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Dual mag failure SIMULTANEOUSLY!

pierre smith

pierre smith

Well Known Member
(Moderator note: because it involves equipment common to RV's, this is RV-related.)

My son has helped build several RV's during his A@P apprenticeship.

Last Tuesday, a Cirrus 22 dead sticked to my airport from 19,000' on a cross country. He tried switching tanks, fuel pump on, to no avail. Fortunately, my stepson is a Cirrus trained A@P. He came home from work and diagnosed it as DUAL mag failure!


The plastic gears had stripped on both mags. The driving gears in the accessory section operated smoothly.

He had two mags overnighted and installed them the next day. The engine fired up on the second blade, ran fine and they test flew it and the gent went home to Tampa.

The airplane has a 4 bladed composite MT prop and the thinking is that there is not much vibration damping and could be a causative factor.

The engine parameters were downloaded to a memory stick and forwarded to Cirrus for examination to try and get to the root of this, since they only had 200 hours on them and the factory rebuilt IO-550.

Amazing,
 
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Yep

The magneto repair facility definitely wanted to examine the mags.

To me, the likelihood of a dual mag failure would be akin to my chances of winning the lottery.

We don't know whether one failed 30 minutes to an hour before the second failure either. We may find out after Cirrus downloads the memory stick.

Best,
 
Oh this should be interesting to hear updates on. The school I rent from has a similar SR22 with the 4-blade MT prop. Keep me updated Pierre, if not in this thread.
 
Continental/Bendix, Dan. My stepson also told me that there's a service letter out to NOT use the gear aligning tool on those mags because it could damage the tooth and then the trouble begins.

Best,
 
What are the chances of that happening?

Pierre,

The obvious take away is that you should buy a lottery ticket, right now!

David
 
Like Dan asked are they Slicks or Bendix Mags?

I would like to think there was a single event like the mother of all backfires that nearly KO'd the gears or an install procedure that did the same.

I once witnessed an R22(Helicopter) with freshly rebuilt bendix mags that settled back on the pad as the internal plastic gears failed right after the first run up. There have been SB's over the years for bad lots of gears that should get thrown out. The mag shop had installed brand new bad gears.

If your ticket wins Pierre, you will need to share with David.
 
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Will do

Cirrus is examining the data downloads and the mag rebuilder is inspecting the failed ones as well.

I'll keep you all updated as my stepson gets the information.

BTW, does anyone on here know if the Lightspeed ignition systems ha a Continental STC?

Best,
 
pierre smith said:
...
The airplane has a 4 bladed composite MT prop and the thinking is that there is not much vibration damping and could be a causative factor.

....

Amazing,
Click to expand...

My neighbor has had three mage failures in less than 150 hours. They have been attributed to the Sensenich ground adjustable prop. He now spins a Catto and the problems have gone away.
 
S20's

Continental/Bendix S20's. The lightweight propped Cirrus's are seeing too many mag gear failures. Specifically the ones with lightweight props (Hartzel or MT composite - the MT is STC'ed the Hartzel is stock) and lightweight starters (ST-3 for example). Usually it is only one mag but I have helped replace a dual mag failure (not an in flight failure) before.

I don't think we have seen or heard the last of this.

DanH said:
Slicks or Continental/Bendix?
Click to expand...
 
More info

I am flying with a sensi ground adjustable prop. Do you have any info about these mag failures. How does he know it had anything to do with the prop? What failed in the mags. Same problem all three times. Would like to hear about this
Cm
 
N941WR said:
My neighbor has had three mage failures in less than 150 hours. They have been attributed to the Sensenich ground adjustable prop. He now spins a Catto and the problems have gone away.
Click to expand...

Anecdotally or with data? Can you please share the data?
 
Locking tool

pierre smith said:
Continental/Bendix, Dan. My stepson also told me that there's a service letter out to NOT use the gear aligning tool on those mags because it could damage the tooth and then the trouble begins.

Best,
Click to expand...

I try to avoid using the locking tool. It would be relatively easy to damage the gear with this tool. I think the main issue with the tool is people using too much force sliding the mag in place while engaging the drive gear.
 
Mags do fail for other reasons. 30 years ago, rented 172, landed at an airport with the same FBO. Plane wouldn't answer mag check, and I was told to, "Fly it home." I refused, took a 182 back, and they tried to fly the 172 the 20 miles, only to have the other mag fail on the way. No damage, thankfully. I had a single failure on my 9A. BTW, no failures so far on my E-Mags.
My guess on those failures was too many hours, too little service.

Bob
 
Slick Mags..

BTW my Cherokee 180 has "cushion blocks" on the mag. Also, when I overhauled the engine at 2460hrs it had gone through 6 mag failures in about
15 years, all Slicks. I think the cushions do help, but the failures were a great deal more than Bendix mags I've flown.
John
 
pierre smith said:
Continental/Bendix, Dan. My stepson also told me that there's a service letter out to NOT use the gear aligning tool on those mags because it could damage the tooth and then the trouble begins.

