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06-19-2005, 02:44 PM
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Join Date: Mar 2005
Location: Northern California, USA
Posts: 537
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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?
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06-20-2005, 05:45 AM
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Join Date: Apr 2005
Posts: 65
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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. 
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06-20-2005, 07:18 AM
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Join Date: Mar 2005
Location: Northern California, USA
Posts: 537
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Quote:
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Originally Posted by GRANT ED
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. 
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Good point. Maybe I should install a second engine...I'd need it to drive the other 21 alternators!
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06-20-2005, 07:48 AM
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Join Date: Jan 2005
Location: Tehachapi, CA
Posts: 538
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Quote:
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Originally Posted by mgomez
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.
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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.
__________________
Cam
Santa Ana, CA
RV-9 at KFUL
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06-20-2005, 12:36 PM
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Join Date: Feb 2005
Location: LSGY
Posts: 3,199
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One pilot, too
Quote:
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Originally Posted by GRANT ED
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.
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Even worse - only one pilot.
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06-20-2005, 06:36 PM
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Join Date: Mar 2005
Location: Northern California, USA
Posts: 537
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[quote=N916K]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.
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06-22-2005, 04:03 PM
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Join Date: Jan 2005
Posts: 68
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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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