Spark plugs. Why you should check yours, today!

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.

Champion RHB32S fine-wire spark plug, with resistor removed

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.

left-to-right: resistor, spring, retaining screw, note the corrosion on each part

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.

Champion RHB32S, one year old, 350 hours total, 35,000 ohms 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.

Champion REM38E, 3 years old, 450 total hours, 200,000 ohms resistance

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!

Champion REM38E, 3 years, 450 hours total, 1.024 megaohms resistance

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.

Champion resistor with 15,300 ohms

Here is another Champion resistor that came out of a completely dead plug. The resistor has five million ohms of resistance!

Champion resistor with 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.)

How Many Ohms of Resistance Are Your Spark Plugs Creating? What? Spark plugs deliver spark energy, they don’t create resistance to energy…do they? Well, they don’t exactly create it – or do they? Read on to find out why more and more A&P’s and IA’s are now checking the resistance as part of their scheduled spark plug maintenance. For the past two years Tempest has been educating the general aviation industry on the importance of checking the resistance of spark plugs. Here are some common myths and misconceptions about spark plugs and resistors: 1. Resistors are used in spark plugs to reduce radio noise. False - resistors, although they may help with radio noise reduction, are used primarily to reduce the electrode erosion effects caused by capacitance after-fire. This is a known after-firing of the plug caused by the residual energy built up through the harness leads and magnetos. 2. Spark plugs are considered in good condition if they spark in a tester. False - Bomb or bench testers can not adequately simulate the conditions of the engine cylinder, altitude and condition of the magneto. It is a fact that plugs with high resistance have tested as “good” in a bench test environment. 3. The resistance in the spark plug doesn’t matter, because my magneto has the ability to put out more energy than is required. False - The typical aircraft magneto will put out about 20 - 25kV. Let’s go back to science class for a moment and revisit Ohm’s Law. It takes 1 Volt to push 1 Amp through 1 Ohm of resistance. Based on Ohm’s Law a magneto can not deliver adequate energy to the spark plug if the system resistance is above 20-25kΩ. Tempest recommends any spark plug over 5kΩ needs to be replaced with a new plug. Experience suggests that any resistance over 5kΩ ohms causes the voltage to bleed through the path of least resistance rather than ionize the gap. 4. All aviation spark plugs are made the same, therefore there is no difference in resistance. False - Tempest uses a 21st century proprietary FISS resistor design which results in consistent resistance values of 1 – 2kΩ. Our competitor uses an old style stack up design which can cause extremely high and inconsistent values, resulting in misfiring and a rough running engine.