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  #1  
Old 10-31-2015, 01:51 PM
Toobuilder's Avatar
Toobuilder Toobuilder is offline
 
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Default Keep an Eye on Those Ring Terminals

Here's a new one on me:

Took off this AM for a quick flight around the neighborhood. Within a few minutes of takeoff I feel a sharp pinch on my bare leg - like a bee sting. A search of the cockpit does not reveal any critters, and my leg looks unscathed. I'm about to write it off as nothing when I catch sight of a pinhead sized metallic object drop out from behind the panel and land right on my leg, with the same painful results as before. I quickly figure out that it's molten solder, leaking out of my instrument panel. OK, time to land and figure this one out. Ammeter shows nothing abnormal, but a careful hand survey of my strobe light switch reveals that the wire is loose and too hot to touch. All lights off and uneventful landing ensued.

Inspection revealed that the screw securing the soldered and crimped ring terminal to the output lug on the switch/breaker was slightly loose, causing enough localized arcing to generate considerable heat. Enough to melt solder. And yes, the breaker is functional.

So the takeaway for me is that you can have a very high localized heat condition that won't show up on the ammeter, pop a breaker, create any radio noise, or show up at all until stuff starts to smoke. I was fortunate in a sense that the builder of my Rocket used soldered connections that gave me an "early warning".

I have a new focus area on my future inspections - just thought I'd share the experience.
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  #2  
Old 10-31-2015, 03:20 PM
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Question

Was there any heat shrink over the joint?
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  #3  
Old 10-31-2015, 03:55 PM
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If a connection is loose, it can arc, creating a lot of heat, and maybe not showing anything abnormal in the ammeter. Snug them up and make that a note for regular interval inspections for a while to look for other possible loose connections.
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Old 10-31-2015, 05:02 PM
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Yes, there is heat shrink on the ferrule of the terminal. But its open on the ring end, allowing the solder to escape.

And, yes, I've known about this in theory, but never experienced it firsthand. I'm reporting it to bring the reality "home".
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  #5  
Old 10-31-2015, 05:14 PM
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Perhaps this is a good time to ask why solder is being used in a crimped terminal in the first place. A properly crimped connection (using aviation-grade crimp terminals, the correct crimping tools and the proper crimping procedure) will result in a connection that is every bit as strong and electrically-sound as a soldered connection -- and perhaps MORE SO, when you consider that solder can wick up into a wire, causing it to become brittle beyond the crimped portion.
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Old 10-31-2015, 05:35 PM
RVDan RVDan is offline
 
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In this case, the breaker doesn't pop because you don't have a shorted circuit and the current flow is normal. The poor connection is dissipating the energy at the faulty connection rather than it going to the load (lights). This is another example of why to use aircraft grade tefzel wire. If it was just ordinary commercial PVC insulated wire, you would have had lots of smoke and possibly open flame to go with it.
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Old 10-31-2015, 05:58 PM
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Quote:
Originally Posted by BuckWynd View Post
Perhaps this is a good time to ask why solder is being used in a crimped terminal in the first place...
With all due respect, the soldered connection is irrelevant to cause. I only brought it up to illustrate the fact that this connection got really, really hot.

I suspect that most builders (with the obvious exception of the guy who built my airplane) know the pitfalls of a soldered connection. My airplane will eventually see a total electrical makeover, and the vast majority of the trrminals will be crimped

Again, the focus is that solder or crimp, a relatively benign condition like a loose terminal can make for big problems.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
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RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C

Last edited by Toobuilder : 10-31-2015 at 06:01 PM.
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  #8  
Old 10-31-2015, 08:30 PM
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Talking

Conventional strobe circuits are notoriously hard on switches. Any kind of resistance will cause thermal runaway. Sounds like you've experienced this. You may have to replace the switch as well.

There was quite a bit of discussion on this about 8 years ago on the Aeroelectric list.

Cheers,
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Old 10-31-2015, 09:38 PM
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Quote:
Originally Posted by Toobuilder View Post

I suspect that most builders (with the obvious exception of the guy who built my airplane) know the pitfalls of a soldered connection.
Actually I think a lot don't. It seems new builders are getting talked into it all the time, so I think it is worthwhile for it to be mentioned.
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  #10  
Old 10-31-2015, 10:28 PM
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Default thank you

Bringing up the fact that it was not good practice to have solder on the ring terminal is like saying the Canary was not supposed to be down in the mine shaft. (ASPCA rules or something) I don't think the owner put the solder there in the first place.
The melting solder alerted the alert pilot to the overheat.
The heat came from a loose screw holding the ring terminal. Or not holding it well, as the case may have been.
We should never have loose screws, but I know it can happen.
The posting is a great lesson for ongoing inspections.
Electrical is the least comfortable area of expertise for almost all the builders and owners I have met.
Thanks very much for taking the time to write about your experience.
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