AircraftOwner Online

Ignition System Maintenance

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 certainly doesn’t mean that the annual inspection is justification for doing all repairs on an annual basis.

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 plugs don’t get pulled and there is no opportunity for inspection of how the overall system is functioning.

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 an 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 upwards of $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.  When new, aviation spark plugs have a resistance of about 1,000-1,200 Ohms.  However, this resistance climbs as the plugs get older in service.  Champion and Unison will not release official values for a bad plug resistance, but I was told unofficially that any plug showing 20K Ohms or more should really be scrapped.

That bad plug even looked different from the other seven when examining the electrodes.  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 before its next annual when the magneto failed.

Preventive maintenance is always the key to safe and sensible aircraft operations.  Spark plugs, oil, filters, etc. are all cheap in comparison to the damage that can occur when problems go unchecked!