Insane impulse coupling issues AGAIN, seeking theories

[Ryan ORL]

31 minutes ago, EricJ said:

Which shop at KRAP is working on it?   KRAP is hometown for me and was home base way back in the day.    I know a little about some of the current operators and have had some experience from various trips home over the years, although nothing too recent.

I wouldn't assume an IRAN is necessary, fwiw.

And +1 to get a better ignition on there.    I *think* there are Bendix mags approved for that installation, which I think would be a good choice.   That said, also getting a non-QAA better Slick model on there would be good, too.

I'm assuming the right mag has no impulse coupler?    One thing to double check is that the right mag is cold (grounded p-lead) during start.   Kickback due to a hot mag at high advance can do all kinds of damage, including fubaring the impulse coupler on the other mag.    Have you witnessed any kickback during start?

 

It's at the mx shop attached to WestJet Air Center.

They are reluctant to do anything with it without an engine shop looking at it for liability reasons and honestly I don't blame them.  I think the engine would have to come off anyway to open the accessory case on my J.

The right mag is indeed no impulse and was already replaced with a brand new Champion Slick, and is no longer a QAA mag.  Unfortunately the left mag was a QAA Slick, and the only reason I had left it that way was because, at the time (no longer), the engine was under warranty.  I definitely will be going SureFly.  In the meantime, I think we will be doing a brand new Champion Slick on there.  I need to discuss w/ the engine shops whether they can do it with the SureFly from the start.

Never had any kickback during start.

[EricJ]

28 minutes ago, Ryan ORL said:

It's at the mx shop attached to WestJet Air Center.

That's good.

28 minutes ago, Ryan ORL said:

They are reluctant to do anything with it without an engine shop looking at it for liability reasons and honestly I don't blame them.  I think the engine would have to come off anyway to open the accessory case on my J.

This might be a case where Savvy might be able to help.

[Ryan ORL]

Savvy seems to agree in this case.  There's a lot of large metal fragments missing into the engine.  They were onboard with a simple repair if the pieces could be accounted for, but unfortunately not.  In fact, when the magneto was removed, there was a mass of metal dust/sludge that can only be described as the consistency of toothpaste.  Plus presumably lots more that went down into the accessory case.  And the gears on the engine side did sustain some minor damage.

In principle it needs just a look through of the accessory case and the sump flushed out, but I don't think anybody is signing their name to such a thing under the circumstances.

[Fritz1]

Sounds like the accessory case needs to come off at least, then gear on engine side can be checked for defects and whether it is the right part, debris will be found in sump and in suction screen and in oil filter, then a determination can be made whether engine needs to be torn down or not, time to consult with your favorite engine shop, glad you got on the ground safely, curious what caused the mayhem

[Ryan ORL]

1 hour ago, Fritz1 said:

Sounds like the accessory case needs to come off at least, then gear on engine side can be checked for defects and whether it is the right part, debris will be found in sump and in suction screen and in oil filter, then a determination can be made whether engine needs to be torn down or not, time to consult with your favorite engine shop, glad you got on the ground safely, curious what caused the mayhem

The impulse coupling locked up at speed and tore itself apart.  How this occurred exactly is still a bit of a mystery, but a cursory examination of the internals revealed that both rivets on the pawls (the hinge points I guess?) were sheared off, with one of the holes being elongated.  From my reading, loosening or failure of these rivets can allow the pawls to shift outwards slightly and grab onto the catch pin when they should be held in by centripetal force.  This occurring at normal engine speed results in the damage you see here.

The scary thing here is, given this happened well under the 500 hour inspection interval, it is not something that can be inspected reasonably at annual or any other way without disassembly.  And this can in theory fail in flight.

[Ryan ORL]

This is the best explanation of my failure mode I have seen

https://youtu.be/Pw0FOB54tV0?si=b45tvQVACVpwxqqf

(b) Pawl rivet wear.

