Is One and Half Mags Enough?

I have logged at least 1,500 hours flying behind one of those combined magnetos, and I have to say, I wondered about the concept, especially when the clouds were on the ground or it was a dark night. Are two magnetos in one box really enough?

The magneto I’m talking about has a single drive from the engine, but contains two totally independent mags in a single unit. The most popular of the series, as far as I know, is the Bendix 3000 family of dual mags.

The single drive dual mag is used mostly on the Lycoming four-cylinder 360 engines, but it is also used on others. The reason for the design is to overcome a shortage of accessory drive pads on the rear of the engine.

Aircraft piston engines need to power several accessories, including magnetos for ignition, sometimes fuel pumps for fuel injection, vacuum pumps for instruments and to inflate deice boots, and alternators.

Within the engine is a set of gears rotated by the crankshaft. The gears typically rotate the camshaft to open and close the exhaust and intake valves in the cylinder heads. And the same gear train in most engines powers the accessories, including the mags.

I understand why a magneto is called an accessory because it is not in the core of the engine like a crankshaft or piston. The mag is bolted onto the outside of the engine. But the word accessory implies that the mag is an option, like adding chrome wheels to your new car. Now that’s an accessory. But a mag is absolutely essential to operation of the engine. That’s why we have two mags because the engine won’t run without at least one providing the spark for ignition.

So, does putting both mags in a single unit and driving that unit with a single shaft from the engine offer the same redundancy as two independent mags? At first, the answer would appear to be no. If that common drive coupling to the engine fails, both mags cease to product spark and the engine quits.

But if you think about that single-point failure potential a little more, the single drive dual mag doesn’t seem so bad. After all, there is only one gear train in the engine to drive mags no matter how many mags there are. If those gears fail, all mags on the engine do too. And essentially all magneto problems involve the components inside the mag, not the drive coupling to the engine. The dual mag has independent points, rotor, coil and so on making the operating components redundant.

Most of my hours flying with the dual mag were in Mooney 201 airplanes powered by the Lycoming IO-360 fuel injected engine. The dual mag never let me down, though it seemed to be harder for the mechanics to time correctly, and it also seemed to need overhaul more frequently than conventional single mags. But those are only recollections with no real substantive data to back them up.

In looking at more than 30 years worth of NTSB accident reports I haven’t seen any pattern of more frequent failure of the dual mag leading to an accident. Of course, if the mag failed and the pilot landed successfully without a lot of damage the mag failure would never make it into an NTSB report because by NTSB definition there was no accident.

But the other day I came across the report of a fatal accident in a Mooney 201 where the NTSB determined that the dual mag had failed leading to a loss of engine power. The Mooney was flying over the inhospitable terrain of a dry lake bed with jagged salt deposits and deep crevasses that ripped into the airplane on landing making the crash unsurvivable.

NTSB investigators found that the dual mag on the Mooney engine performed normally when tested after the accident, but also found evidence that the mag rotated freely in its mounting to the rear of the engine. Magnetos are held to the engine case by clamps. Mechanics loosen the clamps to rotate the magneto so its spark is timed to the correct angle of the crankshaft. Once the timing is set the clamps are tightened to hold the mag in place.

The NTSB could not determine why the mag in the crashed Mooney was loose because all of the proper hardware was still in place. But when the mag rotated out of position it would have changed the spark timing disrupting engine operation, making the engine run rough, and eventually lose power.

The clamps holding any mag could come loose so that situation is not unique to a dual mag. But if one of the two mags usually found on an engine were to come loose and rotate out of proper timing position the engine would run rough because of the mistimed spark. What step does every engine roughness checklist include? Check each mag, right? If one of two mags has come loose and slipped out of time the engine will run smoothly when you turn off that mag. With only one mag functioning the engine won’t produce quite full rated power, but will run perfectly fine to fly you to a runway.

So all my worry about gears grinding up, or that common single drive shaft failing in the dual mag were misplaced. It’s the possibility of the mag clamps coming loose and the mag rotating out of timing that can get you.

This entry was posted in Mac Clellan's Left Seat Blog. Bookmark the permalink.

22 Responses to Is One and Half Mags Enough?

  1. Kayak Jack says:

    If there were two, separate drives, one for each mag, then there would be twice the likelihood of a drive gear-train failure. And, twice the chance of broken parts circulating around inside the engine. Just like there’s twice the chance of an engine failure on a twin versus a single engine.

    On clamps, etc., I’m a strong believer in both safety wire, and in castellated nuts and cotter pins. I’m not a strong believer in nuts with built in friction tabs, such as Ny-Lock nuts. I have more confidence in a nut and lock washer – either a split ring or star – than a Ny-lock nut. NEW Ny-lock nuts are OK. Trouble is, there’s no reliable way of knowing how many times it has been reused, or how badly it has been worn.

  2. Mark Shanahan says:

    My one engine failure… an O-360 with the dual mag. The single bearing supporting the drive shaft began to fail and overheated. The hot cam melted the followers so the points wouldn’t open. Fortunately, there was a useable farmers field nearby.

