Spark plugs, part two

We’ve been analyzing spark plugs for a few months now. From this analysis, we’ve found a continuing problem with Champion brand spark plugs. Fine wire plugs seem prone to insulator cracking after a few hundred hours in service. At first, we believed the problem was confined to Tornado Alley Turbo equipped engines. That was probably because those planes fly a lot, so we see them at 100 hour intervals several times each year. TAT’s September 2011 service bulletin #SB11-05 calls for removal of Champion fine wire plugs from their engines.

See Part One of this blog article

Now we are seeing this same problem occurring in the lower compression TSIO-550-K engine in the 2011-2012 SR22T. Here is a set of Champion RHB32S plugs with 400 hours since new. You’ll note they have higher than normal resistance readings, and three of them have cracked insulators. Two of them are missing large pieces of insulator, which had to exit the cylinder via the exhaust valve.

Champion RHB32S spark plugs, one year old, 400 hours in service, SR22T

Note the un-even resistance readings, on spark plugs that are only 18 months old.  This engine was misfiring during climb, and occasionally during cruise. The number two top spark plug insulator is cracked, but hasn’t lost the loose piece yet.

Champion RHB32S fine wire spark plugs, 18 months old, 400 hours, Cirrus SR22T

The plugs with 484,000 and 668,000 ohm readings test fine on a bench tester, but cause random misfiring in the engine. It’s difficult to conduct lean of peak operations when the spark plugs mis-fire. That wastes a lot of fuel when you can’t run the engine properly. The two cracked plugs are missing large pieces of nose insulator. The hard ceramic has to depart the cylinder through the exhaust valve. Hopefully without causing any damage on the way…

Champion RHB32S spark plug, broken insulator, 18 months old, 400 hours, Cirrus SR22T

Here we have the number two cylinder, bottom spark plug. Note the large chunk of insulator that is gone!

So here we have the worst of both problems: broken insulators, and high resistance. The former can cause pre-ignition and engine damage, the latter can cause rough running and that sudden shake that really gets your attention in IMC, over water, or at night.

The next photo is of a 3 year old Champion RHB32S fine wire spark plug removed from a normally aspirated Cirrus SR22 with 675 total hours. Notice that the insulator is not just cracked, but shifted upwards. This is about to lose the entire half of insulator all the way into the spark plug core.

Champion RHB32S, 3 years old, 675 total hours, SR22 non-turbo

Moving along, let’s look at some nine year old, 1000 hour Champion fine wires removed from a normally aspirated Cirrus SR22. The electrodes are still quite serviceable, and should be, as these plugs should last 2000 hours. This customer had random CHT and EGT problems, particularly on his #2 cylinder. That cylinder’s EGT was running 200 degrees below normal, because both plugs were barely working.

Champion RHB32S spark plugs, 9 years old, 1000 hours, Cirrus SR22

Here you see that four plugs read “open” with a multi-meter. The others are 241,000 and 222,000 ohms, so the magneto has to work very hard to push any spark through these plugs. This engine was very hard to start.

Champion RHB32S spark plugs, nine years old, 1000 hours, Cirrus SR22

Five out of six of the bottom spark plugs read open. And the remaining one is 438,000 ohms. That’s a lot of spark energy lost to the resistor inside the plug. I’d prefer that the spark pass through to the electrodes, where it will do some good. Fortunately, none of these have cracked electrodes. But look at the deposits and oil in the next photo:

Champion RHB32S spark plugs, nine years old, 1000 hours in service, SR22

Here you can see the lead and oil deposits in the spark plugs. Fouled spark plugs cause cylinders to pump oil through the rings. Replacing these plugs restored the engine’s performance and cut the oil consumption back to normal.  Unfortunately, the standard response to plugs like this, would be to clean the plugs, bench test them, and put them back in the engine. When they foul again, you’d be blaming the cylinders and planning a top overhaul. After all, the bench “bomb tester” says they are good to go. But the center electrode resistance check reveals just how bad these plugs are. Throw them out, and install a new set of spark plugs! You’ll be glad you did.

Here is only a partial collection of defective spark plugs accumulated, since we started analyzing spark plugs just a few months ago. Several dozen were already disposed of before this photo was taken. Most of these plugs are less than 3 years old, but were removed due to cracking and mis-firing problems. Champion fine wire plugs were always promoted for their long life, but that is not the case with these plugs. At $85 each, you’re looking at $15,000 worth of spark plugs that didn’t make it past 700 hours (on average) in service.

It took only two months to accumulate a mountain of defective spark plugs

Stay tuned for more analysis as we report back on the engines running Tempest fine wire spark plugs. We’ll look at how the plugs are aging, and how the engine data compares since changing over. Based on initial reports, we’re expecting lower fuel and oil consumption, and more stable EGT and CHT readings.

There are two things we can learn from this problem:

1- A cracked insulator can develop at any time, and it can become a glow plug with out warning. If your CHT rapidly rises above 500 degrees, go full rich, retard power as much as possible, and land.

2- High resistance is not directly related to insulator cracking. It simply makes your engine run poorly, consume more fuel, and damages magnetos & ignition harnesses.

Here’s a great video on how Tempest makes their spark plugs, in the USA:

Part three to follow soon!

Part one is here:


This next set of plugs was removed yesterday from a 2008 Cirrus SR22, normally aspirated airplane with 777 hours since new. We see this plane for 50 and 100 hour inspections so we are very familiar with it. At the last 100 hour inspection at the end of January, the plugs were tested and three were replaced due to cracking. Now, 100 hours later, we have three more cracked. Even more noteworthy, one of the three new plugs already has 42,000 ohms of resistance.

