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  #1  
Old 10-29-2018, 05:31 PM
Dugaru's Avatar
Dugaru Dugaru is offline
 
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Default Recent Power Loss to Avionics Bus

[reposted from general discussion]:

For once, I have a relevant data point to add in an electrical discussion! Although I wish I could have skipped the whole thing...

Apologies in advance for my ignorance, since I'm not the builder and know very little about electrical systems. Although I'm learning more.

Last week, on a night flight on an IFR plan, fortunately in gorgeous VMC, I smelled a brief plastic-y burning smell and then everything on my avionics bus went dark. Except for my battery powered G5, for which I developed an even greater appreciation at that moment.

I later determined that I had blown the 15 amp fuse upstream of my diode.

When this happened, I flipped the ebus switch. Everything came back up, which made me happy, and then... everything on the avionics bus promptly died again.

Because I had now blown the 15 amp fuse upstream of the ebus switch.

I've rooted around under the panel since then, and everything is now working with the fuses replaced. I'm planning my next moves to keep this from happening again. But already I have some key lessons:

1. If installing a G5, pay the extra $ for the battery backup.

2. ATC is good at diagnosing electrical failures. Apparently losing comms and transponder simultaneously is a clear sign.....

3. Check the battery on your handheld before flying. Note that even if you do so, and even if the battery indicator is at "half," that handheld, which you have carried FOR YEARS for JUST SUCH AN EMERGENCY, might still go dead when you push the transmit button.

4. The principal problem with losing all comms at night is that it's hard to turn on the lights at airports. This never occurred to me during instrument training regarding lost comms.

5. Airports are extremely dark at night without runway lights. The green/white beacon can be readily identified but doesn't give you enough light to see.

6. ATC can turn on the lights for you, even at a non-towered field with pilot-controlled lighting! I somehow never realized this. Or at least they could at W96, maybe it has to be close to a big transmitter (which W96 presumably is). Can't tell you how grateful I was for that assistance.

7. ATC, for its part, is very grateful when you call PROMPTLY after ending a no-radio IFR flight. Proud to say that I managed to check that box.

8. An RV-9 can of course be readily landed with no flaps, even when it's still trimmed for 150 knots....

9. If instrument rated, always file an instrument flight plan for night flight, even in VMC. That way, if you lose comms, somebody will know that you need the lights turned on.

10. Flying and navigating in an RV at night with just a G5, and an iPad with Foreflight, is no major problem. Did I mention that the battery backup option for the G5 is a good value?
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  #2  
Old 10-29-2018, 05:48 PM
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RV7A Flyer RV7A Flyer is offline
 
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Quote:
Originally Posted by Dugaru View Post
[reposted from general discussion]:

8. An RV-9 can of course be readily landed with no flaps, even when it's still trimmed for 150 knots....
In the previous thread, you said that you initially blew the 15A fuse for the *avionics* buss, then the 15A fuse for the E-buss. Do you have flaps and/or trim on either of these busses?

I think a schematic would help understand what happened here, if you can post one.
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  #3  
Old 10-29-2018, 07:20 PM
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Dugaru Dugaru is offline
 
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Default I wish I had a schematic!

I’ve written the builder to see if he has a schematic.

Flaps and trim appear to be on the avionics bus. The “ebus” switch appears to turn on the entire avionics bus.

Again, I’m still learning this stuff, so apologies for any idiotic statements. I suspect I have the ability to perplex/infuriate knowledgeable people with my ignorance.

Quote:
Originally Posted by RV7A Flyer View Post
In the previous thread, you said that you initially blew the 15A fuse for the *avionics* buss, then the 15A fuse for the E-buss. Do you have flaps and/or trim on either of these busses?

I think a schematic would help understand what happened here, if you can post one.
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  #4  
Old 10-29-2018, 07:57 PM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
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A 15A fuse might have been sufficient for the avionics buss when the plane was first built, but the addition of new or different avionics might very well now be trying to pull greater than 15A, particularly when the radio or transponder are transmitting.
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  #5  
Old 10-29-2018, 08:17 PM
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Dugaru Dugaru is offline
 
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Default New avionics

I suspect youíre right. Itís worked fine for 18 months in this configuration, BUT the autopilot (which I had added) was working extra hard in big up and down drafts, and I was charging two gadgets on the USB port (which I also had added).

Maybe there was a reason the builder didnít originally have any accessory plugs or USB charging ports....

Iím not really sure what the easiest fix is. I do at least know Iím not supposed to just put in a bigger fuse.




Quote:
Originally Posted by alpinelakespilot2000 View Post
A 15A fuse might have been sufficient for the avionics buss when the plane was first built, but the addition of new or different avionics might very well now be trying to pull greater than 15A, particularly when the radio or transponder are transmitting.
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  #6  
Old 10-29-2018, 08:22 PM
rocketman1988 rocketman1988 is online now
 
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Default usb

Some of those USB ports can draw more than 2.4 amps EACH...
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  #7  
Old 10-29-2018, 08:48 PM
Mich48041 Mich48041 is online now
 
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This is an example of why two fuses should not be connected in series. You never know which one (or both) will blow. If two fuses must be wired in series, then the main fuse should be several times larger than the smaller fuse. For instance, if the load fuse is 5 amps, use a 30 amp or bigger for the main fuse. Or consider using a fuselink in place of the main fuse.
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  #8  
Old 10-29-2018, 08:49 PM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
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Quote:
Originally Posted by Dugaru View Post

I’m not really sure what the easiest fix is. I do at least know I’m not supposed to just put in a bigger fuse.
You can if the wire is sized for it. If not, it should be pretty easy to rewire with larger gauge. It's just two wires: one from main bus to avbus switch, the second from avbus switch to avbus.

FWIW, I put my AP servos on my main bus when I upgraded a few weeks ago for precisely this reason. I put my avbus on a 20A fuse when I first built the plane but have since added enough that I couldn't give 5A to the servos off the avbus. Eventually I'll go from the 14AWG I have now up to 12AWG and that should give me plenty for a 30A fuse to the avbus.
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  #9  
Old 10-30-2018, 01:39 AM
rv8ch rv8ch is offline
 
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Default diagram needed

Quote:
Originally Posted by Dugaru View Post
...
Iím not really sure what the easiest fix is. I do at least know Iím not supposed to just put in a bigger fuse.
My recommendation would be to first do an analysis of what you have today - reverse-engineer a good wiring diagram. From that, the best way to proceed will become very clear.

Even though I firmly believe electricity is the closest thing in the world to magic, there is no magic to wiring our aircraft - make sure that the wires are sized right, the fuses are sized right, and the parts providing power (alternator and battery) are sized and working right, and it will just keep working.

Another plug for spending some time reading Bob's excellent book and website:

http://www.aeroelectric.com/Book/AEC_R12A.pdf

http://www.aeroelectric.com/
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  #10  
Old 10-30-2018, 07:53 AM
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Carl Froehlich Carl Froehlich is offline
 
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Default

I see this as a clear example of the limitations of single E buss and single battery design in todayís power hungry glass world.

Carl
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