Detonation - cylinder destroyed

[Jungle Pilot]

Several weeks ago, on a 30min Encore flight, My #6 CHT went to over 600° as recorded by my JPI 711.  Unfortunately, this went unnoticed in flight until engine roughness was detected on the next takeoff which was aborted. On inspection, all cylinders had compression in the 70s except #6 which was 15. Borescope inspection showed piston damage. The cylinder and piston were returned to the engine overhaul since the engine had only 200hrs since overhaul. The #6 spark plugs all had broken center electrode insulators.  The overhaul shop is contributing toward the new cylinder and piston and says the classic detonation damage is caused by pre-ignition from “cross fire”, either inside the magneto or in the spark plug wire bundles. They recommend not tie-wrapping spark plug wires together. 
1. Has anyone heard of this?

2. If the magneto only fires every 120°. then the only possible pre-ignition crossfire would be 120° BTDC, which if it occurred during the initial compression stroke would do it?

3. Firing order is 1-6-3-2-5-4 so the crossfire pre-ignition would have to come from #1, during #6 compression stroke. Right?

thanks for any comments. 
Dave Ketcham, Mooney 252/Encore

[Jungle Pilot]

4. Why did we not see any warning message on the JPI 711?  The max CHT is set to 450°

[Cruiser]

please see page 24 of the JPI manual for alarm function. Can you upload the JPI files to Saavy Aviation and post the link here for us to see?

 

 

[A64Pilot]

I have heard of crossfire, but only inside of the mag, occurs most often I think at very high altitudes with unpressurized mags.

‘I feel sure if there was “crossfire” from one wire to another there would be ample evidence from arced wiring, you couldn’t miss it. 

[EricJ]

Crossfire in spark plug wires run parallel for long distances (i.e., more than a foot or so) was a thing for automotive engines, which is why it was common to twist cables, preferably more than two at a time.    The shielding on aviation plug wires is much more substantial, so you see aviation plug wires commonly run parallel all the time.   I suppose if a harness was in bad shape and the shields were abraded enough or something there could be cross fire in a harness, but I'd be suspicious of a new harness or a harness in good shape doing it.

I'd think fuel flow, potentially an injector/line issue might be more suspect, but who knows.    

[Shadrach]

2 hours ago, EricJ said:

Crossfire in spark plug wires run parallel for long distances (i.e., more than a foot or so) was a thing for automotive engines, which is why it was common to twist cables, preferably more than two at a time.    The shielding on aviation plug wires is much more substantial, so you see aviation plug wires commonly run parallel all the time.   I suppose if a harness was in bad shape and the shields were abraded enough or something there could be cross fire in a harness, but I'd be suspicious of a new harness or a harness in good shape doing it.

I'd think fuel flow, potentially an injector/line issue might be more suspect, but who knows.    

Imagine how deteriorated a harness would have to be for this to occur… I don’t recall ever seeing an aviation harness where the plug wires didn’t share routing for at least part of their length. 
 

I’ll 2nd the interest in engine data. Should paint a pretty clear picture of when things went south.

[slowflyin]

Interesting, I never heard of ignition lead crossfire either.   My new harness came with a large adel clamp to route the entire bundle.   Interested on my side as I fly high often.  

[Will.iam]

One of the reasons i got a surefly on one side in case my mag loses pressure at altitude i can shut it down until i can get to a lower altitude. Amazing the cylinder held together forthe remainder of the previous flight and was not noticed. 

[Will.iam]

6 hours ago, Jungle Pilot said:

4. Why did we not see any warning message on the JPI 711?  The max CHT is set to 450°

Was another alarm going off at that time? Maybe jpi did not reorder the alarms in order of importance? Also i thought i read where if you cancel a flashing alarm like low fuel or high TIT the jpi will time out for 5 mins before resetting the alarms. 

[ArtVandelay]

4. Why did we not see any warning message on the JPI 711?  The max CHT is set to 450°

That’s why I like newer 900 which has separate LED to get your attention. Does the 711 even blink?

[Shadrach]

30 minutes ago, ArtVandelay said:

That’s why I like newer 900 which has separate LED to get your attention. Does the 711 even blink?

The 700 identifies any reading that’s outside of limits by selecting that reading and blinking. I’m sure the 711 is the same

[Ragsf15e]

Could an injector clogged enough to lean out that cylinder to ~50 ROP do this?  I’d think the cracked plugs are more a result than a cause?

[jetdriven]

There was some kind a thing about champion fine wire plugs cracking the insulators which caused preignition a few years ago.

[A64Pilot]

13 minutes ago, Ragsf15e said:

Could an injector clogged enough to lean out that cylinder to ~50 ROP do this?  I’d think the cracked plugs are more a result than a cause?

Yes, but more likely if your ROP to lean to peak or close to it, if your LOP AND outside of the box a clogged injector just makes that cylinder leaner and cooler and even less likely to detonate, so if your LOP a clogged injector isn’t a problem. I learned after my first one clogged on an IO-520 and me going slobbering rich to keep it cool that I would have been better off to just let it lean itself out, sure I’d maybe have a 5 cyl for awhile but it wouldn’t hurt it.

I read that as -50, did you mean if ROP could it lean to peak, then yes it’s possible.

