In 2013, some 0-200 Continental engines using the C72AE propeller experienced severe failures of magnetos---I am told that failures of BOTH Slick (Champion) and Bendix (TCM Electronics) magnetos occurred--both on one with impulse couplings and without.
Continental claimed the cause was 6th order harmonics from the prop. Sensenich disagreed, but in an abundance of caution, discontinued sales of the prop. See Notification CN-1-13.
Some pilots removed the prop, going to metal or wood. Some did nothing. Many planes, including N998SC are still flying with the C72AE without issue.
In the meantime, TCM has re-issued SB643C calling for a 500 hour TBO on their magnetos and a replacement or overhaul of all pre-2015 mags with over 4 years of operation!
http://www.tcmlink.com/pdf2/SB643C.pdf
The Senenich C72AE is a good performer, but is not any lighter than the wood W72GK-44. The composite prop requires a bit less maintenance (retorquing) and top speed with the C72AE using the 50 cartridge provides about 7 mph more top speed--not much difference in climb. Sensenich tells m that the composite prop, like a wood prop would fail in a prop strike without severe engine damage--not splintering like the wood prop, but breaking at the root.
BTW--Sensenich is an impressive organization--answer phone on first ring--and their Don Rowell is friendly and very knowledgeable.
The Cubcrafters Forums speak of the "infamous" C72AE, but other information is lacking--sort of a mystery. The only posts seem to indicate that the prop is OK if using Slick magnetos. But, other forums (e.g. Supercub.org have vigorous debates regarding Slick vs Bendix---it appears that most think the Bendix mags are heavier but more robust and powerful?)
For the 2019 Annual, we will be checking the mags closely---most likely replacing or overhauling them. More research before then. (See below)
One thing to note---it is very common for a propeller/engine combination to have a "resonant" frequency. The Sport Cub, 998SC included, seems to have a "shudder" in the 1900-2000 rpm range. (It can be seen clearly in my videos, but I really did not notice it when flying.) It can be reduced with "damping" of the engine mounts using visco elastic bushings, and is less of an issue with a wood prop that has inherent damping. If one does not notice it and operates the plane for extended periods in that 1900-2000 range, that might account for mag drive failure issues, but more likely the issue is RPM in the 2600 and above range...see below. Resonant frequency is affected by mass and stiffness. The amplitude or seriousness of the vibration is affected by damping. A heavier prop will reduce the frequency, but metal props have less inherent damping than wood or composite. (Keep in mind these 0-200 4 cyl opposed engines were basically designed in the 1930-1940's and were designed for wooden props and originally designed with 65-85 hp. To get 100 hp, camshafts, valve timing and other things were changed---much of these may affect resonance. And, I suppose the magnetos have had a bit of "value engineering".
For sure--avoid flying a vibrating plane and keep an close eye on those magneto drives and engine mounts. (If mags were known for their reliability, the practice of having two might not have been as common.) But, remember that often resonance produces excess stress in a component, but not a noticeable vibration.
Here is a post I shared in the Cubcrafters Forums:
"First, sound decisions are almost always based on inadequate information and probabilities of less than 100%. That said..facts..
1. My observation (other people's experience) is that mags (Bendix and Slick) have a high probability of requiring significant maintenance over a 500 hour run. Seems prudent to remove/disassemble them for a serious and careful visual inspection at 500 hours--maybe even more often. This inspection should probably include drive gears.
2. All rotating machinery is subject to "harmonic resonance". Every rotating machine and component has a "critical" speed or "resonant frequency" where stress increases to a level significantly higher than desired for long life. Most machines are designed to be operated "under-critical" or at a speed (RPM) below "critical". In the early days of aviation, it was the engine crankshafts that failed! (Read about the Graf Zeppelin that lost four of their five engines due to harmonic resonance. As a nerdy engineer, I find technical articles to be interesting--in the past few weeks I have spent considerable time reading the literature regarding torsional vibration.) Critical RPM is increased by a reduction in mass or an increase in stiffness. So, a mag with less inertial mass will have a higher critical RPM for "resonance".
Conclusion: The Bendix mags are heavier with more inertial mass and probably less damping. Changing to the lighter Slick mags probably increases the "critical" RPM of the mags to a level outside normal operating range and maybe even less stress overall from torsional vibration. The Sensenich 2EK/C72AE composite prop probably changed the torsional vibration input to the mag drive to match the "critical" or resonant frequency of the Bendix mags at certain RPM's. Relatively continuous periods of "cruise" operation at this critical RPM where stress was above normal led to premature failure of the mag drive---with some drives failing earlier due to metallurgy or machining at the low end, or outside of the spec. (Important to consider that Continental's Recommended Max Cruise RPM for a 200-A is 2500 RPM.) The only indication of potential failure or "early warning" would be thru a visual inspection of the drive gears looking for fretting or an actual crack in a drive gear or shaft. Bendix mags with drives at the higher (stronger) range of metallurgical and machining tolerances and or planes operated at "under-critical" or seldom at critical speed have survived--so far--and a certain population have a probability of a very long life--especially if operated at lower RPM. Interestingly, my plane has operated with the Sensenich prop and Bendix mags for more than 470 hours, but at lower RPM since it has used the 52 pitch cartridge since 2009. (Redline RPM on these little engines over the years may be as much related to harmonics and critical resonance as it is to valve train and connecting rod life.)
