Rotax Piston Meltdown

[waterboy2110]

Originally Posted by rv6ejguy

Thanks for posting the detailed photos of the plugs and piston. Plugs look totally fine and heat range is 8 which is stock for this engine. Insulator color looks good too so I don't see evidence of an overly lean condition.

I would say from these better photos and the fact that the ground electrodes are intact and the insulators don't show any cracking, heat distress or aluminum micro spheres on them, this was probably not a pre-ignition event and certainly not a detonation event.

Photo 12 of the piston shows an overheated crown failure very much like the 3rd photo I put up in Post #58 which is of a Subaru EZ36 piston and a typical failure mode on these engines. On the 912, this is hard to explain since they are very well proven at high continuous power levels. I have a customer who's been doing flight training on these engines for over 15 years and has over 4000 hours on them with no issues. His last engine had EFI on it and went right to TBO with no work inside.

One unusual thing I see is that a large portion of the #1 and a smaller portion of the #2 rings seems to be missing. As you can see in photos 3 and 4 in post #58, the rings are intact, even though in photo 4, they are completely unsupported. I've never seen rings melt or break in this sort of failure or even from pre-ignition as the melting points of the rings are around double that of the aluminum. Did you find any large ring bits in the debris? It looks from some of the impressions in the piston crowns that hard, sharp edged objects made some of these marks.

Does anyone know the ring material used on these engines? Are the pistons cast or forged?

Is it possible to get a clear photo of the edge section of what is remaining on the 2 top rings?

Any screws or other bits missing from the carbs?

This is a very unusual failure mode but there has to be a reason for it.

Ross, thanks for looking at the photos and providing valuable input. I will post more detailed photos of the piston for you to examine. There were chunks of ring embedded in the piston - I have a picture of that from my initial borescope that I'll post now. There wasn't anything that I knew of that could have fallen into the intake but I'll recheck the butterfly screws. There's nothing else I could think of that could have made it's way into there. The #4 piston also has pitting on the head indicating that material had made it's way into that cylinder. My take on that was the FOD from #2 made it's way into #4 once the failed piston lost compression.

[bret]

I just replaced a broken piston ring on my 360, removed the ring and reassembled in the bore and measured the ring end gap thinking a ring end butt condition may have happened but the specs came out within tolerances, my only guess would be that whoever did the last overhaul broke the top compression ring during assemble and did not know it. Could that have happened here, sure looks like a good portion to the 1st and 2nd compression rings got past the top land and got chewed up, question, how did they make it past the valves, can you pull the valves and look at the seats and valve face for marks, the seats I believe are harder than the rings so I wouldn't think there would be too much damage, maybe bend a valve a tad? I would argue that the erosion on the piston top is a sign of detonation, is there a hole going through the piston at the top ring land on the left in the photo? looks like a cutting touch path. in any case, this is an interesting failure. Root cause anyone?

[DanH]

No noticeable plug damage, or heat damage to the squish area at 12 o'clock in the area mirroring the piston damage. Hot plug induced preignition unlikely. Detonation still possible, but I remain mystified by the lack of blasting and splatter evidence.

There is an anomaly in the EMS dowloads. Put MP and RPM up on the same screen and align 'em. Compare the plots. They match very well (as they should; fixed pitch)...except at 640 on the time hack, where MP drops about 5" with an RPM rise. I'll need to think about it.

http://912ulsenginefailure.blogspot....is-charts.html

[scsmith]

Originally Posted by DanH

No noticeable plug damage, or heat damage to the squish area at 12 o'clock in the area mirroring the piston damage. Hot plug induced preignition unlikely. Detonation still possible, but I remain mystified by the lack of blasting and splatter evidence.

There is an anomaly in the EMS dowloads. Put MP and RPM up on the same screen and align 'em. Compare the plots. They match very well (as they should; fixed pitch)...except at 640 on the time hack, where MP drops about 5" with an RPM rise. I'll need to think about it.

http://912ulsenginefailure.blogspot....is-charts.html

Other than instrumentation issues, the only way to explain that is that something changed in the environment of the propeller to change its ability to absorb power. The airplane is on the ground at this point, right? A very strong headwind gust or turning from downwind to upwind?

A sudden drop in air density would cause the MP to drop too, as well as cause the RPM to increase. But how do you get a region of much lower air density? taxi through a hot jet wake?

[AndrewR]

We all like a whodunit with the culprit revealed at the end. Unfortunately things aren't always that neat. I stand by my statement in post 75 that it looks most likely that the cause was simple overheating, due to the lengthy wait on the ground resulting in inadequate cooling margin when full power was applied - with possible contribution from factors like RPM and fuel.

Lets translate the engine data to Lycoming equivalents. Rotax temperature limit =120C, Lycoming 500F, so working with normal temperature ranges means 110C is approximately equivalent to 450F. 116C (maximum reached) would be eqivalent to 480F. However the Rotax limit is a harder limit - if you exceed the coolant boiling point it's all over, whereas probably nothing immediately happens if you exceed 500F on a Lycoming.