 The 500hr inspection involves checking the pawl rivets for wear. Excessive wear allows the heel end of the pawl to start contacting the stop pin which will eventually cause the latching end to be thrown out. Centrifugal force at normal engine speeds keeps the latching end clear of the stop pin but when this fault occurs the sudden engagement with the stop pin will destroy the coupling which may then shed broken metal into the engine accessory case. This will likely result in both magnetos being put out of action plus further damage to the engine. Such an occurrence in a single engine aircraft will leave you looking for a suitable field in a hurry.

[Paul Thomas]

5 hours ago, Ryan ORL said:

Pretty good view of the damage here, definitely needs replacement.  Nothing much was visible on the engine side gear, just some small chipping.

The biggest issue is the missing ear of the impulse coupling, which is a big chunk!

Sorry, I mean on the engine side. A loose or wonky (technical term) gear could be a culprit.

I'm looking at the flange and surprised that the material is all the same color; unless those piece separated a while ago, I would have expected darker where it first cracked and a lighter color where the rest of the flange finally came apart. In this case, you have the opposite where it's almost all dark, except for a small piece.

[Ryan ORL]

Unfortunately I didn’t get a photo of that side, but it visually looked mostly ok.  Engine is definitely going for IRAN at this point so maybe we will find something more…

[Ryan ORL]

As for the color of the cracked material, I believe it probably wasn’t cracked at all until it finally destroyed itself in one violent event.

The persistent failing I believe was the pawl rivets wearing and elongating their hole.  And probably the pawls themselves were wearing some on the “heel” end.  Some of that’s visible in the photos.

Once it finally caught at running engine speed, kaboom.

[GeeBee]

Pawl rivets was the progenitor of a crash of an Aero Commander at South Lake Tahoe. It left the owner of Rosedin Electric a well known Bay Area electrical contractor and philanthropist a paraplegic. It resulted at the time (1967) the largest aviation tort liability in award in history (11 million). Small thing, results in big disaster.

Good job recognizing that something was not right. 

[Paul Thomas]

If the failure of the mag can be shown, would the insurance cover the engine's IRAN? It may be worth digging into the policy.

[Fritz1]

This has sent me thinking, I have a 6000 series Slick impulse coupling mag on the Bravo, hot starts were always a problem before I installed a slick start mag booster, I have had a retard breaker mag overhauled by Aircraft Accessories of Oklahoma, not QAA, on the shelf for probably 3 years, just need to rewire the ignition switch for a second P-lead, had an Electroair for about 2 years and uninstalled it for multiple reasons, hot start on the impluse coupling mag even with the slick booster is still not great because the skytech light weight starter spins the hot engine fast enough for the impulse coupling not to engage, will get rid of the impulse coupling now and install retard breaker mag.

[Ryan ORL]

Well fortunately a few engine shops responded quickly and the prices weren’t as bad as feared.  Western Skyways is going to handle the IRAN this time.

[Ryan ORL]

12 hours ago, Fritz1 said:

This has sent me thinking, I have a 6000 series Slick impulse coupling mag on the Bravo, hot starts were always a problem before I installed a slick start mag booster, I have had a retard breaker mag overhauled by Aircraft Accessories of Oklahoma, not QAA, on the shelf for probably 3 years, just need to rewire the ignition switch for a second P-lead, had an Electroair for about 2 years and uninstalled it for multiple reasons, hot start on the impluse coupling mag even with the slick booster is still not great because the skytech light weight starter spins the hot engine fast enough for the impulse coupling not to engage, will get rid of the impulse coupling now and install retard breaker mag.

I have the Slick start unit also, and a Skytec starter… not sure whether it turns over that fast, never considered it.  Anyway, the Slick start is great for hot starts… I do wonder if it’s somehow involved, in that you can get the engine to start catching when ordinarily it wouldn’t (and therefore you can have the impulse engaged, then unengaged, then engaged again in succession if it’s a bit unstable before it finally gets going fully)

[Fritz1]

The Slick Start lights the hot engine off even without the impulse coupling engaged firing at 20 degrees BTC, think the threshold is 120 RPM, the Slick Start solves the mags biggest problem, i.e. low energy at low rpm, best used with retard breaker mag, at cruise rpm the the ignition energy in Joule for the mag is within 10% of the electroair, weight of both systems is about the same, both have their pros and cons