Broken bottom engine mount bolt on 912ULS Vixen

[SWeidemann]

Today, I noticed a new clunking sound while backing my Vixen (Series 5 trigear) back into the hangar. On inspection, I discovered the bottom center engine mount bolt had sheared off. This bottom center weldment & bolt joins the flange for the nosegear shock assembly that takes most of the nosegear shocks. I suspect the prior owner had an issue with this because of the patch on the firewall covering a suspicious hole in the firewall. Unfortunately it looks like the threaded part of the sheared off bolt is still inside the engine mount. Right now, I am hoping I won’t need to take the engine off to get at it. Oh boy, what fun. Has anyone else with a nosewheel had this happen? And before any of you tailwheel guys decide to make a comment about converting to tailwheel, I know all about pros & cons of both configurations and I’m sticking with my machine. I’m looking for feedback and a fix to get back in the air. Thanks folks.

Skot

[PapuaPilot]

If the bolt has a clean break you should be able to just spin it out and install a new bolt. If it is stuck I would suggest using a proper size EZ out.

[SWeidemann]

If the bolt has a clean break you should be able to just spin it out and install a new bolt. If it is stuck I would suggest using a proper size EZ out.

Phil,

Thanks for the encouraging words & advice about an EZ Out. It is a clean break almost flush with the engine mount tube. I have the remaining engine mount bolts loose enough to make a small gap (along with suspending the engine from a hoist). So far I have had no success in rotating the imbedded part by tapping it with a right angle pic. If I can’t make that work, there’s more disassembly.

[VictorV]

I'm curious about what could cause such a failure. Was there any evidence of corrosion or cracks?

[SWeidemann]

I'm curious about what could cause such a failure. Was there any evidence of corrosion or cracks?

Well, I don’t see any obvious corrosion or cracks. I did get the broken bolt end extracted from the engine mount and discovered the AN bolt may have been inappropriate for this application since the (broken) bolt had longer threads than the one I replaced it with (ordered by AN designated on the installation instructions with the engine mount). The break in the bolt was right at the point where the threads stopped and the smooth shaft of the bolt began. Seems to me like this point was likely a weak spot. I am convinced that the original installation of the bolt also may not have been torqued properly. By hindsight, I do recall a clunking noise while ground handling with a towbar, which was not investigated thoroughly enough. I failed to recognize the noises as abnormal and naively assumed the nosegear was making normal movement noises. The four rubber donuts were also old and brittle (unknown age….this plane flew first in 2003). Today I finished the repair installation with new elastomers and a important new AN bolt. Before I fly again I will re-check the bolt torque, safety wire the bolt and upgrade my preflight procedures to include a more careful assessment of that possible trouble spot. I also think (in my case anyway) I should regularly remove the AN bolt (probably 100 hours) to inspect for further damage and/or replacement. The day before I discovered my broken bolt, I was commenting to a friend that everything flight in an Experimental aircraft is essentially a test flight.

[SWeidemann]

Bolt comparison:A2C6DA1E-0389-40CE-B5E0-B342614CC09D.jpg

[VictorV]

Very interesting. It does indeed look like the threads caused the weakness. It seems like there's
no way to avoid that though. You could use the longer bolt but you'll probably still end up with
at least 1 thread inside the hole made by the 2 mating parts. When you said that it was not
properly torqued originally did you it was too tight or too loose? I'm new to a lot of this but
that's such a critical component I would want to make sure nothing like that ever happens
again. Maybe Brandon has some thoughts on it....

Victor

[jrevens]

That's obviously not an AN bolt.

[SWeidemann]

That's obviously not an AN bolt.

Here is the head of the broken bolt. AN or what?

C593736B-315A-4FA7-9029-A3DC3F513AEF.jpg

[VictorV]

LFC manufactures aerospace hardware so it may still be acceptable.