Maintenance questions

There are ADs against many mags, some of them dealing with
impulse failures. Get the mechanic to check for any that apply to this
mag.
Corrosion and sludging often cause sticky impulse couplings.
Neither are good, especially the corrosion, which can cause spring
breakage and power loss. Mags on infrequently-flown airplanes, or those
mags that haven't been off for a long time, definitely need checking.

Dan

All the AD's are complied with. I use Aeroshell 15W - 50 semi-synthetic oil, changed every 50 hours and will be
surprised if things are gunked up or corroded but I might get shocked. I put over 100 hours on the Cherokee this
year including the 2000 nm cross country trip to Canada and back. I've averaged somewhere between 80 to 120 hours
per year on her for the 7 years I've owned her and have installed rebuilt mags once. We are going to pull the mag in
question and take a closer look. Thanks for confirming what my conscience was already subtly telling me to do.



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I would be a bit concerned on that. There are AD's against some mags' impulse
couplings (like mine... O-360 with Bendix). Basically, the little tabbies that are
supposed to catch and load the spring get worn. Also, there is a bad type of rivet
that holds them on the impulse coupling. If the pivot point on the catch tabs are
worn, it could be binding enough once in awhile so gravity won't drop them where they
need to catch. If these couplings fail and fall apart, they'll drop a bunch of little
steel pieces in the accessory housing and likely jam up the gears. Since important
things like the camshaft run on those, it could ruin your day.

Even if they aren't the type with ADs on them, I'd take the mags off an have a
looksee. It doesn't take that long to take them off and look. Also, it's typical for
only one mag on the Cherokees to have a impulse coupling. Does yours have one on
both?

All AD's are complied with on my airplane. I just have the one impulse coupling on the left mag. We'll pull the mag
and look hard at the area you describe. Thanks Cory.



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JJS jschneider@remove socks cebridge.net wrote:
: Even if they aren't the type with ADs on them, I'd take the mags off an have a
: looksee. It doesn't take that long to take them off and look. Also, it's typical for
: only one mag on the Cherokees to have a impulse coupling. Does yours have one on
: both?

: All AD's are complied with on my airplane. I just have the one impulse coupling on the left mag. We'll pull the mag
: and look hard at the area you describe. Thanks Cory.

I thought that too. My IA is particularly thorough on finding ADs on not only
the aircraft, but all the accessories, too.

IIRC, the AD against the Bendix mag impulse coupling is just to look for wear
on the catch pin, not the pivoting teeth and their associated pivot pin. What you
describe sounds like it could be that pivot point binding somehow... not part of the
AD (again, IIRC).

-Cory

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

Given the possible consequences, I would pull both mags and have then
looked over. Why? As you said, you risk a failure away from home at an
inconvenient place and time (at night, on Sunday, on a field with no FBO).

Why both? If you have another year to go before overhaul time (which I
trust you are doing every 500 hours), and one is flaky, there is a
chance that whoever did them last time was less that perfect.

But, that's just me. I take both Slicks in every 400-500 hours. We
usually wind up replacing one and overhauling the other. The next time
around, the new one gets overhauled and the overhauled one gets
replaced. Usually costs around $550 or so. Cheap piece of mind for me.

I have no experience with Bendix mags. So, if you have them, others will
need to chime in.

Good Luck,
Mike

He noticed that the impulse coupler on the left mag wasn't
clicking. He turned it a little faster and it works fine. Tried it slow again and sometimes... no clicky. Since
I've had no starting problems, he suggests I just keep flying it unless it begins to give trouble. The mags won't be
due for overhaul for at least another year. My question is: Is there a slow death type "normal" failure mode in
which progressively harder starts occur or do impulse couplers usually fail suddenly and leave you stranded? I
really don't want to throw away money. On the other hand I'd rather not get stranded after dark at an airport away
from home with no mechanics around or parts to be had.

Long time ago ('82). As best I can remember, slightly unusual EGT readings
and maybe a slight roughness (Continental O-470). The next time I was
directly over an airport I reduced power and did a mag check (at 9000 feet).
One mag was dead. I switched back to both and announced that we would be
landing at that airport, where the spring got replaced.

Another time, after a fuel stop, in doing the runup, one mag was dead. That
turned out to be a defectively manufactured Bendix capacitor (terminal
crimped over insulation) that had worked for almost a year. Later, that day
in talking with my daughter (who was 10, then, and had been riding in the
right front seat on the previous leg), she mentioned that she had noticed
some slight roughness in the flight leg that preceded the failed mag check.
I agreed with her. Apparently, it had not not been pronounced enough to
lead me to do another one of those "enroute" mag checks.

"JJS" jschneider@remove socks cebridge.net wrote in message
...

"DL" wrote in message
ink.net...
If the impulse coupler spring breaks, you have, at best, a dead mag. If
you don't notice the subtlety in flight (as I once did) you will notice
it as a dead mag on your next run-up.

May I ask how you detected a broken impulse coupler spring in flight, DL?
I might learn something here.


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"DL" wrote in message nk.net...
Long time ago ('82). As best I can remember, slightly unusual EGT readings and maybe a slight roughness
(Continental O-470). The next time I was directly over an airport I reduced power and did a mag check (at 9000
feet). One mag was dead. I switched back to both and announced that we would be landing at that airport, where the
spring got replaced.

Another time, after a fuel stop, in doing the runup, one mag was dead. That turned out to be a defectively
manufactured Bendix capacitor (terminal crimped over insulation) that had worked for almost a year. Later, that
day in talking with my daughter (who was 10, then, and had been riding in the right front seat on the previous
leg), she mentioned that she had noticed some slight roughness in the flight leg that preceded the failed mag
check. I agreed with her. Apparently, it had not not been pronounced enough to lead me to do another one of those
"enroute" mag checks.

"JJS" jschneider@remove socks cebridge.net wrote in message

May I ask how you detected a broken impulse coupler spring in flight, DL? I might learn something here.

Thanks, DL. I ordered a copy of "The Magneto Book" from Sacramento Sky Ranch and should be getting it any time now.
Hopefully it will help me with some of my questions about magnetos and impulse couplers. I didn't realize a broken
impulse spring coupler would disable the mag. Obviously, I've never seen one apart. BTW the A&P IA pulled mine
after I asked him too. He found nothing amiss. All tolerances were acceptable and there was no gunk or corrosion.
He lubricated it and reinstalled it. The Cherokee is back in my hangar waiting on me to reinstall the interior. The
annual is complete except for that and the paperwork.



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