Mag failure at runup

[Tony_Hale]

I was starting a flight review with a Mooney M20J owner yesterday.  He has a digital tach, but no engine monitor.  Engine started fine, and we taxiied out to the runup area.  When doing the mag check he went to left mag and there was no rpm drop.  He went to right mag, and engine died.  He got it restarted and we taxiied back to the hangar.

I was telling him about the dual mag set-up and explaining the controversy over a shared single drive.  Do all 201s have the dual mag?  He was asking if it could be converted to a traditional two magneto set-up, and I said I didn't think so, because everyone would do it if possible.

He said he's not sure he would've caught the bad mag if I hadn't been there.  I said it would be hard not to when you have one mag with no drop and the other mag cutting the engine off.  

[Mooneymite]

I was picking up a new-to-me experimental aircraft.  When the owner did the run-up when we flew it together, he did the mag check.  He kind of breezed past a 150 rpm drop and told me, "it's just a plug, it'll burn off in flight".  We went and flew, but when I was leaving after doing the deal, the mag drop was still there, so I went through my repertoire of tricks to fix it, but the 150 drop persisted.

Finally I taxied back and told the owner, I wasn't taking it until the mag drop was within limits.  Long story, short version, the four screws that hold the mag together were all loose!

Pay attention to your mag. check!

[1524J]

2 hours ago, Tony_Hale said:

I was starting a flight review with a Mooney M20J owner yesterday.  He has a digital tach, but no engine monitor.  Engine started fine, and we taxiied out to the runup area.  When doing the mag check he went to left mag and there was no rpm drop.  He went to right mag, and engine died.  He got it restarted and we taxiied back to the hangar.

I was telling him about the dual mag set-up and explaining the controversy over a shared single drive.  Do all 201s have the dual mag?  He was asking if it could be converted to a traditional two magneto set-up, and I said I didn't think so, because everyone would do it if possible.

He said he's not sure he would've caught the bad mag if I hadn't been there.  I said it would be hard not to when you have one mag with no drop and the other mag cutting the engine off.  

Although the duel mag has its own set of challenges, separate mags still have a single point of failure in the drive. And yes, it can be converted but at considerable expense best offset while performing an overhaul.

[KSMooniac]

The conversion to separate mags requires a different accessory case and a few other parts.  There used to be a big up-charge from Lycoming on an exchange for this conversion, but I believe they'll do it now for no up-charge as they want to get rid of the dual mag engines.  It isn't economical to convert until overhaul/exchange, though.  There is at least one good thread on here documenting everything involved with the conversion if the owner wants to research the topic.

If it were me, I'd just get the mag sent to Select Aircraft in Lancaster, TX and let Aaron fix it.  It is a good mag, but needs the 500 hr inspections at a minimum.

[Tony_Hale]

Scott, I recommended he use Aaron @ Select.  

[CaptainAB]

Did he check the "off position" before shutting down? Sounds like he could have a bad p lead. Which could get someone hurt. 

[kortopates]

2 hours ago, Tony_Hale said: I was starting a flight review with a Mooney M20J owner yesterday.  He has a digital tach, but no engine monitor.  Engine started fine, and we taxiied out to the runup area.  When doing the mag check he went to left mag and there was no rpm drop.  He went to right mag, and engine died.  He got it restarted and we taxiied back to the hangar.

I was telling him about the dual mag set-up and explaining the controversy over a shared single drive.  Do all 201s have the dual mag?  He was asking if it could be converted to a traditional two magneto set-up, and I said I didn't think so, because everyone would do it if possible.

He said he's not sure he would've caught the bad mag if I hadn't been there.  I said it would be hard not to when you have one mag with no drop and the other mag cutting the engine off.  

Although the duel mag has its own set of challenges, separate mags still have a single point of failure in the drive. And yes, it can be converted but at considerable expense best offset while performing an overhaul.

What single point of failure are you thinking of with the 2 separate mags? The idler gear? Pretty catastrophic to both mag set-ups. I am probably missing your point but just see the 2 mags in providing added redundancy for some but maybe not all failure points of the Dual Mag

What I find ironic is that it's generally not one of the mags that fails in the Dual mag, but most common catastrophic failure we hear is the entire dual mag assembly falling off that usually was fine except for its securing hardware. The redundancy it provides is probably fine except for its apparent ease in coming loose. Yet it's really rare to hear of a single mag coming loose - but I doubt it's because it's less of an event since it too will lead to a loss of oil pressure pretty soon.

My thought is it probably would have been fine if it was more reliably secured to the case as our single mags seem to be.

[Guest]

It seems that most issues with magnets coming off or loose is not a fault of the mag, it's the fault of the maintainer.  In more than 30 years I've never personally seen a D3000 mag fall off let alone come loose.  

Proper gaskets, clamps, new lock washers, a torque wrench, a willing maintainer and owner are essential to success.

Clarence

[1524J]

2 hours ago, kortopates said:

What single point of failure are you thinking of with the 2 separate mags? The idler gear? Pretty catastrophic to both mag set-ups. I am probably missing your point but just see the 2 mags in providing added redundancy for some but maybe not all failure points of the Dual Mag

What I find ironic is that it's generally not one of the mags that fails in the Dual mag, but most common catastrophic failure we hear is the entire dual mag assembly falling off that usually was fine except for its securing hardware. The redundancy it provides is probably fine except for its apparent ease in coming loose. Yet it's really rare to hear of a single mag coming loose - but I doubt it's because it's less of an event since it too will lead to a loss of oil pressure pretty soon.

My thought is it probably would have been fine if it was more reliably secured to the case as our single mags seem to be.

 

Yes, excerpt from an article from Left Seat below. I know....it's a little weak but it's all I've got.

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.

[bradp]

Btw this was not a bad p (ground) lead this was a bad mag.  To clarify...

In a bad P lead situation you do this: 

Both = normal rpm

L or R = no drop in rpm for the selected mag with the bad p lead

L or R = normal mag drop for the selected mag with functional p lead

OFF = normal mag drop but engine won't shut off as engine is running (ungrounded) mag with non functional p lead 

 

For a dead mag you'd see this:

Both = run up RPM will have lower RPM, higher MAP and higher EGT than normal.   Important to note what your normal EGT is for a given RPM as this would be your first clue.

L or R: good mag selected bad mag grounded = no drop. 

L or R: bad mag selected, good mag grounded = engine dies 

OFF = engine dies. 

Not a bendix guru, just ASEL. 

-B

[Hank]

I had a single (Slick) mag going bad. What I saw was:

• Both--normal RPM, never paid attention to MP or EGT (but I'm about to start)
• Good mag--normal drop, ~50-60
• Bad mag--drop >200 RPM, with coughing and spitting

Flight cancelled, returned to hangar and called mechanic. Sent Left mag to Kelly factory an hour's drive away. Been running great since reinstall and retime last August, RPM drop 50-75 RPM.

< apparently Anthony is rubbing off, we now have a B signed above.   >