The International Cessna^® 170 Association

I am hearing conflicting words of wisdom about magentos on my A/C

Subject is 54 C170B s n 24633 w Cont. 0-300 engine.
Magnetos are Bendix S6LN-21 - 10-151365-39. Less than 400 hrs on mags.

They are performing perfectly. What is prompting my call to action is the knowledge of two acquaintances that had total electrical failure 5-10 minutes after takeoff . Both pilots and aircraft in top condition. Suddenly became quiet. No serious injuries - walked away. One was Piper Arrow and other Cessna 210. Bendix mags on both I believe. Mag checks performed by pilots were O.K.

Maybe I am reading too much but detailed follow up by FAA repeatly comes up w " post crash check of both mag and they supplied spark"
However read a little futher an you begin to read " when bench check mags were brought up to 140 degrees F. they began to exhibit......... failure."

I find this alarming esp when I remember that FAA has mailed me warning notices about vaccuum pumps (I don't have one ) but never advised that the MAG CHECK pre take off is not inclusive about the true condition.

After realizing that for 25 years I was ignorant about the 140 degrees heat / mag failure I became a little steamed.

Now the actual question. Are the mags on my C170 the same on both sides of engine? One tech (ACS ) says so and that a "R" on mag means rebuilt. An IA/Ap once stated that mag would have "R" stamped to mean that it rotates diff from other MAG and should be installed on Right side of engine. What is wrong with this picture?

Rebuilt mag is 587.00 Plus 190.00 Core - Order now w delivery not before MAR 2008. He did ask me to purchase outright NEW and I said outright NO.

First what has happened to other aircraft may have nothing to do with your aircraft unless they had exact the same magneto model as you do and I'm pretty certain an Arrow and a 210 do not have the same magnetos.

As for the temperature thing I've not heard of a temperature at which a mag is to be checked or will fail. In the case of these magnetos perhaps the coils failed when they heated up.

In general we have two magnetos on aircraft for one reason and that is redundancy. If is highly unusual for both magnetos to fail at the same time and that is what it would take to stop an engine. One mag failure won't do it unless the failed magneto in some why jams or falls apart causing other engine damage which causes the engine to fail catastrophically.

Your Bendix S6LN-21 mags are as good as any other mag you could install assuming they are in good mechanical order. This requires periodic maintenance and inspection.

Bendix SB 586A says to overhaul the magneto at engine overhaul or 4 years which ever is first. The 4 years may be a bit short and engine overhaul is way to long in my opinion. I would recommend these inspections take place about every 250 hours as I had a S6LN-21 bearing fail at about the 300 hours from overhaul but this is just me. I would not exceed 500 hours in any case without having a looking inside.

Both of your magnetos should be identical. They both turn to the left as indicated by the "L" in S6LN-21. They can be installed on either side of the engine.

There have been a few ADs against these magnetos over the years and one of them was to replace the rotating magnets. I seem to remember the mags were to be stamped with an "R" when this was accomplished.

So bottom line if your mags are inspected and serviced routinely by people competent to do so they should cause you no more worry that any other part of your engine.

In my non humble opinion, all magnetos should go to a magneto shop for IRAN (Inspect and repair as necessary) at approximately 500 hour intervals. This includes the impulse couplings. For airplanes that do not fly regularly, perhaps a five year interval.

sometimes old mag coils fail when they get hot. shouldn't happen to two mags at once though. I second Bluldr's "non-humble" opinion. have your mags opened up and IRANed every 500 hrs.

it isn't a very tough job, make sure your mechanic has the mag service manual and proper tools for the job (he's illegal if he doesn't) and don't take it to the dumba** that didn't know what the "R" was for

I love it when such competent and knowlegable replies such as Bruce's, Blu Elder's, and HA's are given so cheerfully and freely here. This re-affirms the incredible value of the forums, and the great people who visit here! (And to think that we used to get criticized for spending the money to support the forums.)

If you switch from Bendix to Slick you can get two NEW mags, NEW harness and NEW spark plugs for about $1200.

My IA has had 2 forced landings due too Bendix mags failing when hot after a few minutes into a flight. Go with Slick more trouble free and lighter.

Like Karl said if you would decide to replace your mags for any reason going Slick is good alternative. This is what I've done on both my Cub and 170.

I did not do it because I thought the Bendix was inferior or that the Slicks were is some way better. It came down to new verses rebuild with new harness and plugs verses just rebuild mags for nearly the same price. I say nearly because converting to Slicks will take more time than just replacing the Bendix mags. Time is money. If you could use a new harness and plugs the Slick offer is hard to beat.

