paranoid.

One reason such improbable failures can happen is the aforementioned inability to test the OV relay preflight. It had probably failed years before my flight. So I was just one failure away from an emergency. When the voltage regulator fried, the bus voltage went very high, and sparks and smoke erupted from behind the panel. I shut the alternator off before anything in the panel fried. Read the "Never Again" article I wrote in the 3/99 AOPA Pilot if you want the whole sorry tale.

So I'm seriously considering building two busses into my RV-7, each one having enough stuff in it to get me down (say, an SL-30 navcomm on one bus, a GNS-430 on another; an EFIS on one bus, and Trutrak A/P on another...that kind of thing...still pondering)

What do you guys think? Am I polishing the cannonballs?
 
As I said in an earlier post, you will never be able to eliminate every possible failure point. Just try to eliminate the common ones. Alternators are a (relatively) common one. Equally so is the wiring associated with them. If it really bothers you you could use dual busses. IMO just make sure you don't get carried away. You can have 23 Alternators, 53 busses, 93 backup batteries and 875 backup instruments but just remember that at the end of the day you have only got one engine. :)
 
GRANT ED said:
You can have 23 Alternators, 53 busses, 93 backup batteries and 875 backup instruments but just remember that at the end of the day you have only got one engine. :)
Click to expand...

Good point. Maybe I should install a second engine...I'd need it to drive the other 21 alternators!
 
mgomez said:
My paranoia is based on the inability to know if the OV relay is working before I take off. Yeah, I know it's a very improbably set of failures: first the OV relay has to fail, and then the regulator has to fail, causing an overvoltage. But it happened to me, in IFR at night, in a Cessna, so I'm paranoid.
Click to expand...

Personally I don't think having multiple backup systems is near as important as how you manage the power systems.

From what I read most alternator problems are discovered by the radios going out or the lights getting dim. When that happens you have probably been flying for quite some time without an alternator. All that time could have been used to get down safely.

Last week while flying under the hood with my instrument instructor my voltage regulator went out. The OV crowbar popped the fuse to the alternator field and turned the alternator off. I don't have an alternator light but I did setup my engine monitor so that if the battery got down to the typical unloaded battery voltage (12.3 on my engine monitor) or got as high as 15.5 volts then the master warning light would go off. By having it setup this way I was able to know about an alternator problem as soon as it happened. I actually got a few master warning lights when the voltage went to 15.6 volts before the total failure of the regulator. I stayed under the hood and we used it as part of the lesson. We put about 20 minutes on the plane getting it back to the airport. The voltage got all the way down to 12.2 volts after we landed. I felt that I could have flown for quite some time if needed to get me back on the ground safely. In this case a little bit of knowledge headed off any problems I would have had in landing safely.
 
One pilot, too

GRANT ED said:
You can have 23 Alternators, 53 busses, 93 backup batteries and 875 backup instruments but just remember that at the end of the day you have only got one engine.
Click to expand...
Even worse - only one pilot.
 
N916K said:
From what I read most alternator problems are discovered by the radios going out or the lights getting dim.

Well, in my case, the lights went BRIGHT. But I get your point.
Click to expand...
 
Dual Electronics

I have two batteries, two alternators, two voltage regulators on two seperate busses for my 'glass cockpit'. I can cross-connect the two if necessary due to a single failure. Since I also have duel electronic ignition the two systems were necessary. I have split critical componants on the two different busses, the glass screens have multiple power connections so I have one running to each bus.

Works for me!

David Schaefer
RV6a N142DS "GeekOne"
http://home.kc.rr.com/n142ds
 
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