Working off Lycoming and Rotax MP/RPM charts, Rotax 5100 looks approximately equivalent to Lycoming 2350.

Lets imagine a Lycoming broke it's rings and burned out the side of a piston. The data shows OAT was 107F, a long wait on the ground meant CHT reached 450F prior to takeoff, climb was at 30" manifold pressure and 2350RPM with CHT reaching 480F - what cause of failure would you suspect?

[airguy]

Originally Posted by scsmith

Other than instrumentation issues, the only way to explain that is that something changed in the environment of the propeller to change its ability to absorb power. The airplane is on the ground at this point, right? A very strong headwind gust or turning from downwind to upwind?

A sudden drop in air density would cause the MP to drop too, as well as cause the RPM to increase. But how do you get a region of much lower air density? taxi through a hot jet wake?

Airflow turbulence that changes at certain velocities in the inlet and presents different local conditions at the point of measurement? Could this be a tripping boundary layer?

[waterboy2110]

Originally Posted by scsmith

Other than instrumentation issues, the only way to explain that is that something changed in the environment of the propeller to change its ability to absorb power. The airplane is on the ground at this point, right? A very strong headwind gust or turning from downwind to upwind?

A sudden drop in air density would cause the MP to drop too, as well as cause the RPM to increase. But how do you get a region of much lower air density? taxi through a hot jet wake?

Impressive analysis of the data.
KCCR - Holding short on 32R for landing traffic on 19R (local carrier - Jet) thus the delay. So yea - we most likely went right through the wash. I never really though about it until you mentioned it.

[BillL]

Originally Posted by AndrewR

However the Rotax limit is a harder limit - if you exceed the coolant boiling point it's all over, whereas probably nothing immediately happens if you exceed 500F on a Lycoming.

Where temperatures are measured and where they occur in a liquid cooled engine are very important to what happens. If this temperatures occur at the high heat transfer surfaces, incipient boiling, then nothing much typically happens. The vapor is swept away with coolant flow and re condenses in the cooler bulk flow. It happens all the time and many engines with no detrimental effects. It is better from a design standpoint that the heat transfer and coolant flow keep it from happening, but there is no catastrophic issue. It may result in fatigue and cracks in the long run. Heat transfer increases dramatically with incipient boiling that keeps the temperature of the parent material (head, port) from further rapid rise. Many SAE papers have been written on this effect. Too long ago to provide quotes.

For the cooling system to suddenly cease to function entirely, the coolant pump would have to cavitate and reduce head dramatically. The temp at suction is typically the lowest in the system for this reason. Cavitation can be related to some more easily measured temperature like top tank temps or the radiator/heat-x exit temps. The pump also needs some mechanical head which is hard to do in an aircraft installation. So, if (IF) cavitation had happened then there would have been a rapid rise of temps within the engine that would have caused substantial boiling, temp rise, pressure rise, and exceeded the pressure cap limits very quickly. A cooling system does not recover from this easily. Actual systems testing is needed to quantify cavitation in a system due to several effects that are difficult to quantify in modeling. Then a limit temperature can be provided for a specific coolant, be it water, or EGW mix.

Originally Posted by AndrewR

Lets imagine a Lycoming broke it's rings and burned out the side of a piston. The data shows OAT was 107F, a long wait on the ground meant CHT reached 450F prior to takeoff, climb was at 30" manifold pressure and 2350RPM with CHT reaching 480F - what cause of failure would you suspect?

Good thought about what happens with a broken ring, but each engine, reacts differently to its balance of heat flows and cooling design basics. One recent broken ring event in a VAF members engine had extensive second land damage and down to the oil ring. There was no evidence of detonation or preignition on the piston crown, not even at the top land over the failed area. The rapid flow of combustion air that "blasts" the piston and wall, also has rapid heat transfer to cool the flow temps. It would seem that a Lyc with its air cooling would be less tolerant of broken rings and combustion blowby, but it certainly did not end in this type of failure. It was benign, oil consumption increase and some barrel damage. Each engine design is different.

This failure sure seems reluctant to be fitted cleanly in the detonation or preignition category, yet clearly has failed. If I was the warranty guy and could not see any clear reason for the failure thus could not say it was operator induced, warranty would be certainly awarded.

I have seen many boxers eat pieces and the combustion chamber damage is usually more evident, and extensive on the lower side of the chamber than on the upper. For this reason I had mentally put FOD as a low probability as root cause. Low piston-to-head clearance (squish) elevates the likelihood of to land damage.

I'm still puzzled after seeing those plugs. Is there any possibility for cross fire on this engine?

[DanH]

Originally Posted by scsmith

Other than instrumentation issues, the only way to explain that is that something changed in the environment of the propeller to change its ability to absorb power.

For now I'll take instrumentation issues. Look at the oil and head coolant temperature plot. There's an anomaly at the same 640 time hack...a coolant temperature bump and a oil temperature drop at the same time.

Does anyone know the time hack units?

Waterboy, can you post the raw EMS file for download, or upload it to a tool like Savvy?

[Weasel]

Possibly transmitted on Comm radio at 640?

Depending on radio frequency selection, I get a few data irregularities.