Stay ahead of your spark plugs

One of my pet peeves is the concept of “annual” aircraft maintenance. While the FAA has certainly mandated that our aircraft are inspected annually, that doesn’t mean it’s smart to wait an entire year before inspecting or maintaining your aircraft.

A case in point is the aircraft’s ignition system. The spark plugs, harness, and magneto components all continue wear during the time between annual inspections. Depending on the number of hours that you fly each year, this can be either a very small change, or a rather large one. For most aircraft owners that I speak to, the annual inspection is the only time during the year that they remove their spark plugs for cleaning and inspection. Unless forced to by some ignition issue, such as a fouled plug, the spark plugs don’t get pulled until the annual inspection.

That’s too bad, because some issues can progress without outward symptoms until it’s too late. I recently came across a very compelling example of this when working on the ignition system of a Grumman AA-5 Traveler. The engine had begun running rough, with ignition "misses" fairly frequently.

The magnetos were disassembled and inspected, revealing a burned distributor cap and rotor on one of them. The burning was focused on a single contact in the block. The damaged magneto parts were replaced, which is an expensive proposition. You can replace the cap and rotor on your car for less than $10. Those similar parts in a Slick magneto cost more than $350.

Before calling the problem “solved,” I investigated further to try to track down the origin of the failure. It turned out to be a bad spark plug. Believe it or not, a bad plug with excessively high resistance can cause the magneto to fail as well. New aviation spark plugs have a resistance of about 1,000 to 1,200 ohms. However, this resistance climbs as the plugs get older in service. According to Tempest, any spark plug showing a resistance of greater than 5,000 ohms should be replaced. You can use a good multi-meter to test the resistance between the center electrode and the conductor in the spark plug barrel or Tempest makes a dedicated spark plug tester called the AT5K for a little over $100.

When the plug was pulled on that Grumman, it was fairly obvious that there was an issue because the bad plug looked visibly different than the other seven. It obviously had not been firing well and this might have been noticed if the plugs were examined more than once a year. This aircraft was only a week short of its next annual when the magneto failed.

As the aircraft owner or operator, the airworthiness of the aircraft is your responsibility, and it only makes good sense to inspect your ignition system on a regular basis. It’s simple for an owner to do and definitely one of the best ways to monitor the health of your engine, taking care of small issues before they become big ones.

Next time, we’ll get our hands dirty with some little DIY sparkplug maintenance. Until then, happy flying!

Jeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Jeff is also the creator of SocialFlight, the free mobile app and website that maps over 5,000 aviation events. Free apps available for iPhone, iPad and Android, and on the web at www.SocialFlight.com.