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Found Single Point Failure in Some Heritage Ignition Systems

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PilotjohnS

Well Known Member
I found a single point failure in ignition systems that use a switch having a BOTH position.

I had a ACS FAA-PMA ignition switch in my RV9A short out during takeoff and ground out BOTH mags, engine went dead. Plane had 10 hours on the Hobbs.

During run up, at 1700 RPM all is normal.
But during takeoff at 2300 RPM both mags would cut out momentarily, This was experience as a stutter in the engine.

After convincing myself it couldn't be the switch, I changed the switched to a Cessna 152 part and all is well.

Theory: turns out the P-leads from the magnetos have high voltage on them when the engine is operating. I read something like 300 V at 1700 RPM and 500 Volts at cruise RPM. if the BOTH position develops a high voltage short, then it will shutdown BOTH mags. They are not independent redundant systems with a common switch.

I am changing to two locking toggle switches with a Key start soon.

I would like to find a high voltage tester to see where the breakdown is occurring.
 
PilotjohnS said:
I found a single point failure in ignition systems that use a switch having a BOTH position.

I had a ACS FAA-PMA ignition switch in my RV9A short out during takeoff and ground out BOTH mags, engine went dead. Plane had 10 hours on the Hobbs.

During run up, at 1700 RPM all is normal.
But during takeoff at 2300 RPM both mags would cut out momentarily, This was experience as a stutter in the engine.

After convincing myself it couldn't be the switch, I changed the switched to a Cessna 152 part and all is well.

Theory: turns out the P-leads from the magnetos have high voltage on them when the engine is operating. I read something like 300 V at 1700 RPM and 500 Volts at cruise RPM. if the BOTH position develops a high voltage short, then it will shutdown BOTH mags. They are not independent redundant systems with a common switch.

I am changing to two locking toggle switches with a Key start soon.

I would like to find a high voltage tester to see where the breakdown is occurring.
Click to expand...
Exactly the kind of failure that Bob Nuckolls has been writing about for decades.

http://www.aeroelectric.com/articles/MagnetoSwitchOptions.pdf
 
The dielectric strength of air (how far an arc can travel) at sea level is roughly 3000 volts per millimeter. The minimum voltage that will cause an arc in air is 327 volts with a electrode spacing of 7.5 um. I don’t think the primary voltage would exceed 300V but Even at 500 volts the switch would have to fail in a way that the switch contacts were both within a few thousands of an inch to ground but not touching. Hard to imagine. Particularly since the switch seems to function normally otherwise. I don’t like the ACS switches and used toggles but this is not the type of failure described in the AD or by Nuckolls.
 
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Mikeyb said:
The dielectric strength of air (how far an arc can travel) at sea level is roughly 3000 volts per millimeter. Even at 500 volts the switch would have to fail in a way that the switch contacts were both within a few thousands of an inch to ground but not touching. Hard to imagine. Particularly since the switch seems to function normally otherwise. I don’t like the ACS switches and used toggles but this is not the type of failure described in the AD or by Nuckolls.
Click to expand...

True - the SPOF introduced by having one device (the key switch) that keeps both mags running is the failure mode that can be prevented by using two simple toggle switches.
 
PilotjohnS said:
I. I read something like 300 V at 1700 RPM and 500 Volts at cruise RPM. if the BOTH position develops a high voltage short, then it will shutdown BOTH mags. They are not independent redundant systems with a common switch.
Click to expand...

I believe most permanent magnet generators, like the mag, produce a set voltage (not sure which design elements set the voltage) and the voltage is relatively constant, when measured at it's peaks, regardless of RPM. RPM just changes the frrequency (Hz) of the AC. When I set the governor on my home generator, the voltage is constant and the RPM adjustments are getting the Hz in line with the 60 needed for our appliances. It was my understanding that magnetic generator in a mag is producing around 50-100 volts, but have no idea if that is correct or not.

The effective energy output of the mag does increase with RPM, but that is not due to voltage increase, but a frequency increase. In AC, more Hz equal more energy.
 
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The output voltage of a permanent magnet generator is proportional to RPM.
Its an inductor. E=L dI/dt. Doubling the rate of current change (RPM) doubles the output voltage.
 
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After learning to fly in a few Cessnas with well-worn keyswitches I decided I never wanted one in my own aircraft. When I bought my -6, I removed the keyswitch and replaced it with three toggle switches. Two for the mags, and one momentary for the starter.
 
Mikeyb said:
The output voltage of a permanent magnet generator is proportional to RPM.
It an inductor. E=L dI/dt. Doubling the rate of current change (RPM) doubles the output voltage.
Click to expand...

Thanks for addressing my misunderstanding.
 
Single wafer

Any switch with a single wafer and a both position is suspect.

I think only a two wafer switch, with left and right on separate wafers, would be considered no longer a single point failure. JMHO

Reading the AD posted earlier, seems this has happened before.

I suspect the arc over was not thru air, but rather thru contamination. Dirty Switch. The engine performed the same ( missing) with the switch in Left Right or Both, made no noticeable difference.
 
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