Are dual mags reliable?

I own a Cessna R-182 that has a Lycoming O-540 J3C5D engine. It seems like a great engine and runs well. I have a concern about the “”dual-pack”” type magnetos that run off a common gear. Yesterday, another pilot told me of an engine failure that he had years ago in a Cherokee Six where the common magneto gear on that engine failed, causing him to lose both mags and forcing him to make a deadstick landing. He felt that it was a rare event for that gear to shear but, maybe on second thought, not so rare.

I obviously have a concern that I’m flying an airplane that has a dual pack mag, which is presumably less reliable than an engine with two completely separate mags.

Can you shed any light on the reliability of my O-540 regarding the mag issue? I know there are thousands of engines out there with this type of mag system, but I would like to find out through any statistical data or from someone like yourself if the dual pack mag on my engine is something I should be concerned about. Finally, why did Lycoming choose to do it that way in the first place? Thank you very much for any insight that you can provide.

Brad Miller

via email

Thanks for letting us know of your concern regarding the Lycoming O-540-J3C5D in your Cessna R-182 aircraft, which utilizes the Bendix Dual Magneto.

Trust me when I say you’re not the first person to share concern for this configuration. Even as a long time employee of Lycoming I must confess when this technology was first being developed I, too, was somewhat skeptical, but as time moved forward my skepticism was diminished by the reliability of this system.

Now don’t get ahead of me on this, but as things unfolded there were problems and concerns, but nothing out of the ordinary for something new being introduced into the marketplace.

From my experience I’d say the biggest concern regarding it was the fact that maintenance facilities and/or maintenance technicians failed to educate themselves on the entire system. For some reason there was a predetermined mindset that this system was no good, mainly driven by the same comment you mentioned, based on the fact that it is driven off of a common gear.

Let’s stop right there. Isn’t it a fact that all Lycoming engines have only one crankshaft gear driving the idler gears in the accessory housing? Therefore the engines utilizing two individual magnetos are really driven off only one common gear too, which is the crankshaft gear.

I’d like to back up a bit and explain my comment about the industry in general not being educated on this system. I always compare this to the time back in the early 1960s when fuel injection was introduced on Lycoming engines. These, too, were looked upon with jaundiced eyes by most maintenance facilities and technicians in the field because it was something they were not familiar with. I guess you could say that the fear of the unknown was the culprit here and I feel the dual magneto had the same impact for quite some time in our industry. For some reason our industry was slow to take up that information, which caused all kinds of bad rumors regarding this system and its reliability, most of which were unfounded.

However, as time went by and field personnel caught up and became familiar with these systems, things changed and the systems were accepted.

From what I can recall, this type of technology was driven by several factors that Lycoming took into consideration before moving towards certification. One was cost of manufacturing, i.e. less machining during the manufacturing process. Of course another was cost of components. There also was a weight advantage, which all airframe manufacturers seek religiously from engine and other component manufacturers supplying products for their aircraft.

I’ll go out on a limb here and state that “”if”” your dual mag system was maintained in accordance with the manufacturer’s maintenance requirements, this system is as good as any in the industry. So if you know for a fact that your mag has had the proper care according to the Bendix maintenance manuals and all Service Bulletins, AD Notes, etc., complied with, there should be no concern on your behalf regarding its reliability.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:


  1. James Tiner says

    The mag drive failed on my purchased new 1977 Piper Lance with around 200 hours on it. Lucky for me VFR mid day and I was able to get it down. After researching and learning this was a known problem with AD’s out on it, I sold the plane. Not sure the cause, but the buyer died in a crash about a month after purchasing the plane from me.

Leave a Reply

Your email address will not be published. Required fields are marked *