The International Cessna^® 170 Association

Flying Young Eagles today. I was waiting on the ramp for the departure line to clear. Engine at an idle I thought for the second time the first being the last flight that I was hearing the impulse coupling on a mag snapping just a little louder than I thought I'd ever heard it before. I had taken my head set of to hear it better when snap squeal something went. Because I was thinking there was a problem it took less then a second to kill the mag switch and the kill the engine.

After getting the kids in another airplane not making a big deal that we had a problem I went back to the plane. Turning the prop produced a grinding and snapping.

After the event we pulled over the prop slowly while feeling the mags to see if we could determine if it was one and which one. Definitely a problem with the left mag , there was no doubt.

Pulling the mag expecting to see the impulse coupling all screwed up but it didn't look bad. But what why is the shaft canted to one side.

The front bearing had failed.

Further examination of the mag gear shows one tooth with a chip out of it so we will at a minimum be inspecting the cam gear before we replace the mag.

After disassembly of the mag at home we noticed after splitting the mag halves that one of the tabs that aligns the half had recently sheared off.
I don't think this could have been because of another failure but I'll continue to look at it.

Anyway perhaps the tab broke off for some unknown reason sending debris into the front bearing causing it to fail but there doesn't seem to be any damage to the rear bearing which I'd think likely with this scenario

That just leaves bearing failure which then caused all the rest of the damage.

Time on this mag since overhaul by an overhaul shop is about 500 hours with about 90 since it was removed visually inspected without splitting the mag case. In other words the impulse coupling, points and overall feel of the operation of the mag where looked at.

So the question is for those experienced people out there. Have you seen Bendix mag bearing failure before? One mechanic that did see it say's he's never seen it in his career. Perhaps because had I been in the air, or not listening at an idle and able to quickly kill the engine the mag and the rest of the airplane may not be in as good of shape as it is to look at.

I thought impulse couplings only worked during start and higher rpms disengaged them?

tye

True they do operate at low rpm. I was at an idle, 700 rpm I'd think. Don't know when they cut out.

I said I though I was hearing impulse couplings not that that was actually what I was hearing. One thing that I was trying to figure out was that the beat was not consistent missing more beats than hitting. perhaps if it was actually the impulse I was just at the cut off point.

Or now that I've seen what was wrong the shaft was wobbling allowing the impulse to hit sporadically.

The impulse coupling itself does not look damaged. And I didn't mean to imply the impulse caused the problem.

I was just describing the event as I heard it.

I pulled my mags off a few weeks ago. They hadn't been looked at since new in 1983. At least nothing in the logs. The bearing on the left one was spinning in the case, making an awful mess. Other than a distributor cap and capacitors, nothing else was too far gone. Little scary about the bearing though.

I understand what you meant now. It's good you're in tune with the normal sounds of your engine. Could've been much worse. You make a good point about mag failures in flight not leaving as much to examine. Could be this is a common failure and there isn't normally enough left to examine. Is there a way to search FAA service difficulty reports for Bendix mag problems?

tye

Bruce,
It's just my opinion, but where I used to work NO! mag went more than 500 Hrs. befor beeing pulled for OH. It's not just the points or the cams but every thing else in the mag that can go wrong! Bearings are number one! Espicaly Case and Sinctilla / Bendix mags. 500 Hrs on most of the equipment I used to work on could happen in less than a year. How long has your Mags been running? Time (in the perspectave of years) is another constraint that needs to be addressed! Please let us Know how many years it has been between O/H's thanks,
brad

Well Brad I'm looking closely at the overhaul ticket and they where installed in 5/1/98 but looking close they where actually overhauled 8/21/95. Nearly 3 years in storage.

It looks like I was mistaken on the hours there are fewer with 361 hours since overhaul. This is why I didn't have them inspected further when I overhauled the engine 150 hours ago.

The grease on the rear bearing didn't look very good. After all it's been there since 1995. It is well old and stiff.

I'll be pulling the other mag for closer inspection before to long after I get it home.

Bruce, are the impulse coupling parts intact? I'm reading the Skyranch book "The Magneto Ignition System" and there is a section on the Bendix impulse coupling where they mention the effects of wear on the flyweights. Apparently they can come apart and get into the gears. They don't mention bearing damage specifically.

tye

Check out Bendix Service Bulletin No. 556C concerning Sintered Iron vs Machined Steel bushings.