All of these plugs tested below 5,000 ohms only 100 hours ago.

Champion RHB32S, 777 hours since new (2008)

Three of the top plugs are cracked. Two are missing large chunks, and one is broken but the piece hasn’t worked it’s way out yet. Only the wire is keeping it from being eaten by the cylinder.

Champion RHB32S, 777 hours since new (2008). Broken insulator ready to fall out.

It’s not the best situation when you can’t trust your spark plugs.

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13 Responses to Spark plugs, part two

  1. Trip Taylor says:


    Excellent blog and follow up on the first spark plug article. Thanks for the good “real world” knowledge and experience.


  2. Pingback: Spark plugs. Why you should check yours, today! | Platinum Aviation Blog

  3. Trip,

    We’ll be reporting back on how the engines are doing with these new spark plugs. So far, we’re seeing real improvements!

  4. Glenn,
    Your follow-through on this issue is a real service to the industry. Kudos to you, to Tornado Alley for pushing through to an AD on this, and for Cirrus Aircraft for supporting it with a Service Bulletin. I hope the light you are shedding on this issue moves Champion to take decisive action on this safety of flight issue.

  5. Peter H says:


    I have just tested 11 RHM38S iridium plugs which came out of a rebuilt IO540-C4 engine (not a Cirrus) in 2008 after ~700hrs and all but one measure several megohms. One measures 290k. That engine was running perfectly. The mag is a D3000 single shaft dual mag.

    I did some googling and cannot find any data on what the resistance of this plug (or any other similar plug) should be.

    The Cirrus plug is a longer reach type than the RHM38S. The insulator is probably a lot longer. Maybe this is why it is more prone to cracking?

    I have one old style Champion plug which measures 1.1k so clearly the internals are very different there.

    However, with all plugs but one measuring > 10M with a digital meter (I am an electronics engineer) I wonder if there is something very odd going on.


  6. Peter H says:

    Correction: just measured some more. Most are > 20M. One was found at 800k, and one at 2M.

    Where are the manufacturer figures?

  7. The resistor nominal value is 1100 ohms when new. They will vary from 800 to 1700 ohms, new. 5000 ohms is considered the upper limit by Tempest, and Champion says 5000 in some literature, and no limit in others. You can do the math and figure how much energy is absorbed by the resistor when there is 1 million ohms in the way!

    Look at the springs on the ignition leads, the corrosion you see is a product of high resistance. It is sparking there, because of the high resistance inside the plug. You’ll see this inside the magneto as well. Lycomings tend to run cooler CHT’s so the ignition doesn’t work as hard as a turbo Cirrus system does.

    If you have an engine analyzer, record data before and after changing the plugs. It will be noteworthy. Even though the engine seemed “perfect”, it will run much better when you remove the resistance bottleneck by installing new plugs. Especially at altitudes above 8000 feet.

  8. Peter H says:

    I’ve just re-measured all the plugs with a 1000V insulation tester and except one (which remains open) they read completely differently. None were over 1M.

    So clearly the “resistor” is not a resistor as we know them. Whether this is intentional, who knows? Maybe there is a tiny air gap at the ends, of the order of 0.01mm.

  9. Stephen Wilson says:

    I am an A&P/IA with 40 years in the business, 35 of those in General Aviation. I worked for years in a twin Cessna dealership, and saw a bunch of fine wire Champion plugs run 1000 hrs in those days. I am personally aware of a C421C that ran through two OH periods on one set of Champion Fine wire That’s over 3000 hrs, and that’s not hyperbole. (with the third set, the pilot, a very knowledgable old mossback Colonel, didn’t want to press the issue on the third shipset OH) That changed a couple of years ago, with the plugs failing, one after another, loosing insulator ceramic, cracking inside the barrel & so forth. Saw the same thing on a PA34 with new TCM remans, in this time period. Something changed in the design, or mfg of these plugs a few years ago. What was it? I’m looking at a set of RHB32S plugs, pulled from a TA Cirrus, with less then 100 hrs that 5 of 12 wouldn’t pass my elementary “bomb” test at 80 psig. This is the second set of Cirrus plugs I have swapped out in a year.

  10. Jan K. Thomsen says:

    Although not in the aviation business (I once was), a got triggered by Stephen Wilson’s comments. Please bear with me.
    “That something changed” he wrote. After my retirement I began playing with motorbikes again, which I hadn’t done for many years. My favorite for the moment is a Moto Guzzi, that has not been running very well lately.
    Today I checked the two spark plugs (again) and wondered why one insulator looked longer than the other. Big was my surprise when I found out that the insulator could slide up and down the center elektrode and actually close the electrode gap when the plug was installed in the engine. Obviously, the insulator did not suffer an axial crack like in the photos but, had cracked radially.
    I showed the plug to a friend of mine who has been doing motorbikes all the life. Neither of us has ever seen anything like this. And the plug (NGK) has only done about 10 hours of service.

  11. Thomas says:

    What a great blog. I have been chasing my tail for some time re an ever increasing miss problem in my Turbo Arrow IV. Had mags done, harnesses checked, P leads checked, ignition switch checked. Plugs tested OK on tester but obviously something wrong with them we found out. Lots of frustration! Been running Champion RHM38E’s for years. Have had that “shudder” many times which turned to finally an aborted T/O recently. In checking the plugs with the meter, 6 were open circuit, one was 13000 ohms, Five were around 5000 ohms. Over here in NZ these plugs cost around $NZ150 each!
    Now fitted a set of massive and runs like a dream.
    What have champion done!!!!!!

  12. Victor says:


    Are you or any of Platinum’s Techs going to be at the Cirrus Service Center symposium in two weeks?

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