Bendix SN621 Bearing Failure

lowNslow · Post by **lowNslow** » Wed Dec 13, 2006 5:21 pm

gahorn wrote: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.

And when your sitting in a flaming pile of wreckage this is important because.....

Bill Hart · Post by **Bill Hart** » Wed Dec 13, 2006 5:27 pm

gahorn wrote: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.

A little off subject but was he trying to indicate that both engines were producing power at the time of the accident or that even though all 4 mags were producing sparks they were not producing enough sparks to keep the engines producing enough power to keep the airplane in flight?

Post by **Bruce Fenstermacher** » Wed Dec 13, 2006 6:06 pm

Haven't made a final decision what to do with my mag situation. But I'm learning a lot about mags. One thing I know is no mag if it's not turning will produce a spark.

Mine froze. Had I been in the air the frozen mag gear might have taken out the common cam gear stopping the other mag or the cam or both. it wouldn't have been a pretty picture.

I'm surprise now that I'm thinking about it that the mags don't have some coupling designed to break like a vacuun pump when a jam occures. Maybe they do I'm just missing it in the design and mine didn't appear to work.

As it is when the bearing failed parts of it jammed the rotating magnet and probably due to the slow rotation of the idling engine and quick response to shut of the engine just bent the mag shaft and gear out of the way of the cam gear.

Most of the Bendix parts are reasonably priced. The two exceptions are the the parts I need the rotating magnets/drive shaft and the mag gear. The magnets list at over $800 and I've found one new for $459. the mag gear is $479 from Spruce.

I've got a spare mag gear but the rotating magnet is another story. Seems an 1994 AD made most of them produced earlier than that unairworthy. I have 5 examples of these magnets that I can't use.

I have a friend who claims to have a good rotating magnet but if that doesn't turn up I'll probably be going with new Slicks which will cost $1200

Besides the compelling story George just told about Bendix mags,

can anyone think of a reason to keep them?

One other thing. I think I'll be servicing the bearings and other mysterious parts inside my mags about every 250 hours regardless of how well their working from now on.

blueldr · Post by **blueldr** » Thu Dec 14, 2006 3:49 am

BRUCE,

If you're looking for opinions on magnetos, I consider one Bendix magneto worth six Slicks.

Post by **Bruce Fenstermacher** » Thu Dec 14, 2006 4:06 am

BL

Would you elaborate on that a little.

jrenwick · Post by **jrenwick** » Thu Dec 14, 2006 11:24 am

I know two great mechanics, one who likes Slicks, and another who likes Bendix. I think it's like Chevy vs. Ford trucks. But the Bendix guy says Bendix mags, when in tip-top condition, will throw a longer spark, as in, when it's out of the airplane with the harness cap off, it can spark all the way from the contact, around the insulating tower, to the case. The Bendix guy also thinks Slicks are more prone to early failure, whereas he can repair a Bendix and keep it running well for 500 hours. I don't know of any real reliability statistics, so that's just one guy's opinion, based on anecdotes.

On the other hand, I guess Slicks are priced such that you just replace them with new ones after 500 hours, and I can see some point to that -- starting out with new mags each time, rather than trusting somebody's rebuild.

I really like the Bendix' hot sparks on my J3; it makes hand-propping very easy. It had Slicks on an A65 before the present C90 with Bendix, and right now I'd say it starts easier than it ever has. I prop from behind with one arm, and you really don't have to put any oomph into it at all. Just pull the prop through until the mags click and it will start.

Best Regards,

John

N1478D · Post by **N1478D** » Thu Dec 14, 2006 5:57 pm

Light Plane Maintenance states that it is not good for the mags to be fired when not hooked up to the harness/plugs. They can very easily arc on the inside.

jrenwick · Post by **jrenwick** » Thu Dec 14, 2006 6:00 pm

Thanks! I didn't know that.

John

N1478D · Post by **N1478D** » Thu Dec 14, 2006 6:21 pm

You are welcome,

I didn't know it either, and like many people I have seen, enjoyed twisting the gear and feeling the impulse coupling wind up, etc.

I don't do it anymore, and don't hand a mag to somebody who might do that.

That least path of resistance stuff apparently can really cause some damage and cause a mag to fail sooner than it would normally.

George can probably fill in the why's and how's.

Light Plane Maintenance for years has been the most enjoyable and usefull subscription that I have. I save the articles that relate to the 170 for reference so I don't have to remember all of the technical info that they provide.

Like it has been stated here before, a GREAT mechanic might be a little less knowledgeable about electrical matters, for instance, than other skills. My inspector has twisted my mags when they were off of the airplane before, and I have seen other mechanics do it too.