Best,
Click to expand...

Pierre,
Could you get me a link or pointer to this service letter?
Thanks.
Johan
 
Possibly we should stop confusing Bendix and Slick type mags, they are significantly different designs.
Bendix mags use a red mark on one tooth of the nylon / composite distributor gear, which when lined up in the witness hole, is the #1 firing position. The SB concerning the use of a lock tool only applies to Bendix type mags.

I have an old type brass lock tool that screwed into the witness hole. The issue with it was that when you screwed it in, it was possible for the gear teeth to get damaged if the lock tool teeth didn't line up properly.

Slick mags use a pin inserted through the back of the distributor which aligns the gear in the #1 firing position and the is no way to damage the gear teeth with the alignment tool pin. (unless you jam it in or turn the mag with it installed)

Use of "non-flexible" gear lock tools has been covered by the SB for a very long time. Most current looks are now made of flexible material that do not screw in (press in) and should not damage the gear teeth. In practice, you really don't need the lock tool, unless you bump the main drive gear out of position during installation of the mag to the engine.

TCM IO-550s (6 cylinder mags) use a two eared male drive couplings instead of gears, which fit into the female coupling with rubber dampers. The rubber dampers are supposed to reduce shock loads from the engine drive to the magneto.

It is not that uncommon for the nylon distributor gears in the magneto to get shock damage from the engine drive due to starter kick back or impulses from the engine. Vacuum pumps sometimes suffer the same fate.

I spoke with a Slick mag engineer once about using nylon / composite distributor gears and I was told that it has to be a non conducting material, so nylon seems to work the best.

That is why there is a 500 hr inspection, to inspect the gears and points etc.

The failure at 200 plus hours might be attributed to a starter kick back issue or even excessive RPM excursions or possibly faulty drive gears. I had one fail in less that 20 hrs on a Baron after a 500 hr insp due to installation error.
 
N941WR said:
My neighbor has had three mage failures in less than 150 hours. They have been attributed to the Sensenich ground adjustable prop. He now spins a Catto and the problems have gone away.
Click to expand...

What does Sensenich say about this?
How were you able to trace this back to the prop with 100% certainty out of curiosity.
 
Too many hours, tool little service

videobobk said:
Mags do fail for other reasons. 30 years ago, rented 172, landed at an airport with the same FBO. Plane wouldn't answer mag check, and I was told to, "Fly it home." I refused, took a 182 back, and they tried to fly the 172 the 20 miles, only to have the other mag fail on the way. No damage, thankfully. I had a single failure on my 9A. BTW, no failures so far on my E-Mags.
My guess on those failures was too many hours, too little service.

Bob
Click to expand...

Not necessarily. Had Slick mag fail on my RV-7 with TMX-0-360 at about 940 hours. Ordered new mag, ran very rough. Was in a hurry to get plane going so ordered another mag. This one really ran rough. Called Slick factory and got permission to open up first mag. Reset E-gap, reinstalled, ran just fine. Sent second mag back on warranty and was reimbursed. I think they may have some quality issues. Anyway so far so good.
 
Just thinking out loud, but could the common cause for this type of failure be a nasty kickback from a flooded engine? I know that is ripped started castings apart (skytech). It wouldn't surprise me if it could strip a plastic gear. But I don't have direct experience with that, and hope not acquire any.

Nice to hear that it ended uneventfully. Our engines are so reliable these days that most of us never get a forced landing so most of us don't have any experience with them. We just hope that our training was good enough and we are current enough and we are lucky enough that if it does happen it will be a situation with some options.
 
ijustwannafly said:
What does Sensenich say about this?
How were you able to trace this back to the prop with 100% certainty out of curiosity.
Click to expand...

They blamed the engine and the engine manufacturer blamed Sensenich. He gave up and bought a Catto and the problem went away.

What no one could answer is if the prop caused internal damage to his engine and no one wanted to pay for it to be disassembled, inspected, and put back together.
 
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Makes a good case for eliminating the mags and installing electronic ignition!
I went with EFII ignition only and couldn't be happier. I know that's not an option for certified birds......too bad!:eek:
 
Update

After a two hour flight with the new mags, the engine became rough and upon inspection, several teeth had been lost on the mags!

A mechanic in Florida found what he thought to be the problem. The airplane has air conditioning and the drive pulley has two rubber cushions to soften the engagement torque of the a/c compressor, when it's engaged. the rubbers are 7 years old and not soft anymore. The jarring of the accessory gears as the compressor is engaged, causes the backlash that damages the plastic mag gears!

So they went out and flew several flights without engaging the a/c compressor and examined the mag gears...no damage.

They flew a few more times and engaged/disengaged the compressor and came back and found damage to the gears in the mags. The cushioning effect of the rubbers in the drive had hardened with age and seem to be the culprit!

