Tuesday, October 06, 2009

Magnetos and propeller strikes

 Hi John, a couple of us mechanics have been talking about what a Slick mag needs after a prop strike, I've been on the Unison website and can't find any info, where could I look for something in print?

 "Magnetos must be overhauled after a lightening strike on the aircraft, a sudden engine stoppage, prop strike, or immersion." Champion Aerospace, LLC Maintenance and Overhaul Manual L-1363 section 4.0

In my experience of doing propeller strike inspections (approx. 12 per year for 10 years), magnetos were never part of the inspection. This was before Continental/Bendix added their magneto inspection requirement. My recollection (it is often faulty at this age), is that they blamed distributor gear tooth breakage in the magneto on sudden stoppage/propeller strike. Picture below is the distributor gear with broken teeth:

bendix distributor gear with broken teeth

Our opinion is that this breakage is caused by worn bushings creating a conical oscillation about the center axis (whirl or shopping cart wheel flutter). Load variations are generated within the gear itself that result in non-uniformly distributed loads across the gear tooth surface. This is a very rare event but we have witnessed it on a magneto test bench.

For more opinions on propeller strikes:


  1. Hi, I am an accessory tech in Canada, I deal on a regular basis with Slick and other magnetos.
    On a propstrike we would inspect for damage on the housing, rotor, distributor gears and anything that rotates, or is connected to the engine. A sudden stoppage really puts ALOT of strain on a magneto. We've had sheared rotor shafts, and the pilot doesn't even notice it for 3 - 4 months! The manual does state that for a propstrike the mag should at least be inspected, if not overhauled!
    Its virtually the same for Bendix [TCM] mags, the difference is really in what you'd replace if anything goes wrong.
    If you have access to the Champion Aerospace website, you should ask them BEST, they are the new owners and manufacturers of Slick magnetos, Unison no longer deals with that stuff.
    Good luck

  2. Also note:
    In the latest and greatest version of the Slick Magneto Overhaul & Maintenance manual, the torque on ALL slick magneto drive nuts is 120 - 180 INLB, NOT 120-320 inlb!!!
    The over-torquing of the drive gear nut HAS caused major cracks in the drive assemblies, especially in 4309 & 4310 magnetos.