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Single shaft dual magnetos e.g. D3000 - why were they used?

This article suggests that incorrect maintenance might explain most of the (few) failures seen.

Administrator
Shoreham EGKA, United Kingdom

This SB is simply about a new style attachment clamp and iterating which gasket is the right one.

Magnetos are very simple products. It’s not difficult to inspect, adjust and install them. I’ve had a 5° difference in my dual magneto a week ago which requires removal and internal adjustment. I’ve had the chance to study it in detail and how the adjustment is done. Really simple stuff, any A&P with some training can do it.

Still I wished the FAA would never have certified dual magnetos. It’s a perversion of redundancy and it’s not good for the engine because they are never 100% aligned and if they are, not for very long.

They are accurately aligned if overhauled by a decent firm, not a monkey. Mine was done by monkeys, JAR145 monkeys, every time, until I started using QAA and they have always been perfectly matched.

Regarding redundancy, my view is that the biggest risk is if the mag comes off the engine, and that usually is a poor installation issue. I preflight inspect the flange+clamp before every flight (it’s visible via the TKS refill hole). The other failure scenarios mostly involve a failure of the drive gear, but that will usually go together with a failure in the accessory gearbox which then stops the camshaft, and then you will have a really bad day. Statistically there are very few failures of these mags which do not involve bad workmanship, or a cracked magneto case (which was probably also a maintenance failure).

Two mags are of course better, but we have only the one prop

Last Edited by Peter at 03 Jun 20:09
Administrator
Shoreham EGKA, United Kingdom

I’ve had a 5° difference in my dual magneto a week ago

Advanced or retarded? Did you notice it in the flying?

Mainz (EDFZ), Germany

They are accurately aligned if overhauled by a decent firm, not a monkey.

It changes over time and you have little control over that. There is a cam in the middle which is driven by the engine and this one operates the contactors. There is steady abrasion and this is not identical on both so timing slightly changes. This is why you can loosen a screw and reposition them. Not difficult at all.

Under EASA you do not have to overhaul magnetos. Only an inspection is required and some parts have to be replaced every 500h. The Part 145 needs to have magneto training to do this. The dual magnetos are rare and a lot of shops prefer to not touch them due to lack of experience.

See also Mike Busch’s magneto webinar. There is “internal” and “external” timing, IIRC. Some timing creep is normal.

Last Edited by boscomantico at 03 Jun 21:00
Mainz (EDFZ), Germany

With the single magnetos, you just loosen them and adjust the angle while having your magento tester connected. Piece of cake. With dual magnetos, you have to remove the magneto (can be a major pain depending on the installation), open it, adjust the contactors and then put it on a test stand to verify the timing and correct operation and install it back again. What takes 15 min on a single magneto installation, can easily take 3h with a dual magneto and the result won’t be as good as the timing is never 100%.

Have I mentioned that I think dual mags should never have been certified?

This is why you can loosen a screw

It is very important (and mandatory according the manual) to replace the cam screw on all Bendix magneto’s. On most single magneto’s there is a spline, so the cam can not rotate on the axle, only the contact assy is adjusted.

On a dual magneto both the contact assy is adjusted (for opening) and the cam set for correct internal timing. On a dual magneto the cam / axle mounting is tapered. Very important to have to screw renewed everytime, and not just loosen, adjust, and retighting.

Failures on magneto’s (with good maintenance) is really rare. The weak point on dual magneto’s is the flange mounting especially on aircraft which have poor accessibility, another weak point are the capacitors, which are often neglected, as they are inside the cover that stays with the aircraft during overhaul for example.

JP-Avionics
EHMZ

Good point about capacitors. I once had one side of a dual mag fail halfway between Scotland and Iceland (in a single engine) due to a capacitor failure. It was very worrying because one half ran progressively rougher and then failed, without knowing it was the capacitor I imagined all sorts of broken bits circulating inside the common casing. It was a very long 2 hr flight (Faeroe was out due to fog).

Other than that I have had few problems with the dual mag (2000 series), although it seems to be difficult to get the internal timing right. I believe that a difference of 3deg is acceptable. One problem is slight rough running above FL100, probably due to low air pressure causing arcing.

I would much prefer 2 separate mags.

I sympathise with Ted.P as I had a similar issue whilst flying above a mountain range. Not as bad as Ted because I had 2 seperate mags and at lease a hope of landing somewhere below. Never the less when there is a failure in the air there is no way of knowing what has failed. It was simply a capacitor shorting out the mag.
BTW the nearest airport was a large commercial and when I called to explian our problem the response was immediate and superb.

UK, United Kingdom
44 Posts
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