Spark plugs

[Mike Davis]

Interesting exchange from the Cessna Pilots Association forums
"All modern aircraft spark plugs have a resistor built in between the center electrode and the metal piece one sees when looking into the hole what the spark plug lead goes. This resistor should have about 500 to 800 ohms resistance when new when measured between a clean center electrode and the internal center piece.

Over time these resistors go to a much higher resistance which causes the voltage/current from the magneto to back up to a spot in the ignition system where it can flash over and destroy/burn something. This spot is usually inside the mag somewhere unless the mag harness is weak. Slick mags are much more susceptible to damage as they are built lighter and the spark can arc across the distributor arm and or the carbon rod that connects the spark to the distributor arm. Bendix mags can also suffer distress, particularly if they have not been serviced regularly like every 500 or so hours.

There is no cure for the plugs going to a high resistance, but one can measure the resistance when the plugs are cleaned and throw away any plugs that read much higher than 4000 ohms. For whatever reason, not much literature has been published by Champion or the magneto manufactures as to this problem; maybe liability problems?

Next time you have a new plug, measure the resistance to get a base number. When you clean the plugs in 100 hours or so, take another measurement. Make sure the center electrode and the internal piece is squeaky clean when you take the measurements.

I would be interested in feedback from anyone investigating this issue. I learned about this from the fellow who has been doing my mags for 35 years up in Seattle.

I had a couple hundred old plugs I have saved so I decided to test all of them for their resistance. About half of the massive electrodes plugs had resistances of over 30K ohms so I threw away anyone with over 4000 ohm resistance. All of the real worn fine wire plugs had resistances of less than 1000 ohms even though the center electrode was severely worn. 12 old BG44 plugs that had about 800 hours on them all tested less than 1400 ohms.

I cut one massive spark plug apart with a band saw to see how they were put together. The resistor is about one-half inch long and had very high resistance even out of the plug. It is a mystery to me how these resistors can grow resistance over time. On Champion plugs, you can unscrew the aluminum cap inside the plug and the little spring and resistor will fall out. Unison plugs do not have a screw in theirs. Has anyone seen any literature from Champion on this resistance issue?

It is possible that some mag problems can be caused by the high resistance of spark plugs.

Below is a response I finally got from Champion. If what they say is true, maybe we will have more consistency in the future with these expensive plugs. "

'Mr. Welsh,

I apologize for the delay in getting back to you. There is no specific reason that the Fine Wire plugs should have any different resistance characteristics then the massive electrode plugs. It is more likely a resistor batch issue. We have seen batches of resistors that behave with very consistent resistance over long periods of times and batches that for some reason seem to climb rapidly. As I alluded to earlier we have a process improvement project underway that will likely allow Champion to manufacture a more consistent resistor initially and over its life. I know that this is not a specific answer but Champion is aware of the variation and is actively working to identify the cause and implement controls.

Thanks for your input. Its questions like yours that help keep us on the right track.

______________________

Adam McNine
Quality Assurance Manager
Champion Aerospace LLC
(864) 843-5353
(864) 843-5467 Fax'




-----------------------------------------------------------------
List archives located at: https://mail.dcsol.com/login
username "rebel" password "builder"
Unsubscribe: rebel-builders-unsubscribe@dcsol.com
List administrator: mike.davis@dcsol.com
-----------------------------------------------------------------