There is a central wheel driving both sides. If a part gets loose internally, it can get anywhere and destroy the whole setup. If the magnet gets loose and shifts, both sides are hosed. That is actually a common scenario, magnets do move and clamps get loose. Usually a dual mag is held in place by only two clamps and there is no safety wire or anything.
Still, I am not aware of a statistically larger risk of having an engine failure with a dual mag compared with two single mags. Yet it was an incredibly stupid move. In my aircraft, it allowed Cessna to install a turbo charger (oil scavenger pump), in the TB20 the motive was to save $5 because it has an unused accessory drive.
I am not aware of a statistically larger risk of having an engine failure with a dual mag compared with two single mags
I think that is correct.
Yes, if the magneto comes loose, you lose the engine. But it seems that more common failures are in the individual magneto(s), leads, and spark plugs.
I recall reading maybe two reports, ever, of the D3000 mag causing a forced landing.
The biggest issue with mags is that the pilot needs to get his brain around the fact that if one mag fails, it may be necessary to switch off that mag, and fly on the other mag only. This was never mentioned in any training I had, FAA or JAA. It isn’t a case of a mag failing and you just carry on flying. It may be that way but perhaps in well below 100% of mag issues.
I check the magneto clamps on every preflight check.
I recall that the IO540-C5D5D being about $1000 cheaper than the IO540-C4B-something (two separate mags). Also, the -C4D5D has a bit more room behind it, for e.g. nose gear retract mechanism.
It was claimed that Socata chose that engine for the extra room but that was proven wrong when one US pilot was told by an FSDO inspector that he can install any engine in his TB20 which shares the same TC as the -C4D5D, so off he went and installed the “B” model with the two separate mags! Then he was told in the Socata owners’ group that this is 100% illegal, so he stopped replying to posts and PMs 
I am sure he can’t be prosecuted because of that concession he got, but it would not be good for anybody else.
I overhaul mine every 500hrs by QAA in Tulsa. I would never let any European shop anywhere near them. One UK shop did it and then refused to give me the work pack; I strongly suspected they used secondhand parts and reported them to the CAA. The CAA was completely uninterested (despite this shop being “well known” to them for past “form”) so I sent the mag to QAA in Tulsa to be re-done. 800 quid wasted but worth it for the peace of mind. QAA did think the impulse coupling spring was secondhand, which made sense since this UK shop was at the time about the only place which had that part in stock (this was during the Bendix mfg stoppage which made some D3000 mag parts very rare). I now have a spare magneto on the shelf – $2500.
it may be necessary to switch off that mag, and fly on the other mag only. This was never mentioned in any training I had, FAA or JAA.
You are right, Peter. And I can confirm that it was never mentioned in the SEP training I had before in Europe and in the US.
It was in the mid-eighties when I was a passenger (2nd row) in a C210 departing Düsseldorf RWY 15 when at 1500ft MSL/ 1350 GND (90s after T/O) the engine suddenly sputtered like hell and lost all power. The pilot pulled the throttle back to idle, turned the aircraft around, checked fuel system etc, notified ATC and we were even a bit high when landing in opposite direction on our departure runway (33/15 – which is a taxiway today).
Trouble shooting revealed a bad magneto, misfiring into the good one. If you are at a higher cruising altitude you would have time to consider that. In our case the pilot did the right thing to turn back.
That design was a complete joke, and for my taste it’s against all principles of intelligent aircraft design. Is it possible to install a backup electronic ignition or maybe replace one of the mags with an electronic ignition?
I overhaul mine every 500hrs by QAA in Tulsa. I would never let any European shop anywhere near them.
Somebody did a great job selling you the idea that mag overhaul is black magic only to be performed by illuminated members of the special guild It really is fairly trivial. I’ve participated in both an overhaul and an internal adjustment of mine. It’s a very simple piece of engineering, very few parts, few adjustment options. It takes diligence and knowledge of the procedures (which grease to use etc.) but it’s anything but exceptionally difficult. A lot of small 145 shops here have authorization to do it. You need a test bed and I’ve seen the most ingenious designs out there — usually done decades ago by a mechanic in his spare time.
