The lowly spark plug is one of those aircraft parts we all take for granted. It’s not exciting like a new glass panel, and it doesn’t have the ramp appeal of a new four-blade prop. But when a spark plug fails, you’ve got some real trouble that focuses your attention on what matters most – a smooth running engine.
Do you have to lean aggressively to keep your spark plugs from fouling during taxi? Do you have persistent trouble with your pre-flight magneto check? Do you experience engine roughness or vibration during flight? Maybe the problem isn’t your leaning technique, or the magnetos. Maybe it’s your spark plugs!
Over the last few years, sophisticated engine monitoring has given us a wealth of information on the health of our engines. The data allows us to diagnose problems that used to go unresolved for far too long. Using this data, we are now able to pinpoint problems with spark plugs. These plugs seem to work fine in the shop’s bench tester, but give trouble when installed in the engine. At Platinum, we can analyze your engine data and determine if your engine can benefit from new spark plugs.
Spark plugs contain an internal resistor to absorb excess energy, which protects the electrodes from premature erosion. This resistor is subject to extreme heat from the cylinder head, and the stress of 25,000 volts of electricity passing through. If the resistance increases, the spark energy at the electrodes decreases. Resistors dissipate energy in the form of heat. This results in intermittent roughness, elevated cylinder head temperatures, and completely fouled spark plugs in some cases. So the ideal resistor must remain stable throughout the service life of the spark plug.
Let’s examine the two different styles of resistor. Champion uses a carbon-pile design. This resistor element is found inside the spark plug body, held under pressure by a spring, which is in turn secured by the screw.
As you can see in this photo of a two month old Champion RHB32S fine-wire spark plug with only 110 hours in service, there is corrosion on the conductive surfaces. When it was new two months ago, the resistance was normal, about 1500 ohms. But today, this spark plug’s resistance is over 12,000 ohms! That means some of the spark energy isn’t passing through to the electrodes. This spark plug was causing elevated CHT’s in the affected cylinder. Installing new spark plugs resolved the CHT problem, and stopped the intermittent vibration problems at low rpm.
The second plug is also a Champion RHB32S fine-wire spark plug. It is one year old, with 350 hours in service. It has over 35,000 ohms of resistance.
The third plug is a Champion REM38E massive electrode spark plug. It was installed in 2009, and has 450 hours in service. The resistance is a whopping 200,000 ohms! This plug was constantly fouling after landing, and also during taxi. Replacement with a Unison spark plug solved this persistent problem.
This next plug is another Champion REM38E, from the same engine as the previous plug. It was constantly causing trouble with both pre-flight and in-flight magneto checks. Here you’ll see it has an astonishing 1.024 megaohms of resistance- that’s over one million ohms!
The last plug is a Unison REM40E. Notice the corrosion-resistant nickel finish. This is highly beneficial in South Florida, where corrosion is a constant concern. This used plug has 1454 ohms of resistance, right in the middle of the “new” specification. We have several planes running these spark plugs, and even after several years and hundreds of hours, the resistance remains steady, between 1300 and 1500 ohms.
The Tempest (formerly Unison/Autolite) design spark plugs use a fired-in resistor. It’s completely encapsulated so corrosion is never a factor. This proprietary resistor remains stable during the life of the spark plug. I’ve personally run Unison spark plugs in the right engine of my Piper twin since 2002. That’s ten years and over 900 hours in service, with no change in internal resistance values, no fouling issues, and minimal wear of the electrodes. They still are within new resistance limits at 1450 ohms. Even more noteworthy, the massive electrodes are still within wear limits as well. I’ve rotated the plugs every 100 hours, and they’ve outlasted competing spark plugs by a two to one margin. (The Champion REM38E spark plugs shown here came from the left engine of my Piper twin.)
Here we test the actual resistors removed from two Champion RHB32S fine wire spark plugs. The first resistor has 15,300 ohms by itself. Once assembled in the spark plug, the assembly has 35,000 ohms total resistance.
Here is another Champion resistor that came out of a completely dead plug. The resistor has five million ohms of resistance!
Unfortunately, Champion doesn’t have any guidance regarding acceptable resistance values for their spark plugs. We do know that plugs with great enough resistance fail to work in the engine. On the other hand, Tempest has published a guide to evaluating spark plugs that can help solve the persistent CHT and fouling problems that you may be experiencing. (The publication is reproduced here, courtesy of Tempest.)
Recently, Tornado Alley Turbos published a service bulletin, SB11-05, calling for removal of Champion fine-wire spark plugs. TAT has experienced many failures of these spark plugs when installed in the turbocharged Cirrus SR22, among others. They are recommending removal of these plugs to head off potential engine failures due to detonation / pre-ignition. Here is the link to the service bulletin: http://www.taturbo.com/TATSR22-SB11-05%20fine%20wire%20spark%20plugs%20initial%20release%20sept%2023%202011.pdf
We don’t fully understand why spark plug insulators crack at this point in time. What we do know is once an insulator cracks, the spark plug may become a glow plug under the right conditions. This leads to destructive pre-ignition that can destroy a piston in a matter of minutes. This danger alone is reason to consider replacing your spark plugs today.
While TAT is only calling for removal of fine-wire Champion plugs due to cracking, we are also experiencing cracking of Champion massive electrode plugs. I’ve seen a recent failure that nearly caused a piston to fail. The aircraft was flying overwater, but fortunate to be close enough to land, so being able to reduce the power stopped the damage before the piston failed completely. It did melt the edges, and resulted in a cylinder replacement. The pilot saw an elevated CHT reading, just like the one depicted in the TAT service bulletin. The temperature rise is rapid, and left unchecked, may cause failure of the affected cylinder, resulting in engine failure in less than 10 minutes. Since many planes in South Florida operate over open water, this can be a serious safety of flight concern. Here is the spark plug, and the piston with melted edges:
- Champion RHM38E massive electrode spark plug, with cracked insulator
So if you’d like a smoother-running, more reliable engine, I would recommend installing all new spark plugs during your next maintenance visit. Tempest has taken the time-proven Autolite / Unison design and brought it back into production. Tempest spark plugs are available for all aircraft, in both fine-wire and massive electrode types. (Fine-wire types are considered to be 1-2% more fuel-efficient, and they last 2-3 times longer than massive electrode plugs. Plus they are highly resistant to fouling, offering lower total costs over their life span.)
Platinum has Tempest spark plugs in stock, and we can fine-tune your complete ignition system for maximum performance and fuel economy. They are in limited supply, so contact us to reserve a set for your airplane today.