When do spark plugs get changed?

AggieMike88

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The original "I don't know it all" of aviation.
Never thought to ask this question until today...

When are spark plugs changed on an aviation engine? Is it a tach time item? Annual item, or an IRAN item?

Are they changed as a set or individually?
 
They are normally changed as a set when they are worn out using a wear gauge. Some owners is a set number of hours. Individual plugs are replaced when they go bad. Plugs last different intervals in different engines.
 
Usually the a an p will look at all plugs during the annual, you can read the plug condition to tell you much about how that cylinder is working. Then clean them as needed, carefully with the abrasive, or just solvent, and test them by firing under air pressure, before reinstalling. You may find a plug that the electrodes are worn out, but plugs will go for hundreds of hours before wearing out.
Now if you came in with any problems running , it may be traceble to a particular cylinder or plug, so may need to replace that one. I have never heard of any tbo in hours or calendar listed for a plug. I have had platinum fine wire ones last 1000 hours.
Now if the engine was running fine,you can service and continue to use the plugs, good idea to rotate top to bottom etc.
Or, if the engine was running fine, but you have Champion plugs, and you are easily marketed to, you may buy a whole new set of other brand plugs to install. By the way fine wires cost twice as much and last twice as long esp for turbo engines up high.
 
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Never thought to ask this question until today...

When are spark plugs changed on an aviation engine? Is it a tach time item? Annual item, or an IRAN item?

Are they changed as a set or individually?

I run only massive electrodes and use an erosion gauge to judge that individually. Cheap to buy and keep in the toolbox:

12-00791.jpg
 
Agree with what others said above. Also check for physical damage and measure the resistance tip to electrode. There are lots on online video demonstrating how to do this and normal resistance ranges.
 
Mike Busch recommends "on condition" replacement.

https://www.aopa.org/news-and-media/all-news/2018/july/pilot/savvy-fire-in-the-hole

Apart from replacement, there's also cleaning, gapping, and rotation. Those things, according to that article, are traditionally done based on time (100 hour), but he would do that on condition as well, using a lean-of-peak mag check while in flight. (Maybe he's thinking of a plane that flies a lot more than 100 hours per year; he didn't mention that the plugs come out in an annual inspection, so they might as well be cleaned and gapped then.)


He has a video also, kinda long, so 1.5X playback speed might be good:

 
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I'm replacing all of mine now, to get a baseline. I have a 182H (1965) that I bought in the spring. I recently came in with some rough running issues, particularly on one mag that I couldn't clear @ run-up, thought might be fouled plug. A/P dug into it and evidently I have one mag with loose capacitor, and one coil was close to spec. Both mags were just overhauled at my annual (a different A/P) only 18 hours ago. Also my wiring harness is clearly very old with more than one frayed spots. My spark plugs were clearly a mix of brands, age, and condition. Decided to replace my wiring harness, and also replace all plugs to have a good base line starting point (while the old plugs were already all out). Did I have to replace them all, well no not really. but in my case I felt like it would be a good idea given "fresh mags", and new wiring harness.
 
I clean my plugs using a dental pick or similar to pick pout the lead balls but would never use an abrasive. Fine wire plugs are the only way to go and cost less in several ways on a per hour basis. First you never need to waste time re-gapping as the gap will barely increase over the life of the plug which is essentially until TBO and time must have some value. Second some dyno tests say fines increse BHP by 3% which should equal a 3% reduction in fuel consumption at the same power output. Do the math and the fines come out free compared to massives. If your fuel costs $60/hour in 2000 hours you will have saved $3600.
 
If you are running massives, Go-NoGo erosion gauge hole is 0.260" in diameter. If it fits over the electrodes when properly gapped the plugs are beyond acceptable wear.

Get a gapping tool too.
 
I clean my plugs using a dental pick or similar to pick pout the lead balls but would never use an abrasive. Fine wire plugs are the only way to go and cost less in several ways on a per hour basis. First you never need to waste time re-gapping as the gap will barely increase over the life of the plug which is essentially until TBO and time must have some value. Second some dyno tests say fines increse BHP by 3% which should equal a 3% reduction in fuel consumption at the same power output. Do the math and the fines come out free compared to massives. If your fuel costs $60/hour in 2000 hours you will have saved $3600.
If you need to rationalize using fine wire plugs as a gas saving method you're delusional. If you have plug fouling issues with massive plugs, then fine wires would help. You will spend three to four times the cost of massive plugs. Drop one fine wire plug and your "savings" will take many many hours before you ever break even. You will never notice a 3% increase in power even if there is one of which I am skeptical. Abrasives are fine with massive plugs as long as you don't go crazy and are meticulous about ensuring the plugs are clean of contaminants before installing. I, too, would avoid abrasive blasting fine wires.
 
Not having to re-gap fines pays for the additional cost since I am a very highly priced, brilliant engineer. I did not do the dyno test but if you can get more power by changing plugs then efficiency must rise. I had many boxes serviceable massives i threw away. I guess being delusional is a sign of my high intelligence since by placing a pad under my engine when removing and installing plugs prevents breakage. I have never broken a plug in almost 70 years of cleaning.
 
Not having to re-gap fines pays for the additional cost since I am a very highly priced, brilliant engineer. I did not do the dyno test but if you can get more power by changing plugs then efficiency must rise. I had many boxes serviceable massives i threw away. I guess being delusional is a sign of my high intelligence since by placing a pad under my engine when removing and installing plugs prevents breakage. I have never broken a plug in almost 70 years of cleaning.

Well, if you started your career removing and installing plugs at the age of 12, you'd be "almost" 82 years old now, so who's going to argue with that. :p

And the time savings not having to re-gap any plugs since you installed those first fine-wires in 1949 must surely have added up, I will agree. :rolleyes:
 
I recently talked to my mechanic about this and he said they just replace them on an as-needed basis when they are damaged/worn out.
 
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