    Mac is right about the lack of data on failures. My incident isn’t in the NTSB database, but it should have generated and Service Difficulty Report (SDR).

    My lesson learned – know your ignition system! My early training on mags led me to believe that all aircraft engines would have two separate and independent magnetos. The dual mag doesn’t meet that standard, and isn’t always clearly identified in the POH.

  3. The Bendix D-2000/D-3000 dual mag definitely offers less redundancy and more possibilities for common-mode failures than two conventional magnetos. Your example of loose clamps is a perfect example.

    But you failed to mention the biggest problem with these dual mags: They have become orphans! Consider:

    – Bendix is now owned by Continental Motors Inc.

    – The only engines that use the D-2000/D-3000 dual mags are Lycomings.

    – CMI announced more than a year ago that they would no longer manufacture these mags, nor supply replacement parts for them.

    What’s wrong with this picture?

    There are third-party aftermarket sources for many of the D-2000/D-3000 magneto components (such as breaker points and condensers and bearings), but if you develop a cracked case or a bad rotating magnet assembly or a bad impulse coupling spring, you’re out of luck because those parts are now unobtainable. And they aren’t making any new D-2000/D-3000 mags. So eventually anyone who depends on these dual mags is going to be in a world of hurt.

    Lycoming engines whose model designation ends with a “D” suffix require the dual mag. Most of these engines (but not all) can be converted to use conventional mags by replacing the accessory case, but this is costly and is the sort of thing that can only be done practically at major overhaul.

    Incidentally, the magneto isn’t really an “accessory.” (The proper FAA word is actually “appliance.” The term “appliance” is defined in FAR 1.1. as a component that is part of or attached to an aircraft but is not part of the airframe, engine or propeller.)

    The magneto is an engine part, and is listed as required on the engine’s Type Certificate Data Sheet. It’s no more an “accessory” than a spark plug or ignition harness, because it’s a required engine component and the engine won’t run without it.

    That’s as opposed to appliances such as an alternator or vacuum pump or hydraulic pump, which are not engine components and are not required by the engine’s type design.

  4. SkyGuy says:

    If you have a problem away from your mechanic….you may not find a large supply of knowledable mechanics on this mag. And parts can also be a problem.

    As a working A&P…..I would not buy an aircraft equipped with that mag.

  5. David Light says:

    From an outsider looking in, the point is that this is a single unit that can fail. If there are two separate units and one burns out or fails, the likelihood of the other getting you safely on the ground is greater if they are independent of each other. Yes, that is twice the upkeep etc. I have a nice GPS but I also have a paper chart and a flashlight.

  6. Pingback: 1.5 Magnetos is not quite 2. | High Altitude Flying Club

  7. Earl Turne says:

    Engine parts for which there are only one of (for example the crankshaft) are typically built strong enough that they rarely fail. Things that have a tendency to quit (magnetos, fuel pumps etc.) are usually installed in pairs for redundency. You would hope that the common elements on a single-drive, dual mag would fall into the first category, and that is likely the case on most of them, but some of those suckers are driven by an impulse coupling, which I think is a bit of a mickey-mouse thing to be counting on. We had a rash of impulse couplings wear out pre-maturely a few years back that, had they been installed on a single-drive set-up, would have caused engine failures. In each case, however they only affected the left mag. – not both mags.

    Now, one of the airplanes I fly has the single-drive, dual mag, but it has two engines. Does that mean it has 3 mags in total?

    Just one more thing to worry about, I guess.

  8. Brent says:

    I buddy of mine has a Mooney and when he did his overhaul, he switched to a more conventional dual mag setup. He just never felt comfortable with it.

  9. Jethro says:

    The answer clearly is that one and a half mags is/are enough. According to the FAA.

    All other opinions don’t count.

    Can we now have a blog on the downwind turn syndrome?

  10. Richard says:

    Actually, the pilot’s opinion counts for more than the FAA’s opinion. I have two friends who have had forced landings due to the drive failure on the single-drive dual mag. They don’t care what the FAA says, they will not fly a plane with the dual mag. One of the two has only 130 hours total time, a somewhat less reassuring experience than Mac has had.

    Now with seperate mags, a drive failure may be twice as likely as a single drive failure. But ,if you do have a drive failure you fly home on the good mag, which is the whole point.

  11. Hod says:

    Great information Mac. I’m certain that most of your readers here had no idea of how mags are timed and the internals of said mags. I’m looking forward to a post from you on the relationship between power, airspeed, and altitude when flying over Lake Wobegon at night in a light twin. Keep your nose up in the turns as you write for us simple EAA folk. I might even renew.

  12. John Veltman says:

    I only have about 5600 hrs flying in all sorts of weather behind an 0540j3c5D. No horror stories to report but I routinely replace the D mags at 500 hrs and then use silicone caulk /rtv around the hold down clamp bolts once they are torqued to prevent any shifting. I have no fear of dual mags, but like every airplane component, they need to be carefully monitered and maintained.

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