Help troubleshooting potential causes of crankshaft failure

[Ethanm]

Hello!

As some of you may know I experienced an inflight crankshaft failure last fall with my older model AeroVee engine (not an AeroVee 2.1)

I was hoping to get some help assessing why this might have happened. Any theories are welcome and helpful. I'll try to provide as much information on the incident as possible.

About 2 hours into a flight I started experiencing minor vibrations. Adjusting mixture and running on either ignition made no change to the vibrations which within 2 minutes had increased linearly to the point that the instrument panel was unreadable. At this point the crankshaft broke at the front main seal, resulting in the entire propeller, spinner, and flange departing the plane (never to be found, lost somewhere in the woods of western NY). Luckily I had a long straight road underneath me to land on.

Facts about the plane: The plane (Sonex 0888) had been completed in 2007, it had remained in phase 1 flight testing until I completed it in 2023. Before I started flying the plane, the engine and airframe had around 30 hours. A condition inspection was performed by an A and P in 2023 right before I started phase 1 testing. The engine only had 1 hour on it since it's last condition inspection in 2019. I think it's very unlikely that the plane ever experienced a prop strike since the previous owner had done all of the phase 1 testing leading up to this point, and there was no damage history with the aircraft. When the crankshaft failure occurred, the engine and airframe had 101 hours. All of which I had flown since I bought the plane. I had torqued the prop bolts ~20 hours before the incident. One thing I had noticed previously was that the front main seal tended to weep oil over time. I didn't think this was anything major as it didn't seem like a significant amount of oil. The engine had plenty of oil before departing on the incident flight.

At one point I inquired to Sonex about the turbo upgrade kit. I found out this would require me to replace the current crankshaft with the AeroVee 2.1 crankshaft. Here's the response I got from Sonex:

"Also, we noticed an AeroVee serial number of 0303 listed for this order. This upgrade requires use of the AeroVee 2.1 crankshaft, which was supplied with AeroVee engines serial number 0435 and later. If you do not already have the AeroVee 2.1 crankshaft and prop hub assembly installed, you will need to upgrade for compatibility with the Turbo. This is most easily determined by the color of the prop hub: AeroVee 2.1 hubs are zinc plated (silver) whereas all previous AeroVee prop hubs have a black oxide finish (black in color). The AeroVee 2.1 crank also uses the ACV-H01-17 prop hub bushing and smaller ACV-Z01-74 bolt (M12 bolt vs. M20 bolt on older AeroVee prop hubs). You can see the current crankshaft and prop hub installation in the current versions of AeroVee manuals, downloadable at: http://www.aeroconversions.com/support/manuals.html "

Video of where crankshaft broke: https://drive.google.com/file/d/1TGO-6N ... sp=sharing

Pictures and closeups of where the crankshaft broke:

https://postimg.cc/gallery/Dwbw8TD
https://postimg.cc/gallery/nBwXgpj

Thank you all for your insights!

[kmacht]

You will need to disassemble the case and remove the crank to really tell the method of failure. It’s tough to tell with it still in the case. A couple of possibilities come to mind. The first would be that it was misassembled. It’s possible that the builder didn’t get the hub fully seated or used a press to incorrectly install or remove it. Do you know if the builder assembled it or if they paid for the factory assembled option? Depending on where it failed it’s also possible that the bearing behind the hub wasn’t clocked correctly which would keep oil from getting to the bearing and the crank faking. It’s also possible that it was a bad crank but with so few hours on it I would think oil starvation or misassembly to be a more likely cause.

If you keep the plane you may be able to salvage the case by going with a force 1 hub and crank. It uses a much bigger bearing that the prop hub rides in. The nose of the case gets machined out to accept the larger bearing. The owner of Hummel aviation did my case machining for a very reasonable price. The cost of the crank was actually a little cheaper than Sonex as well.

Keith
#554

[Kai]

Well, if it is any consolation, the same thing happened to us. We were Sonex #0037 with an Aerovee, and our photos show the same as yours.

We could have written the exact same two first paragraphs as you did, exept for the last sentence- we were not so lucky: the front end of the crank with prop, bolts, and scull cap spinner slammed down into the parking lot of a very busy shopping mall on a sunny saturday afternoon. To put it mildly, we were a little apprehensive when we went to the police to report the matter. However, as pure luck would have it, the incident was hardly noticed, no one was hurt and there was no material damage. This was early spring, and the plane right after the incident plonked down in a newly sown field- only the gear legs were bent.

Notice that we did not offer any running hours. We don´t know. The plane was purchased second hand from the original builder/pilot in Ohio. He must have been lying out of the back of his teeth before he was paid. When we got in contact with him about the incident, he shut up like a clamshell. Much later, at Osh, we learned that the plane might have experienced a minor (??!!) prop strike.

