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Single shaft dual magnetos e.g. D3000 - why were they used, and are they safe?

I suspect that there were problems with your ignition system when you made the test.

I agree 100%. Bad harness insulation, which breaks down at the lower air pressure, a bad plug, or even a badly assembled mag. This may be of interest too. I had a bit of fun here too, with the UK CAA backing up the dodgy company.

But discovering that sort of thing is a big plus of a mag test at altitude.

Dual mags are such a pain because you can’t really adjust them individually

Mine have always been overhauled by Quality Aircraft Accessories, Tulsa, Oklahoma, and have always been so close to each other on the mag test that one can barely see the difference. Next time, if I remember, I will post a video of the RPMs during the mag test.

The type of clamp used to secure the mag was changed a few years ago. I vaguely recall there was an AD, but my plane (new 2002) always had the later clamp. The older clamp did cause breakages of the mag housing. I suggest checking that AD (or SB, or whatever). It is well known in the industry.

Administrator
Shoreham EGKA, United Kingdom

The current-type clamp

Administrator
Shoreham EGKA, United Kingdom

Nice and clean engine but your Tempest oil filter is not safety wired and your ignition harness not connected and in case it is, the tie wraps are not aviation grade

The safety wire on the filter really makes sense, because if it turns only a little you’ll have a leak with the high pressure inside the filter

Guys, this is a shot of the engine when it came back from the crank swap, before it was installed.

Administrator
Shoreham EGKA, United Kingdom

I was almost sure that you would not fly it like that :-)

Some of the later Aerostar Superstar II’s had the 350hp Chieftain engines mounted with the dual magnetos. They’re a constant source of trouble for the poor owners. Just a poor, plastic design. That said, when they work, they are the fastest Aerostars there are.

Just a poor, plastic design

Which mag was that? The D3000 (the pic I posted) has an aluminium housing.

I recall reading of about two failures of this single shaft dual mag on the Socata TB fleet, 1980 onwards, about 2000 aircraft. There would have been far more than two engine failures due to other causes during that time – mostly fuel system issues.

Administrator
Shoreham EGKA, United Kingdom

The single cog that drives both magnetos is as far as I know made out of plastic. It’s a single fail point.

The single cog that drives both magnetos is as far as I know made out of plastic. It’s a single fail point.

I took some photos of my spare one. The central gear is hard to see but it is possible to see a bit of it.

Maybe some are plastic but mine isn’t.

This one was overhauled by QAA and most of the stuff is new

unlike the same one which was done in the UK and which came with an apparently secondhand impulse coupling spring

IMHO, the main single point of failure is debris from one mag buggering up the other or, of course, the whole magneto coming off the engine.

To get that central gear to stop (it is on the end of a steel shaft about 2cm diameter) you would have to have a massive failure of the gear in the accessory gearbox from which it is driven, and then you will prob99 lose the camshaft drive too, since all those gears are joined together. That’s probably why the two plastic gears are plastic – so they will shred individually of their respective mag seizes. You have the same on a vacuum pump – also driven via a plastic coupling.

Administrator
Shoreham EGKA, United Kingdom
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