Caution on dual magneto with single coupling

[Skybrd]

I've heard warning about this mag being dangerous due to the mounting hardware coming loose and having a failure but I have found another issue that is as dangerous as the other one. It's failures of the cam followers due to heat and melting. I just had a partial failure of my dual mag with one side mag failure. It was overheated nylon cam follower partially melted and shorting it's length. This causes the points to not open when it finally got to that point. Here is a NTSB report of a Mooney that had a crash landing due to this issue. http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20050805X01172&ntsbno=SEA05LA153&akey=1  I talked to a magneto shop in Texas and the tech told me that the dual bendix single drive has an issue with this problem and this doesn't show up as much with the single Bendix mags. My take on this, is somewhere is an engineering problem especially due to the material of the cam follower and the heat inside of the magneto. Our Mooneys are packed tight and maybe we need some extra cooling to the magneto area. 

[Bolter]

My annual starts on Monday and I am pulling the dual mags out (500 hours) for inspection and repair.  If I see that the components are in good shape, I will assume that I am running things well enough to keep this bit from overheating.  Assuming a relationship between CHT's, Oil Temp, and MAg temps, I don;t expect a problem.  I live in a warm climate, but I am fastidious about keeping low CHT's on my 4-channel JPI and...wait for it...typically running LOP.   

And if I find issues with my mags, I will take pictures and share it as a data point. 

-dan

[jwilkins]

If I am following the NTSB report correctly it appears that the cam followers / points had about 700 hours without logged maintenance. The report states that the cam follower felt oiling pads are supposed to be oiled and checked every 100 hours.

I don't disagree that this may be a design / selection of material issue, but it APPEARS ( if I read the NTSB report correctly ) that the recommended maintenaince was not performed. Again, IF I am reading this correctly the take-away I get is to do the recommended inspection and oiling every 100 hours.

Personally I never understood the logic of having dual mags for redundency and then running both of them off a single drive, but this failure looks like it may have been prevented by following the recommended maintenance. 

I don't have the dual mag single drive, but I DO get the recommended 500 hour mag inspections done. I've never had any issues spotted, but we check.

 

[Skybrd]

Update on my right magneto failure, on further inspection of points and cam follower, found. The cam follower assembled that has the nylon t part was installed backward. The metal spring steel part of the cam follower should be on the side of the movable points and not on the other side. By having the cam follower part backward, it makes the nylon part ride very close to the points and can burn them. This problem was not due to low lubricant on the felt part.  This type of parts assembly problem can easily happen since the cam follower can be put together either way and it's not very noticeable. Again, the problem occurred due to the cam follower being installed backward making the nylon part almost touch the points. The correct way is to have the spring steel part of the cam follower be on the inside next to the movable points. I hope this information helps.