Of course elephant in the room thinks maybe it could have been too lean at too high a power and lean ,but not lean enough

Edited May 19, 2022 by A64Pilot

[EricJ]

FWIW, a friend at a race tuning shop posted this today.    Same kinda thing, just a bad tune.    High boost with bad tuning is what does this on a car.   Customer brought it in asking for a tune.  "Unfortunately we can’t tune aluminum back into its home ."

[Cruiser]

35 minutes ago, EricJ said:

FWIW, a friend at a race tuning shop posted this today.    Same kinda thing, just a bad tune.    High boost with bad tuning is what does this on a car.   Customer brought it in asking for a tune.  "Unfortunately we can’t tune aluminum back into its home ."

now that is what you call a HOT SPOT

 

[Tony Starke]

A little bondo will make that good as new.

 

[Pinecone]

15 hours ago, EricJ said:

Crossfire in spark plug wires run parallel for long distances (i.e., more than a foot or so) was a thing for automotive engines, which is why it was common to twist cables, preferably more than two at a time.   

Hmm, when I was running distributor based auto ignitions, we used little clip things that kept the wires separated by 1/2 - 3/4 inches.

https://www.rvautoparts.com/72175-Moroso-Performance-Spark-Plug-Wire-Separator-7-Millimeter--9_p_363186.html

 

 

[A64Pilot]

Back in my drag racing days detonation failure often resulted in a dime size hole in the piston, too much timing, too low octane and or too lean a mix ( too close to stoichmetric) 

Forget the 103+ octane boost too, I had to buy a 1200 Wiseco kit for a motor I detonated to death on the test ride thinking it would raise pump gas octane high enough.

[A64Pilot]

3 hours ago, Pinecone said:

Hmm, when I was running distributor based auto ignitions, we used little clip things that kept the wires separated by 1/2 - 3/4 inches.

https://www.rvautoparts.com/72175-Moroso-Performance-Spark-Plug-Wire-Separator-7-Millimeter--9_p_363186.html

 

 

Yes but that was for much higher energy ignitions and much poorer shielded leads, if nothing else the outer shield on aircraft leads should short any leakage to ground. Don’t over think this running them together has worked for 100 years.

If we ever get actual high energy ignitions we may need better wires, or maybe do it right and put a coil pack on each wire or maybe at least one per cyl with two short leads.

Edited May 20, 2022 by A64Pilot

[Yetti]

The Tempest Fine Wires from a couple years back had welding issues with the center electrodes.   But when you get to 600 bad things are going to happen to the plugs.   The question would be what plugs were they?

[philiplane]

A melted piston only comes from pre-ignition. Pre-ignition will come from a spark plug that has lost its insulator, which turns it into a glow plug. Now you have an out-of-time ignition source, which lights the fuel that appears the moment the intake valve opens. The piston is now trying to compress the expanding gases, rather than being pushed down by them. The heat rise is rapid and off the scale. Melting occurs in a matter of minutes.

Mixture settings, no matter how improper for the power setting, cannot melt the piston. 

Regarding the shop's though of crossfire: There is no way for plug wires to misfire into one cylinder. It would affect two or more at the same time. And this thought is a relic from the old days of cheap automotive plug wires, which have never been used on airplanes.

While they may be a competent overhaul shop, they don't understand the principles of pre-ignition. Pre-ignition rarely comes from the ignition system, it's usually from a hot spot in the cylinder. There are a few good books on engine damage, and what causes it. The best one is:

https://herringbonebooks.indielite.org/book/9780768008852

Edited May 20, 2022 by philiplane

[A64Pilot]

Pre ignition by definition cannot come from the ignition system, pre ignition is a normal flame propagation, just occurs prior to the spark from some heat source, glowing carbon, some piece of metal etc. normal flame propagation occurs then proceeds through the combustion chamber, just earlier than it should. It differs from detonation in that in detonation the entire charge goes off at once, there isn’t an ignition source, the charge detonates.

pre ignition can lead to detonation from excess heat.

Lycomings explaination

https://www.lycoming.com/node/17607

[Shadrach]

17 hours ago, A64Pilot said:

I read that as -50, did you mean if ROP could it lean to peak, then yes it’s possible.

You misread the tilde ~ as a minus sign -.  Very different meanings. The "Tilde (~)" is used in engineering circles as a symbol for approximation.  ~50 means about 50 or around 50, not -50 (minus).  It's a way of stating that you're not being precise but close enough. I believe @Ragsf15e was suggesting that a clog might lean a cylinder from a healthy ROP mixture setting (≥100°) to ~ (approximately) 50° ROP, which is a not so healthy ROP setting for a turbo operating at a high manifold pressure.

[Shadrach]

29 minutes ago, A64Pilot said:

Pre ignition by definition cannot come from the ignition system, pre ignition is a normal flame propagation, just occurs prior to the spark from some heat source, glowing carbon, some piece of metal etc. normal flame propagation occurs then proceeds through the combustion chamber, just earlier than it should. It differs from detonation in that in detonation the entire charge goes off at once, there isn’t an ignition source, the charge detonates.

pre ignition can lead to detonation from excess heat.

Lycomings explaination

https://www.lycoming.com/node/17607

Agreed, but detonation from whatever cause can damage the cylinder and plugs in such a way that causes pre ignition then contributing to the uncontrolled combustion shitshow.  I would not think a flame front propagating from a damaged, overheated spark plug components, a sliver of metal from damaged spark plug threads/ helicoil  or the like would be considered normal flame propagation.