3. TCM's SB643C (July 2017) with the recommendation to overhaui or replace pre-2015 Bendix mags after five years sends a signal that there is a higher than acceptable (to me) failure risk of a certain group of their mags. These recommendations are almost always a compromise joint decision of engineering, marketing, and legal. One should always remember that although "recommendations" may not carry a legal obligation for the pilot to comply--a manufacturer essentially creates a legal defense and avoidance of liability in the pilot's "failure to comply with recommendations".
4. The nature of a mag drive failure is different from an electrical failure. A broken drive gear tooth can damage the crank cluster gear and lead to complete engine failure--having a second functioning mag might not be any benefit. Yikes! While I practice engine off, dead stick landings and try always try to be prepared---I hope that I never have to actually experience a complete engine loss of power.
5. Other pilots have had good experience continuing use of the Sensenich composite prop after changing to Slick mags--even when running the 48 cartridge with higher RPM's.
Decision: A) Remove and replace my 2007 Bendix mags at 475-500 hours (this year's annual). Even if no signs of drive gear fretting are evident, the probability of possible failure is too high for me. The reduction of risk is worth the $ cost--to me. B) Carefully examine the 36066 drive gears (visually and magnaflux or other crack checking). These gears are quite expensive---the TCM (655845) ones around $800, where Superior PMA "will fit" ones, SA36066 being priced just under $500 each. SA36066 C) Replace the mags with new Slick mags--new harness and new plugs. (I considered buying new post 2015 Bendix mags---but though I guess they are "stronger" than the "questionable" batch, they are still heavier and likely have a very similar critical speed issue. I think the probability is high that the Slick mags are "under-critical" if engine RPM is maintained below 2700, meaning I can use the awesome for climb 48 cartridge with the Sensenich propeller. (I will still "cruise" in the 2200 to 2400 RPM range.)
Note: IMHO, It may very well be that Bendix mags "in general" are more "robust" than Slick mags in most applications (Just generally heavier and more "meaty"--but in the unique case of the 2EK/C72AE prop in a low and slow plane like the CC11-100 running a 0-200-A, they are not the ideal. "
BTW--Cubcrafters has confirmed there is no MRA required to replace the Bendix mags with Slicks.
Follow up as of 6/19/2019
Just some feedback that may be of help to others.. at 456 hours on my S/N 28 CC11-100 Sport Cub, I went ahead and replaced the Bendix mags with new Slicks--ordered from QAA. The A&P was concerned about axial end play and found no spec in the Continental Overhaul Manual's Table of Limits. Many phone calls to QAA, Slick, Cubcrafters and Continental and much discussion later--the axial end play is supposed to be around 0.020" and the drive gear used for the Bendix mag is the same as to be used on the Slicks. End play is "necessary" to insure proper operation of the impulse drive. Still---according to Continental Tech Support--this "necessary end play" is not written in the Overhaul Manual or other bulletin that they were aware of---Joe Logie at Slick turned out to be the best source of info. Seems this info is "known to many" but not to all.
A careful inspection of the mag drive gears and the mating engine mounted gear showed no wear pattern and no spalling--looked almost new. (My guess is that my plane has spent most of it's life with the 52 cartridge in the Sensenich Ground Adj Prop and RPM never exceeds 2500... lower RPM...less harmonic resonance--or none.)
Next came some concern over the hold down clamps---the ones from Slick are a bit larger than ones used on older 0-200-A engines. The new ones from Slick hung over the edge of the machined boss on the engine. Solution, used some older ones--slightly smaller that fit the machined boss perfectly.
Mag change part of 500 hour Mag Inspection/Overhaul which coincided with my 2019 Annual.
Plane had a small vibration at 1900 RPM all during 2018. Thought it was perhaps prop related (There has been some discussion in the forums.) but turns out the right lower engine mount "attenuator" had failed--cracked in half. (This is the silicone colored part that slips over the spacer inside the mount assembly.) Both lower mounts had sagged. We replaced all the engine mount "soft" parts with new items and using new AN bolts and nuts--reused other metal parts.
I had planned to change from the 52 cartridge to the 50, but decided to run with the Slick mags leaving the prop alone to see if there are any performance differences. (I don't expect any.) The plane takes off in a very short distance here at 600 MSL in Central Pennsylvania and is overall a great performer (has performance exhaust) so I may stay with the 52.
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