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Author Topic: Magneto failure  (Read 7715 times)
Larry DeCamp
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Posts: 46


« on: August 15, 2010, : 06:57.45 AM »

I recently had a magneto failure that may be informative for all general aviators.  A misfire developed only after the oil temperature reached 140F, and finally, on the hottest day of the year so far, it quit ! Magneto coils are well known for this failure mode as they heat up, if the insulation is weak. We all confidently do a mag check on "run up" as taught as student pilots, right?  How many folks do a mag check in the air when the engine is warm? Just trying to stimulate some conversation, and hopefully a magneto expert will emerge to be helpful ?
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Doug Frantz
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Posts: 4


« Reply #1 on: August 16, 2010, : 05:28.18 PM »

Hi Larry-
Good point!  I have had three magneto failures while flying, and in all cases both mags checked good during the run-up.  I routinely do as you recommended, that is switching mags in flight to see how they are performing.  The first two failures were on a J-3 that we had re-covered, but did not do any engine work.  It had Eisenmann mags, and the first one quit on the test hop.  I landed immediately and we rebuilt it, and the other one quit on the very next flight!  The other failure was a bad coil in a Bendix mag on a Stinson.  As you said, if they work during the mag check, we assume they are OK.  And while the magneto is a very reliable piece of machinery, we should not expect it to last to TBO.  I take mine off the engine periodically to feel the bearing clearance.  I have replaced a few mags on customer airplanes due to this failure mode.

Thanks for this post.  It reminded me its about time to look at 'em again!

Doug
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Larry DeCamp
Newbie
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Posts: 46


« Reply #2 on: September 12, 2010, : 07:05.11 AM »

I did some magneto research at the Blakesburg fly-in.  Thanks to the generosity of Tom Wottreng ( A&P/AI) and his magneto repair shop ( Aircraft Systems in Rockford, IL), I can pass on some info that will hopefully make antique flying safer and cost less in the long run. There is a broadly held opinion that replacement parts for old mags ( coils, condensors) are new parts that were made long ago with marginal materials available at the time.  There has been opinion offered in print that this true, but I don't have the details of the test conditions applied.  So use your own judgement and I hope I don't get any threatening calls from ACS legal staff !!

One benchtop  stress test used in the industry is as follows:
     The magneto is heated to 165F
     The leads are connected to plugs with a gap of 8mm ( 5/16" ).  This is to simulate the spark resistance created by piston compression of the air.
     The mag is turned at 2000 RPM
     A steady fire confirms a healthy condensor and coil. This is a short test because the mag shaft seal is not lubricated by engine oil on the bench.

The best advice I have gotten to date is from David Schober and TJ at Aircraft systems.  That is to replace my old Bendix S4 mag (s) with a new Slick.  The materials and processes used in manufacture are modern and superior to the old mags, and they do routinely pass the above stress test.That said, a new mag will die quickly if overheated.  Have you put a thermocouple on your mag in flight on a hot day Huh  If the temp is over 160F,  you may be shortening the like of the mag and last time I checked, they are not cheap!
Larry
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