I don’t see why i should send – any – mechanical device from Germany to the USA, except for selling it to the Americans :-)
or maybe replace one of the mags with an electronic ignition?
That’s the future and they are working on it. http://www.electroair.net/stc_ignition_kit2.html
I remember they mentioned the end of 2016 for a Bendix 3000 certified solution.
Q & A
16. My engine has a dual magneto, how do I adapt my engine to use the EIS?
EXPERIMENTAL ONLY! We use a split ring collar on the crank shaft for the electronic ignition timing pick-up. There are some single magnetos that will adapt to the dual mag hole, or alternatively you can operate two electronic ignition systems. Call us and we’ll review the details.
CERTIFIED AIRCRAFT: We have begun looking at engineering projects that will develop into replacement kits for certified aircraft using the Bendix Dual Magneto (D2000 & D3000 series). We will keep the website posted with progress reports as these projects develop. Unfortunately, our dual magneto solutions are limited to experimental aircraft at the moment.
Is it possible to install a backup electronic ignition or maybe replace one of the mags with an electronic ignition?
We did that e.g. here There is no current solution for single shaft dual mag engines (or there wasn’t then).
I think Nobbi has beaten me to it 
Still… no solution.
It takes diligence and knowledge of the procedures (which grease to use etc.) but it’s anything but exceptionally difficult. A lot of small 145 shops here have authorization to do it
I agree
Somebody did a great job selling you the idea that mag overhaul is black magic only to be performed by illuminated members of the special guild It really is fairly trivial.
I do agree with Achima, there a several shops in Europe which are very experianced and good at this, in most cases far better than factory new of factory overhaul, as they seem to throw the together.
Yes, if the magneto comes loose, you lose the engine. But it seems that more common failures are in the individual magneto(s), leads, and spark plugs.
This is true, however this doesn’t happen if it gets torqued and the correct gasket is used. Sometimes the wrong gasket is used, when used, the clamps will rest of the gasket as well, this is not good.
Gears in general do not cause problems, gear damage is quite common on helicopter ones, but even with several teeth missing it will still go, these are often brought in without complaints. Some helicopter users replace gears at every inspection as well, which seems to help.
On dual magneto the real risk from my experiance is:
- Capacitors, as these will stay with the cover at the aircraft, and most mechanics don’t seem to test them, while these do fail
- Broken coupling spring, will lead to random firing. Coupling spring should replaced every single time it gets removed, some people don’t do the 4 year overhaul interval, and with low flying hours / year can cause the spring to break due to rust.
- Loose cam. Their is one single screw which holds the cams to open the points. The screw should be torqued and replaced every time. Timing will be off.
- Sparkplugs and leads, just like with any other ignition system, though not a real risk, as it would not stop the engine.
It was claimed that Socata chose that engine for the extra room but that was proven wrong when one US pilot was told by an FSDO inspector that he can install any engine in his TB20 which shares the same TC as the -C4D5D, so off he went and installed the “B” model with the two separate mags! Then he was told in the Socata owners’ group that this is 100% illegal, so he stopped replying to posts and PMs I am sure he can’t be prosecuted because of that concession he got, but it would not be good for anybody else.
Peter, I think there is zero chance of anybody being prosecuted by an FAA FSDO for that, the enforcement mechanism is simply that without a documented field approval the owner won’t be able to find somebody to sign off his annual inspection. Beyond that it can get very complicated for some types – Piper was for instance notorious for putting zillions of planes/engines on the same TC. There are surely planes out there that have been flying around for many years with a slightly ineligible engine installed on a logbook entry, getting through every annual inspection because the IA doesn’t check the engine against the aircraft TC engine eligibility list.
