132 – Real Inflight Magneto Failures & Troubleshooting

Airplane Owner Maintenance - By Dean Showalter

Note: scroll to the bottom of this post for a special announcement.

Have you ever experienced an inflight magneto failure? If not, you might some day… but if you take action on the recommendation in today’s episode, you can dramatically reduce the likelihood of that happening.

The left mag on this Cessna 172RG failed in flight, and after opening the mag, it became obvious what happened.

The finger electrode that is part of the distributor gear inside the magneto, had gotten loose from its mounting position, and was able to rotate freely 180 degrees, while still occasionally resting in its proper position, which made it somewhat intermittent. Take a look:

The finger electrode should stay firmly in it’s mounting point as in the following picture:

The LAA, or Light Aircraft Association, put out an excellent, one page Airworthiness Alert February 21, 2019, about this very issue. Here’s a link for that document:

http://www.lightaircraftassociation.co.uk/engineering/Slick%20Magneto.pdf

That document also has a link to the Slick SB1-15A that also addresses the issue. Here’s that link:

http://www.lightaircraftassociation.co.uk/engineering/SLICK.pdf

The bulletin recommends changing the copper electrode in certain Slick magnetos to a monel electrode, which is much stronger. This is definitely a worthwhile service bulletin to comply with for the affected Slick mags.

Now back to the Cessna 172RG… it also needed a new ignition switch kit. The Gerdes ignition switch is subject to AD 93-05-06 which requires inspection and lubrication each 2000 hours, although I sometimes wonder if this interval is frequent enough. Take a look at the carbon tracking and wear inside the ignition switch:

This condition can cause the switch to malfunction… it’s easily remedied by installing an A-3650-2 Ignition Switch Kit, which includes replacement of this contact plate, as well as the 3 contact cups in the switch housing. Notice the difference between the old cups and the new cups… any contacts that have the silver plating worn off, must be replaced… most of the switches I’ve inspected, have needed a new kit.

Replacing the switch plate is fairly simple, but slightly tedious, by transferring one wire at a time, from the old switch plate, to the new one:

The second story we talked about in today’s episode is about an inflight magneto failure in an Aerostar. The pilot did some excellent inflight troubleshooting, and landed safely after determining the right engine left mag had failed, and was able to fly to a safe destination with the right engine selected on right mag only.

So after today’s episode, I have several tips about magneto troubleshooting:

• When you select one mag, if the engine immediately runs rough, it’s most likely a spark plug issue.
• When you select one mag, if it’s a more smooth, but large RPM drop, it may be a magneto problem.
• When you select one mag, if the engine dies immediately, it might be a mag, or a shorted P-lead, or the ignition switch, (the switch being the most unlikely, in my opinion.)
• Causes of ignition system troubles, in the order of likelihood:
  • Spark plug issues.
  • Magneto issues.
  • P-lead wiring.
  • Ignition lead, or corrosion in magneto attachment.
  • Ignition switch.

In fact, I seriously wonder if both issues talked about in today’s episode could have been avoided. I don’t have information on the 172RG’s magneto, but the Aerostar’s failed magneto had about 1000 hours since overhaul.

Find a reputable magneto shop, and keep your mags in tip-top condition.

My favorite magneto shop to recommend is Aircraft Ignition Services LLC, operated by Kevin and Leah Herrington in Honey Grove, Texas. They’ve been doing excellent work for us at Classic Aviation LLC. Check out their website:

http://aircraftignitionservices.com/

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Announcement: Carl Valeri is the host of the Aviation Careers Podcast and the Stuck Mic AvCast. He recently interviewed me for an episode of each of these shows (ACP255 and SMAC229), and it was so inspiring to talk with him. You can find these here:

http://www.aviationcareerspodcast.com/

http://stuckmicavcast.com/

I highly recommend the podcasts, career coaching, scholarships guide, and other resources Carl provides. Thanks Carl!