Heat soaked mag drop?

Chip Sylverne

Final Approach
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Jun 17, 2006
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Quit with the negative waves, man.
Ok, let me preface this by saying it's not really a problem, more of a curiosity.

Bendix mags, Lyc O-540. Mags iranned maybe 150 hour ago. On first start of the day, after warming to the oil to 100 deg or so, mag drop both sides is a dead even 61 rpm on a digital tach. All egt rise, no roughness. If I fly to lunch, runup on the way home gives me the usual 61 left side, but right side will give me an extra 20- 30 rpm, to 80 or 90, and the number bounces around a little bit but never exceeds 91, still the same egt rise on all jugs.
Next day, first flight, they are back to dead even.

So it's pretty clear something changes in that right mag when it gets hot, but not enough to throw it out of spec. Most likely suspect?
 
I had a bad coil on a Slick mag that would start missing once it got to a certain temp. It took sticking a temperature probe to the mag while flying it to track down the problem.
 
Ok, let me preface this by saying it's not really a problem, more of a curiosity.

Bendix mags, Lyc O-540. Mags iranned maybe 150 hour ago. On first start of the day, after warming to the oil to 100 deg or so, mag drop both sides is a dead even 61 rpm on a digital tach. All egt rise, no roughness. If I fly to lunch, runup on the way home gives me the usual 61 left side, but right side will give me an extra 20- 30 rpm, to 80 or 90, and the number bounces around a little bit but never exceeds 91, still the same egt rise on all jugs.
Next day, first flight, they are back to dead even.

So it's pretty clear something changes in that right mag when it gets hot, but not enough to throw it out of spec. Most likely suspect?
Doesn’t have to be the Mag. Something else could be going that rears it head when running on only one particular spark plug. I got no idea what it might be.
 
I had a bad coil on a Slick mag that would start missing once it got to a certain temp. It took sticking a temperature probe to the mag while flying it to track down the problem.
This is a good guess. Coils can get breaks in the windings that get worse with thermal expansion and vibration from running. Since it's not a safety of flight issue now (have a 2nd mag and you wouldn't even know that the issues exists without a post flight mag check), might do nothing in the short term and fly it. If the coil is starting to go, it will get worse over time. EGT rise even across cylinders suggests it's not a plug or wire issue.
 
The other thing that was done to the left side, but not the right, was that the mag drive bearing was replaced at 700 hours per lycoming's SB. Maybe there's a touch of wear in the right side that gets sloppier as it heats up?
 
Sounds like every bad Slick mag coil failure I've had in the making. It starts with an intermittent larger mag drop, then it becomes an intermittent failed mag drop (which mysteriously disappears when you take it to the shop and the mag cools down), then it eventually starts to regularly fail when hot, like on the second leg of a long trip. With my previous set of mags, it coils failed often enough I carried an extra coil on long trips. (Of course then the mags behaved.)
 
Sounds like every bad Slick mag coil failure I've had in the making. It starts with an intermittent larger mag drop, then it becomes an intermittent failed mag drop (which mysteriously disappears when you take it to the shop and the mag cools down), then it eventually starts to regularly fail when hot, like on the second leg of a long trip. With my previous set of mags, it coils failed often enough I carried an extra coil on long trips. (Of course then the mags behaved.)

Maybe time to start saving up for one o' them electronic thingies for the right side....
 
The Bendix SF4 manual says you don't return a new coil to service until it's been heated up to 165 degrees and then the mag is tested again. That was with a factory new coil. So it's been an issue for over 60 years. I've gone through 3 NOS coils on mine for this very issue. I wouldn't know if I didn't do a post-flight mag check.
 
So the mag drop is still within specs ? Does the engine run rough ?

Yes it's within spec and no roughness. As I said, it's not a safety of flight issue right now, more of a curiosity. Doesn't mean I won't keep an eye on it, just speculating on cause.
 
Those of us without a digital tach would not even notice a slight 20 rpm difference unless there was engine roughness.

I would check mag timing at some point just to confirm both mags are setup properly.
 
I just had a coil replaced on the left mag of my right engine that failed when warm causing the engine not to start when hot. The coil tested fine when cold and failed testing when hot.

Jim
 
Tracked the source of the problem to the silicone leads from the right mag laying too close to the muffler, like nearly touching. Secured them further away, mag checks are back to steady and even.
 
I would question the condition of the Leads if they actually are the cause.

Heat damage to the silicone insulation could allow “leakage” from the center

conductor to the shielding ground.

However; how did it “ heal” after repositioning?

A high voltage Lead Tester will tell the tale.
 
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