Inspecting the magneto coil

I received this email inquiry:

John, Recently we have found, during a 500hr Slick Mag Inspections, defective coils. The engines in both of these cases were running OK when brought in for the inspection. When coils were tested found primary coil to be within tolerances but the secondary coil showed open resistance. In both cases we replaced coils. We just want to understand why the mags were still operating so we can give an "intelligent" answer to the customer as to why we had to replace coils when all seemed to be operating fine.

Cross-section of Slick coil. Large primary windings next to core. Smaller secondary windings on outside.

Interesting question as sometimes inspection standards seem arbitrary without an explanation of what their intended purpose. by "open resistance' I assume that you had infinite resistance or a open secondary circuit.

Your ohm meter does its check by flowing a tiny bit of current through the secondary wire at a fraction of a volt. Any broken wire will stop the current flow and indicate an open circuit. However, when the engine is operating the voltage in the secondary wires is high enough to jump a spark plug gap so it will easily jump across a small break in the secondary wire inside the coil and the magneto operates fine - for awhile.

The arching inside the coil causes the coil to get hot. On a high-voltage coil tester when you pick the coil up it's like a hot potato! The burning inside the coil starts to melt insulation and burn the secondary wires. Gradually the number of secondary windings reduces as the current shorts across adjacent wires. Now the ratio of primary windings to secondary windings is reduced so the transformer effect of stepping-up of voltage is reduced.

The first indication for the pilot is hard starting; at the slow rotation speed during start there is not enough voltage to spark the plug. Get it started and it runs fine for awhile as the high rpm creates enough current in the primary to off-set the degradation in the secondary. Eventually, the burning inside the coil reaches the outside by burning a hole out through one end of the coil.

It is amazing when you see this because you know that the aircraft was operated with all that internal coil damage. Eventually, enough secondary wires are damaged that the magneto won't produce enough voltage to fire the plugs. At that point the aircraft is grounded in some hole-in-the-wall airport with an angry wife and screaming kids.

Checking secondary resistance on a Bendix S-1200 series magneto

Editorial on inspections: The I-35W bridge that collapsed into the Mississippi river and killed 13 people was operating fine the moment before it collapsed. It had failed previous inspections and was rated as "structurally deficient". These inspections were ignored because it was operating fine. Operating fine - don't fix it or "if its not broke don't fix it - kills innocents