Teach me about airplane spark plug wear

First annual with new to me 182 with a carbureted o-520 and 7 out of 12 spark plugs were replaced due to wear beyond spec (pre-buy last october did not find or mention this as a potential issue and they were looked at during pre-buy). I have worked on many auto/small/farm engines and know how those spark plugs work. I know airplane spark plugs are different but I don't know how they are different. How are these different than say an auto plug? What is being worn out on them (electrode, ground bar, something else)? Is there anything I can do to prevent excessive wear (run ROP instead of LOP, keep EGT below 1400, run at 65% power instead of more/less, etc). Just looking for knowledge to prevent future surprises and to keep a healthy engine/plane in as best of shape as I can. Thank you

 

Plug wear is mostly erosion caused by the spark. The direction of spark travel determines what erodes--the center electrode or side electrodes. The magneto generates an alternating current, so the spark is positive to one plug, negative to the next one, the third positive, and so on. So we rotate the plugs when we clean them; top to bottom, one cylinder to another, so that the erosion doesn't eat up all of one side or the other.

Automobile plugs used to wear out just like this, too. Then automakers started putting in platinum plugs and now they last way longer. You can get similar plugs for your 520, they just cost a lot more.

Go here: http://www.championaerospace.com/wp-content/uploads/2015/06/AV6-R-Aug20141.pdf

 

The AV6-R guide Dan Thomas linked above is a good reference. TCM also has a Service Information Letter SIL03-2C on the subject.

http://www.david.it/it/area-riservata/documenti/continental/service-information-letters/SIL03-2C.pdf

Pay attention to the plug rotation diagram/table in these references as some owners just swap the plugs top and bottom on the same cylinder, and that will not give you the same plug life (assuming all other things are equal) as following the rotation pattern in the documents.

Champion (and probably some others) makes a go/no-go gauge to test spark plug wear (massive spark plugs). It's not an expensive accessory ($4.35) in the tool box if you do your own spark plugs.
http://www.aircraftspruce.com/catalog/topages/ct482gauge.php?clickkey=674167

A pre-purchase inspection is not the same as maintenance. Unless you specified the plugs be cleaned, regapped and checked for wear I suspect the reason the plugs were pulled for inspection during the pre-purchase was solely as one information source regarding the health of the engine. If you flew the airplane from then until now, and didn't notice any problems, then continuing to run on those plugs for another 6 or 8 months seems a reasonable and economical decision.

 

Thank you everyone for this info. It looks like a tool will be purchased and next plugs will be a bit more expensive but last longer.

 

Does anyone have a favorite or suggested interval for spark plug rotation? 100 hr, or at every annual?

 

Rotate the plugs whenever they're out. That's usually at 100 hours. Some engines, especially those used in flight training, will foul their plugs more often (circuit work and so on) and will sometimes need cleaning at 50-hour intervals. We had an O-235 in a Citabria that needed the plugs cleaned at 25 hours until i finally put REM37BY plugs in it; they're very foul-resistant and I like to put them in any engine that qualifies for them whenever we replace a whole set.

 

Thanks! 100 hours it is.

 

Don't drop a spark plug, especially on concrete, the insulator can be cracked and you won't know it till later when you're trying to figure out why the engine is idling rough. The main difference in automotive and aircraft spark plugs, I believe, is the insulation. A spark can jump further, the thinner the air.

 

Had an IA once that said " you can drop a spark plug twice.........Once on the floor and once in the trash" ;-)

 

How does mixture have any thing to do with plug wear?

 

Plugs are changed when the nose of the plug will fit into the tool that you use to measure them, time, has no factor. Champion says that their plugs are designed to run 500 hours

https://www.aircraftspruce.com/catalog/topages/ct482gauge.php

 

Good info. We were talking about the rotation interval, though. I'll use the plugs as long as they're in spec.

 

The nose of the plug will loose a molecule of metal every time the plug fires, thus reducing the size of the nose of the plug, no matter which side of the electrodes lose that molecule. so, why rotate?

 

you did not tell how. just more " it does it" when it really doesn't.

 

It's an old tale, electricity takes the path of least resistance so as the plug wears it will just continue to wear in the same area. There's a reason it's wearing in the spot it is.

 

resistance does not change just because of the polarity changes

 

Page 6 of that Champion manual I posted talks about it. We've been over this before. When a plug is fired positive, the ground electrode suffers. When fired negative, the center electrode suffers. A magneto generates an alternating current as it rotates, which causes, as I said before, alternate plugs to fire with opposite polarities. So if we don't rotate the plugs, we'll have one electrode totally shot and the other still healthy, but the plug will be worn out in half the time that it would have if we'd rotated.

Last paragraph of this: http://okigihan.blogspot.ca/p/spark-plug-inspection-and-maintenance.html

And a quote from this: http://www.aviationpros.com/article/10381828/aviation-spark-plugs

"Spark plugs should be rotated periodically to even out the electrode wear," Hansen notes. "Magnetos fire the spark plugs with negative and positive charges. The negative charge will wear the center electrode and a positive charge will wear the ground electrode. Just like rotating your car's tires, rotating your spark plugs will make them last longer by using up both electrodes."

 

What Dan said. Also, a longer spark plug lead with greater capacitance will wear the center electrode more, so, short of having all your leads the same length, that's another reason.

 

Nope, because you will close the gap each time you clean them. when the tool fits over the nose of the plug by using one or the other or both, it will be worn out. xx,0000 billion sparks the molecules are gone doesn't matter where they came from

there is only one reason to rotate spark plugs, and that wasn't it.

 

My vote's with Dan on that one Tom.

The other reason has been previously mentioned - the capacitance differences due to differing ignition lead length.

 

The real reason you rotate plugs.

at any given time half your plugs are firing negative the other half are firing positive. What happens to the plugs that are having the molecule removed from the minus electrode?
A... It will get smaller, when it gets to a point that it can no longer dissipate its heat, what happens?
A... It turns into a glow plug.

So,, you want to use up the minus electrodes on all the plugs evenly.

 

Seems to me that's what we've been saying.

 

Here is a question for ya..
Why doesn't the radial engines need to have their plugs rotated ever?

 

Then you should have been more explicit. you said you should, but never why.

didn't know there was any election.

 

Another vote for Dan. Dan 2016!!!

 

I've always believed the polarity difference in each plug position causes the wear that eventually erodes the particular electrode. Rotating plugs evens out the wear caused by the spark. Poor leaning may accelerate the wear because the plugs may have to be pulled more often and mechanically cleaned. Depending on how the cleaning is accomplished, abrasive blasting for instance, the plug may have more material removed and thus its service life is decreased.

 

This is true when you run leaded fuel.

 

The typical sparkplug cleaner uses silicon carbide grit. Very sharp, very hard, tends to erode the plug. Glass bead works well and does much less damage, but one has to make sure there's none trapped in the plug. It's harder to see. If either of those grits falls into a cylinder, especially the carbide, terrific scoring results.

 

It must be a miniscule amount of capacitance, hardly mentioned in textbooks.