The Annual Inspection and The Bad Magneto

Let's cut to the chase.  The bill for my last annual inspection was $1800.  Yes, it was a bit of a shocker since I was used to paying less than half that amount.

The unexpected issue that came up had to do with a bad magneto that was discovered after the annual was complete.  The bill for repair, including re and re was around $600.  So the annual ran about $1200 which was what I was expecting now that I was using a new AME at Edenvale.  But I could not help but sigh--the halcyon days of cheap annuals were now long gone.

Here's what happened:

Once the annual was complete, we pushed the plane outside and I did a run up.  Everything seemed to be working ok, including the mag drop for both mags--less than 50 rpm for each.  The magneto timing is checked at each annual so its important to validate each mag is working ok before you go flying.

As I was shutting down, I checked the mags at idle speed of 1000 rpm just to make sure everything was ok.  Usually, you get an rpm drop but the engine contines to run, albeit a bit roughly on the active mag.  The left mag was fine, but when I isolated the right mag, the engine died right out.  Which was a bit puzzling.  I ran them up again at 1700 rpm.  Both were fine.  But at 1000 rpm, the engine continued to run on the left, but wouldn't on the right.

I shut down and had a chat with Chris.  He said it was probably a fouled plug.  We didn't have time to pull things apart that late in the day, but he said he would come over to my hangar the next time I was on the field and have a look.  I said ok, but was a bit unhappy in that I am perfectly capable of checking the spark plugs myself but with Chris doing it, the meter was running at $85/hour.  But since it was still part of the annual, I figured it was better letting Chris do the work.

The next day we met at my hangar.  I  fired up the plane with the cowl off and Chris went around and tested the heat coming off each cylinder exhaust with a special tester.  If a plug is fouled, the temp of the offending exhaust is usually lower.  That way, you don't have to pull all 8 spark plugs out and test each one.  But they were all ok (which made sense since we had cleaned and gapped them during the annual)

Chris then thought it might be a bad ignition lead (which we didn't test during the annual as there is usually no need).  So I taxied the plane over to his hangar and he tested all the leads--no joy.  They were all fine.  We scratched our heads and decided to pull all the plugs anyway, just to make sure they were ok. We did and they were.

Therefore, by simple deduction, it had to be the magneto since the other components of the ignition system were ok.  So Chris pulled the mag and we arranged to have it sent to an overhaul shop in Orillia.  Just to make sure, he had a spare mag in his shop, so he installed that, timed it up and I did a runup.  It worked perfectly, no engine cut out on the right side.  So it was nice to have the confirmation that we were on the right track.  And I could still use the plane while the mag was being repaired.

A few days later, I flew to Orillia to pick up the overhauled mag.  The tech had found a burned out condenser (I think it was), replaced it and it was good to go.  $475 worth of good to go.  Ouch!

Chris had it installed on the plane the same day and took back his loaner mag.  We updated the logbooks and the annual was finally complete.

Looking back on the process, I had a few observations.

First, the magnetos had about 500 hours on them.  They were installed new with the major engine overhaul done a couple of years ago.  Slick, the manufacturer recommends the mags be overhauled every 500 hours.  Yeah, I can see why.  On the other hand, many mechanics say there is no need to overhaul the mags that frequently.  The argument is since there are two, even if one fails in flight, you can still get home.  My old mechanic Frank said he had seen mags run to TBO or 2000 hours on some engines.  Maybe they don't make them like they used to.

My sense, after overhauling quite a few mags over the years, is to do them at 750 hours.  That's when I'll do the left one.

My second observation:  while the inspection was generally as advertised, I was not all that surprised that something came up that wasn't expected.  It's complicated machinery.  Stuff happens.  Still, after 26 years of doing annuals, I've found that most are a non-event, especially if the small stuff is being looked after throughout the year and you keep a careful eye on things.  But looking back, it seems that about every 4 years or so, you hit a big one that's requires more $$.  Hopefully, not $$$$$$.

To this point, other than 2 scheduled engine overhauls in over 5000 hours of flying, I've never had the dreaded big one.  Hopefully, never will.