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Intermittent mag failure


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Here's another maintenance report! :-)

Got a squawk that a significant RPM drop was occurring in flight on an aircraft. Couldn't reproduce on ground, so I asked that they be very careful when flying and try to report as much info as they could next time it happens.

It happened again a few days later, and they tried to figure out which mag. Engine stopped on one mag. They asked me to replace the ignition module with the used one that I keep as backup, but I said I don't think it's the module, suddenly quitting and coming back usually isn't the symptom of these modules. I said I suspect ignition switch or wire getting an intermittent ground. Owner insisted and I replaced. Repaired a couple connections in the process into the modules as a precaution, the wire going into the pin was getting work hardened due to improper zip tying to vibrating fixtures. Also briefly checked wire security on ignition switch (one was a little loose).

A couple weeks went by and it happens again. I reiterate that I think it's the ignition switch. They ask me to order one. Well, I pulled the one off mine and installed it for now, and took the old one and sat it on my desk for testing.

Tests seem to be OK, but these switches are industrial rotary switches, which can be customized to a whole bunch of different switching setups. There is no schematic to speak of unless flight design wanted to provide their order specs. So, I really was not sure what I was testing for...

So I take a gamble. I disassemble the switch. Springs fly EVERYWHERE. and on one small contactor, I see this:


This is a burned contactor with considerable carbon build up caused from arcing. When arcing occurs, it deposits carbon, which makes arcing in the future easier to do as it can follow the carbon tracks. There was also carbon flakes blown around the switch. I strongly suspect this is the cause, as the spark from the ignition modules may have been jumping the gap under the right circumstances instead of going through the coil packs primary side. Note that the high energy secondary does not go anywhere but the engine itself; rather magnetos and rotax modules route the primary side spark to ground to deactivate the ignition system without putting a ultra high voltage through the system. However, the voltage coming off the primary side is still quite high.

I do have a wiring diagram of the aircraft electrical systems, so I sat down to study it carefully and checked switch function on the one i temporarily installed to make sure it works with how the diagram depicts.

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Corey, not sure if you fixed the switch or did you give up after crawling around the floor with a flashlight trying to find the springs?  I'm thinking you put it all back together.  Is the arcing normal?  Can one can open up a switch and see this? Or, was the arcing in your switch due to something amiss in the modules or coils which allowed a high voltage to back feed into the switch? 

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Soon as i saw the burned contactor and carbon flakes everywhere, it went in the trash. I wouldn't throw out a switch just because of a little black mark, but this was a lot.


These switches are complex and very easy to put back together wrong. I do not recommend trying to service them unless you can get a schematic, because the cam positions are critical and the cams themselves are custom made. I'll post the other photos I have in a bit.

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