Even though the compressor is belt driven, the rubbers still play a big role.

Whodathunk?

Best,
 
pierre smith said:
A mechanic in Florida found what he thought to be the problem. The airplane has air conditioning and the drive pulley has two rubber cushions to soften the engagement torque of the a/c compressor, when it's engaged. the rubbers are 7 years old and not soft anymore. The jarring of the accessory gears as the compressor is engaged, causes the backlash that damages the plastic mag gears!

So they went out and flew several flights without engaging the a/c compressor and examined the mag gears...no damage.

They flew a few more times and engaged/disengaged the compressor and came back and found damage to the gears in the mags. The cushioning effect of the rubbers in the drive had hardened with age and seem to be the culprit!

Even though the compressor is belt driven, the rubbers still play a big role.

Whodathunk?

Best,
Click to expand...

The drive pulley is at the prop end of the crank, or is it an accessory drive item?

POSTSCRIPT: Found it:

http://cirrusengineering.blogspot.com/2010/11/enviro-air-conditioning-bushings.html

This photo from 2010; apparently they started out with no soft elements in the drive pulley.

http://1.bp.blogspot.com/_6NMZbtOd12I/TFrJu2INWsI/AAAAAAAAAI4/A3fxGlxHNwg/s1600/IMG_7354.JPG

Don't know if this is the latest:

http://cirrusengineering.blogspot.com/2012/03/air-conditioning-washers.html

Given that the problem has been going on four or five years, I question that this particular mag failure was caused by an AC drive bushing issue.
 
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ijustwannafly said:
What does Sensenich say about this?
How were you able to trace this back to the prop with 100% certainty out of curiosity.
Click to expand...
Somebody determined that the carbon fiber prop on the little 4 cyl Continental engines was suffering from 6th order harmonics being transmitted all the way down the crankshaft ending up causing chattering of the mag drive gears.
The little 4 cyl Continentals have a design flaw... they have no flywheel!!!
The carbon fiber prop on Lycs are not experiencing this issue because Lycs have flywheels which dampen out these high order harmonics.
 
Neal@F14 said:
Somebody determined that the carbon fiber prop on the little 4 cyl Continental engines was suffering from 6th order harmonics being transmitted all the way down the crankshaft ending up causing chattering of the mag drive gears.
The little 4 cyl Continentals have a design flaw... they have no flywheel!!!
The carbon fiber prop on Lycs are not experiencing this issue because Lycs have flywheels which dampen out these high order harmonics.
Click to expand...

The prop is the "flywheel". It has a mass moment of inertia far higher than any other rotating component.

In general, the very large prop MMI is the anchor against which the other components oscillate, so yes, the amplitude of torsional vibration tends to be worst at the accessory end of the system.

A prop is typically modeled as a spring-mass system, with the blade roots being the spring and the blades themselves being the mass. Very stiff blade roots would tend to increase oscillation amplitude at the accessory end.

The Lycoming flywheel has little or no effect on torsional vibration in the first vibratory mode, because it is located near the node, the point in the system with no oscillation. Its effect at higher orders would require analysis, but again, it is a relatively small inertia located in proximity to a huge inertia with no significant connecting spring between them, other than the blade roots. I would predict its effect to be pretty near zip.

IIRC, pendulum absorbers on Lycoming cranks are tuned to the 6th and 8th orders. I don't know about Continental cranks.
 
SDR's

I hope everyone is filling out Service Difficulty Reports. I went to a IA conference a few years ago, and this kind of equipment/matching/problems is something the FAA may be able to make manufactures figure out.

http://av-info.faa.gov/sdrx/
 
Small world

Hey Pierre
It's been a few years seeing you at the AG conventions I'm managing a facility for Prime Turbines in Wynne AR and not so much in sales now.
Who could for get you though.
I just had a 0470 mag drive gear failure 300 tso it seemed that the oh facility put the mag drive idler gear in backwards lost the rt and the left was very soon to go.
Safe landing and the 100% warranty was great.
If you need any pt6 work give me a call
PS sale that -20 while it's still running
LD Blake
 
The good news is the pilot did an excellent job handling this emergency. Goes to show, altitude becomes your best friend when the engine stops producing.
 
It was close..

It was so close that he landed on the numbers, just past a 300' embankment on the approach end!

Guys, practice and practice forced landings more. His engine stoppage was sudden and totally unexpected.

Best,
 
pierre smith said:
It was so close that he landed on the numbers, just past a 300' embankment on the approach end!

Guys, practice and practice forced landings more. His engine stoppage was sudden and totally unexpected.

Best,
Click to expand...

Agreed, Still the pilot had a Plan B. The BRS system,it's been deployed over 50 times and saved more then a few hides in Cirrus aircraft. I've always wondered why Vans didn't add one to the 14,Powerful Safety/Sales tool.
 
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