Yes mags fail and there are lots of reasons. A bad coil which may only show itself when hot is one of them. Again that is why we have two magnetos.

They don't run for ever. Most often mags will tell you when they're getting tired and most often the signs are over looked till the second one starts to fail and the engine stops. I've seen it happen as well and the pilot couldn't believe the mags only lasted 1400 hours with NO service.

Routine maintenance and inspection is the key.

Two things I believe about converting from Bendix to Slick mags, and someone please correct me if my thinking is messed up!

1. Slick offers a bargain price if you can give them Bendix cores. This way they can take many of the Bendix mags off the used/rebuilt market. Once you've converted to Slicks, that bargain deal for new mags/harnesses/plugs is no longer available to you.

2. You replace Slicks with new units every 500 hours, whereas Bendix mags can be rebuilt. (I absolutely agree that 500 is "TBH" for magnetos, and it can be dangerous to run them much past that.) Replacing Slicks vs. having your Bendix mags overhauled might not be a large difference in cost, so it becomes a philosophical matter, of replacement vs. overhaul -- whether you have more faith in the factory or your mechanic (which might be yourself).

As far as I know, Bendix and Slick offer mags of equivalent quality, reliability, etc. -- however my own mechanic, and others, say that Bendix mags will throw a longer, hotter spark. In my J3 that I hand-prop in winter and summer, on water, on ice, snow and on land, this is important to me; so I'll keep using the Bendix mags on the Cub.

I'd appreciate your knowledgeable feedback on these opinions, because mags always seem a bit of a black art to me. Thanks!

John,
I was of the same thought as you. But slick mags can be rebuilt. altho it costs less to get new mags than it costs to have them rebuilt! The 500 hr is just a referance point, just as brought up before. 500 Hrs or five years is the way I look at mags. when I worked at a university the mags were changed at 600 Hr. intervals. over the 500 hr idea, but usualy under the 5 year (we usualy run the aircraft 600 hrs in three years) Inactivity is usualy the reason for anything to fail (mags, cylinders,and instruments) Keep flying and keep things used! that is the best way to twart bad stuf.
something I forgot to mention...we timed the intervals so that no mag was over 300 hrs old on a single engine .....unless it was a bendix 3000 mag... they were done at 500 hrs no mater what.(as per AD)
brad

John

Slicks are just as rebuildable as Bendix. Slick at one time about 30 years ago made a mag that was not rebuildable. The idea was it was a through away mag. Problem was people ran them 1000 hours and found out they could not be rebuilt and were not happy as rebuildable mags were the standard. It was a marketing blunder for Slick that they still have not lived down though these model of unrebuildable mags haven't been available for nearly 40 years.

Doesn't matter whether you have Slicks or Bendix depending on what you need at inspection time buying overhauled units might be more economical.

It has been my experience that there are more Bendix parts available in the field than Slick. Probably due to greater market share that Bendix once enjoyed. I doubt if it's because Bendix break more.

It is true that the best deal on Slicks is with the rebate that they give for competitors magnetos and harness. This does not have to be Bendix mags but most often it is. I traded in two Case mags and two Bendix mags that were very old and unairworthy .

I to have heard Bendix throws a hotter spark. Don't know if this is true or a wives tale. What I can tell you is that my Eismann mags could start my hand propped Cub at a slower rpm or so it seems. I have about 150 hours now hand propping my Slick mags on both my A65 and C85 and can tell you I'm satisfied with their performance and starting ability. I have seen absolutely no difference between the Bendix and Slick on my 170 with the fancy electric starter.

Thanks, Brad and Bruce, this is interesting.

I work with two mechanics, one of whom clearly favors Bendix, and the other likes Slicks. These guys are both first-rate in my opinion, which leads me to think that either brand is fine.

I've used both on my J3, and to be honest, the only problem I've had is with the impulse coupling on a Bendix. Since putting the Cub on floats, I really want something that will start the engine reliably. Once you've cast off from the dock in a breeze, you're at the mercy of those mags!

Brad, you seem to be saying that it's more expensive to overhaul a Slick than to buy a new one; or is it more accurate to say "to have a Slick overhauled"? Either way, if true, it means that Slick is encouraging owners to buy new units rather than have their run-out ones overhauled.

Bendix Mags----Slick Mags

Fords---- Chevies

Personally I'd rather have a loathsome social disease than a Chevie or a pair of Slick Mags.

Really BL.

Siding with the Fords and Bendix one might think you already have a social disease.

I think you need to check on what Bendix is doing for deals. Last fall they where running a deal with new mags and harness and the next set down the road for a fair price. I can't remember the price. I have never been a fan of Slicks. I have seen just as many of them go bad as Bendix....