Post by **GAHorn** » Thu Dec 14, 2006 6:27 pm

Bill: The point was (I believe) that the Bendix mags are so dependable that they will continue to work despite extensive damage, and both engines were producing power at the time of the crash. (This occured last Sunday at Waco, TX. The aircraft descended below glidepath and struck the ground short of the airport and burned. Low altitude alerts were issued to the aircraft, but perhaps too late.)

I used to be a Slick fan. Over the years I've come to appreciate the more durable construction of Bendix. There's a reason they're more expensive.... they're better constructed with higher-quality parts. If you absolutely intend to overhaul/replace mags at their recommended TBO's...then it probably makes little difference which you have. But be aware that the Bendix are usually repairable when the same problems in a Slick make it a total loss.
And if you're a "restoration nazi", be aware that certain engine dash-models specify Bendix and others specify Slick. (The O-300-C was the first originally equipped with Slick, most earlier models, Bendix. Everyone was beginning to shop price. Some models of Slick are even described as "throw away" magnetos. There are some who argue that Slicks are better because they have fewer AD's against them. Bendix-lovers reply, that's because Slicks don't last long enough to be repairable.)

cfiatzph · Post by **cfiatzph** » Thu Dec 14, 2006 6:44 pm

Go with slicks. During the overhaul on the -D for the '54 170 we Iran'd the Bendix mags. The C-172 '56 -A had slicks installed. One had a zero time engine (the 170) and the other had a 35 year old overhaul with 1000 hours on it. Slicks won out without a doubt, much better mag check drops and differences, same timing. I had a friend that had a -200 that sat for a while in 150. They ferried the a/c home changed everything out rebuilt the mags did'nt do anything to the coils. Airplane had a engine failure 4-5 times before they were able to figure out that when the mags got hot one would drop out without notice and then the other would follow. Replaced the mags with new slicks never a prob again. Don't try to save money on something that is in the 4 cycles of a engine. IE cylinders/carbs/igntion/plugs. As it looks for the price of those coils you might as well by some slicks.

Thanks

Post by **GAHorn** » Thu Dec 14, 2006 6:50 pm

cfiatzph wrote:Go with slicks. During the overhaul on the -D for the '54 170 we Iran'd the Bendix mags. The C-172 '56 -A had slicks installed. One had a zero time engine (the 170) and the other had a 35 year old overhaul with 1000 hours on it. Slicks won out without a doubt, much better mag check drops and differences, same timing. I had a friend that had a -200 that sat for a while in 150. They ferried the a/c home changed everything out rebuilt the mags did'nt do anything to the coils. Airplane had a engine failure 4-5 times before they were able to figure out that when the mags got hot one would drop out without notice and then the other would follow. Replaced the mags with new slicks never a prob again. Don't try to save money on something that is in the 4 cycles of a engine. IE cylinders/carbs/igntion/plugs. As it looks for the price of those coils you might as well by some slicks.

Thanks

Comparing mag checks on different engines in order to evaluate mag mfr'ing differences is not a valid comparison. Nor is a coil that fails due to heat/cold. Defective coils commonly fail in such a manner, without regard to mfr. "Don't try to save money on something that is in the 4 cycles of a engine." would imply that Bendix is the way to go.

I've owned airplanes with both types of mags and never had a problem with a Bendix. I've had nylon gear failures in Slicks, but they were easily repairable....at $500 each, including the labor. I could have bought replacements for that kind of money. This is likely a Ford/Cadillac thing.

blueldr · Post by **blueldr** » Thu Dec 14, 2006 7:18 pm

My experience on a magneto run in testing machine is that a Bendix will come in at a lower RPM and fire a hotter spark.

N1478D · Post by **N1478D** » Thu Dec 14, 2006 8:03 pm

Slick would have to offer enough free spark plugs and harnesses so that I could sell the plugs & harnesses for enough money to buy new Bendix mags before I would go the Slick way. Have heard that they demand the Bendix mags on trade. Even then, I would miss the original Bendix mags with the older consecutive serial numbers.

I really enjoy and like the Bendix mags, they are tough, well built, lots of info about the Bendix mags is available free on the web, it is straightforward to time them, and they look good.

Like other time sensitive accessories, it is not fun to spend the money every 500 hours, but it makes good cents!(pun intended) The two mags are completely independent of each other except for the drive gear in the case. Other than that gear, how in the world could one mag be influenced by the other mag to make it fail?

jrenwick · Post by **jrenwick** » Thu Dec 14, 2006 8:39 pm

N1478D wrote:...That least path of resistance stuff apparently can really cause some damage and cause a mag to fail sooner than it would normally.

This implies to me that it would be just as hard on a magneto to pull a plug from a cylinder, leave it connected to the mag, and see how far away from the engine you can hold it and still get a spark. Or a similar trick with just the plug wire would be just as bad. In other words, there is no safe way to test a magneto by making it throw a very long spark, because if there's an internal path to ground with less resistance, it will take that instead.

John