Anyhow, since once was enough, we went to Great Plains for our next 2170 engine: assembly was done professionally by them, and we were assured that every precaution, including the installation of a Force 1 hub and front bearing, had been taken to prevent a repeat of the calamity. We had a little over 70 hours on the engine when this crank too gave up the ghost: it broke across the rearmost web and everything up there became bloody quiet. This time we were in the middle of autumn and a newly harvested field became our salvation. Again we had to swallow our pride and order a set of new gear legs!

This last incident led us to believe, that VW-1 aeroderivatives were perhaps not for us after all: the things were just not sufficiently robust for operation in aircraft. So we decided to go for a different engine from another make, and the rest is history- now, 15 years later we are still happily homping around the sky with this new engine up front.

[karmarepair]

In order to do any further failure analysis, we will need VERY closeup picture of the failure surface itself. Different types of failures leave different marks on the failure surface.

Example: the relatively smooth surface, with lightly stained "beach marks", indicates fatigue. The granular surface is the final failure, which happened rapidly.
http://assets.rbl.ms/27669005/origin.png

It will take at least a 4X macro image to discern much of anything, and even then, "smearing" of the fracture surface during the final failure might have wiped the evidence away.

My engine is serial number 0133, so I have the 2.0 crank. I have about 3 hours running time on it. I did NOT assemble my engine. I asked Sonex if I can upgrade my crank without modifying my current case. There has never been a prop strike on my tri-gear airframe.

[kmacht]

You can swap out the 2.0 and 2.1 cranks without any case modifications. Sonex won’t do it for you as they only sell parts and “kits” but the two cranks fit into the case exactly the same. I would suggest doing some research and understanding exactly what the 2.1 crank gets you over the 2.0. The differences are very minor and the hub attachment method which is the weak link in the design (but still sufficient) is still the same between the two cranks.

[Area 51%]

The 2.0 crankshaft can be used in an automotive application. What that means is material is removed from the crankshaft to accommodate a distributor drive gear and spacer. The diameter escapes me right now, but the bolt holding the belt pulley in place on a car is seemingly much larger than it's application would require. Once again, more material is missing from the aviation business end of the crankshaft.

The 2.1 uses a custom crank without the undercut for the distributor drive gear, and the prop hub bolt is smaller in diameter than it's automotive counterpart, leaving more "meat" to support the prop loads.

[daleandee]

The 2.0 crankshaft can be used in an automotive application. What that means is material is removed from the crankshaft to accommodate a distributor drive gear and spacer. The diameter escapes me right now, but the bolt holding the belt pulley in place on a car is seemingly much larger than it's application would require. Once again, more material is missing from the aviation business end of the crankshaft.

The 2.1 uses a custom crank without the undercut for the distributor drive gear, and the prop hub bolt is smaller in diameter than it's automotive counterpart, leaving more "meat" to support the prop loads.

I could be mistaken but didn't the newer cranks eliminate the keyway? Seems there was only .060 between the keyway and the threads for the end bolt and it was thought that that area would be where a failure might begin. Apologies if I'm in error on this as my Aerovee was an earlier model 2.0 ...

Dale
3.0 Corvair/Tailwheel

[Bryan Cotton]

I could be mistaken but didn't the newer cranks eliminate the keyway? Seems there was only .060 between the keyway and the threads for the end bolt and it was thought that that area would be where a failure might begin. Apologies if I'm in error on this as my Aerovee was an earlier model 2.0 ...

Dale
3.0 Corvair/Tailwheel

The manual and video both show a key.

[daleandee]

The manual and video both show a key.

Thank you sir ... I sit corrected! 8~)

[karmarepair]

New, assembled 2.1 crank http://www.sonexaircraft.com/eshop/cart ... Crankshaft. $1275.

Long article on prop hubs and cranks http://bobhooversblog.blogspot.com/2007 ... -hubs.html

Stock crank flywheel end: http://1.bp.blogspot.com/_JU6RC7jJfRc/R ... GURE+1.jpg

Stock crank failure mode http://1.bp.blogspot.com/_JU6RC7jJfRc/R ... GURE+6.jpg This is a FATIQUE failure, starting at the bottom of the outboard keyway groove.

Looking at the OPs picture of the crank nose, https://postimg.cc/PpNH1CDn it looks like the oil slinger is still in place, but the crank has snapped just behind it, as predicted by Veeduber's illustration above of the stock crank failure mode.

The Aerovee 2.1 crank has MUCH shallower keys, and the smaller diameter hub bolt avoids the buildup of stress concentration near the outboard circlip groove and the outboard key. The keys, both for the prop hub and the cam gear, are FLAT, and not the half-moon woodruff keys, so they take up much less of the cross section of the crank nose. It looks like the circlip groove that secures the cam gear is moved back closer to the number 3 bearing, as there is no distributor gear with the AeroVee magnetron ignition, but there MIGHT be a groove where the oil slinger sits, or it may just be a shoulder, no groove, and thus less of a stress concentration.