Teledyne Continental Service Bulletin SB643 outlines Maintenance Intervals for those uninformed, it is 500 hours on the mags. The main reason that I posted this is because a very good friend just spent weeks chasing a fuel problem and after replacing parts, etc could not find why his engine was stumbeling on takeoff. As a last resort he sent his mags, which had over 800 hours since overhaul to the shop. You guessed it, after spending weeks working on his plane, once the rebuilt mags were in place it ran fine. His logic when I questioned him about the mags at the beginning of the process was that it couldn't be the mags, the runup showed the mags to be working perfectly.

Also, Teledyne Continental SB658 outlines distributor block bushings lubrication at or before 500 hours.

Sure am glad you caught it on the ground Bruce. Doesn't sound like anything got inside the case.

Good Luck! Hope the other mag checks out ok.

Well the plane is home after installing a loaner mag. The cam gear looked fine during inspection.

Funny even with a quick timing which wasn't as precise as I normally insist on the engine ran great. And that slight miss or roughness that we thought we were imagining seemed to disappear. Or may we figured nothing else could go wrong on the short flight.

We've pulled the other mag and will know be considering the following options I have.

1. Find a mag overhaul manual and rebuild/service the loaner mag I got and bring it up to speed as well as inspect and service the mag that didn't fail.

2. Send the whole mess to an overhauler and pay them for their expertise.

3. Replace the Bendix S6LN-21 mags with new Slick?

Slicks replacements will cost the most but I'll get new plugs and harness that I can use later.

Bendix are found just about everywhere and so are parts which is what saved me with a bad condenser (same mag) coming home from Petit Jean two years ago.

What to do what to do?

Like you guys have been indicating, a good run up does not indicate good condition of the mags. I once did an annual on a 140A a year after it was in a flood. The run up was great, but being the curious sort I measured how high the water got in the hanger and measured how high the mags were. When I pulled the mags I literally poured a full handful of water out of each mag. They also had illegal rotors and coils to boot.
Good save with the quick shutoff Bruce.

Mag "overhauls" will also include the "RE-magnetizing" of the magnets in the unit. A bench "overhaul" at home will really be just a repair.
Not that a bench repair isn't a good thing. Just pointing out the difference, and why real overhauls are not cheap.
Ask your magneto overhauler about re-magnatizing the magnets to see if he really knows/does his job.
Savage Magneto in Oakland, CA does it right. So does Hatfield in Houston.

Thanks George. I don't have an overhaul manual yet so I don't know what is required to call it an overhaul. Wish I had "repaired" or "serviced" the mags about 20 hours ago.

I need to look at the overhaul manual and review the costs of having the overhaul versus "repair" versus new Slicks.

I'm thinking that a "repair" might be in order using serviceable parts from my 361 Hrs SOH mag and a donor from my friends box of stuff.

N9149A wrote:Well Brad I'm looking closely at the overhaul ticket and they where installed in 5/1/98 but looking close they where actually overhauled 8/21/95. Nearly 3 years in storage.

It looks like I was mistaken on the hours there are fewer with 361 hours since overhaul. This is why I didn't have them inspected further when I overhauled the engine 150 hours ago.

The grease on the rear bearing didn't look very good. After all it's been there since 1995. It is well old and stiff.

I'll be pulling the other mag for closer inspection before to long after I get it home.

Gee Bruce aren't the mags (overhauled or new) on the list of mandatory replacement parts to call it an engine overhaul? Not that I've never done something like that for myself but I've become a true believer in doing all I can to see that the engine continues to get spark and fuel. My suggestion, leave them to the experts to repair or buy new Slicks.

I flew right-seater for the FSDO Inspector's recurrency ride today in the Hawker 1000 and afterwards he described his experience in a recent accident investigation of a C-310. The airplane was consumed by postcrash fire and the induction and most aluminum was consumed in the fire's intensity. Even the impulse couplings were melted.
But when the almost vaporized harness was removed and the Bendix mags were spun up with a drill, every one of them still produced sparks. The accident investigator indicated that only Bendix mags do that despite such intense damage.