Dual mag failure explained

[Skyland]

As you may recall, last year on May 9, 2022, I experienced a complete flame out and off-field landing just short of Double Eagle (KAEG) in Albuquerque.  

The root cause of the D-3000 dual mag failure has haunted me - until now.  The mag failed at 209 hours after the overhauled exchange.  Mags checked good upon departure but both point gaps closed to zero during cruise 1.5 hours after takeoff.  The FAA sent an IA the next morning to remove the mag.  The inspector found that both point gaps were at zero.  Also, inside the cap was a condenser wire with melted insulation where it had been rubbing on the cam.  The common thinking seemed to be that the burned and shorted condenser wire would have simply grounded out one mag leaving the other unaffected.  The glaring evidence that everyone seemed to ignore was the burned and shorted wire was causing a much bigger problem.  

With the help of my local IA and his mag tester stand, we mounted up a dual mag and shorted out a wire from one point connection to the rotating cam.  Sparks flew and temperatures rose quickly on the cam.  So, what happened in my mag turned out to be an installation error with a condenser wire touching the cam that took 200 hours to finally wear through the insulation.  The cam heated up softening both point rubbing blocks nearly simultaneously to the point of closure.

It does baffle me that the mag is designed that it is even possible for this wire to be routed to cause such a problem.  At least there's some satisfaction in knowing that the mag itself is rather robust as long as precautions are taken not to misroute the condenser wires in the path of moving parts.

 

[EricJ]

Wow, that's interesting.   Thanks tons for sharing that!

The points cam follower is designed to wear at the same rate as the expected wear on the point contacts so that the gap stays consistent.   I can see where unexpectedly elevated temps could affect the wear rate of the followers.

 

[cliffy]

Thats interesting  thanks for writing

I'm trying to remember as its been 40 years or so since I worked on them but I think I remember either the manual or a SB specifying

EXACTLY how the wiring should be run inside the cam box. IIRC it was quite specific.  

 

[N201MKTurbo]

36 minutes ago, cliffy said:

Thats interesting  thanks for writing

I'm trying to remember as its been 40 years or so since I worked on them but I think I remember either the manual or a SB specifying

EXACTLY how the wiring should be run inside the cam box. IIRC it was quite specific.  

 

There aren’t a lot of choices. Getting the wires on the points is a PITA and usually makes you bleed. Once they are on, there isn’t much to inspect. The only way I can think of making that happen is if you twist one, which would be hard to do.

[N201MKTurbo]

So, was it a coil wire or a capacitor wire that shorted?