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]

As a follow up to my follow up, I have discovered that the Vixen engine mount tubing is different from the later model nose wheel equipped Kitfoxes. Through FB Kitfox folks, another Vixen owner has had a failure of the nose shock strut and a collapse of the bottom center engine mount pad (the top, receiving end, support of the rubber stacked shock piston). He provided pictures of the end result, as well as a photo of a different (later model...I assume) engine mount which has an added vertical engine mount tube that obviously has taken up much of the vertical shock load from the shock piston. That (new..different than ours) tube goes right through the space where we currently have a muffler. I assume the later models have the muffler further towards the front. At any rate, I am now convinced that my broken bolt was a result of frequent bending loads of the shock piston pad, causing bolt fatigue from bending. The best solution (I think) would be to completely convert the front FWF to the later model engine mount, muffler locations, and everything else to acquire that beefed up engine mount. In the real world (my world) I am planning on frequent inspection and replacement of the all-important bottom center engine mount bolt.

Skot

[jrevens]

I wonder if that is possibly a prop bolt (AN76?). They generally have a longer threaded length.

[PapuaPilot]

I was waiting to see the pictures wondering if the bolt was too long and somebody "shanked" the nut (i. e. tightened the nut to the bolt shank), but that is not the case.

Shanking a bolt can also happen if we don't use enough washers. The problem is the nut feels like it tightened up, but it is still loose because the nut stopped turning at the shank. Cyclic loads will eventually cause the bolt to break.

IMO that bolt should be suspected as a bogus aircraft part. It is not an AN bolt:
1. Unfortunately the markings on the head are correct. Common AN bolts are identified with an X on the head, plus the manufacture's stamp/letters.
2. What's wrong is the thickness of the head and the amount of threads that are cut on the bolt. Compare it to the new bolt in the picture. It has the correct amount of threads and has a thinner (proper) head.

I don't see evidence that somebody cut more threads on the bolt. DON'T ever do that to an aircraft bolt, just get the correct length. Most likely it failed due to the repeated stresses that occurred on the treaded part of the bolt where it sheared. A proper bolt will have the shank go entirely through the engine mount and fuselage, and the threaded end will only have washers and the nut on it. The other problem with the threaded part of a bolt is that it is dimensionally smaller then the shank. It doesn't fill the hole and will quickly enlarge the hole.

Hopefully the new bolt will fix the problem and there isn't any secondary wear in the engine mount and fuselage.

[SWeidemann]

My conclusion is the bolt that broke was not the right bolt for this application. I do not know why this one was used, and hopefully the right one (the one I put in last Friday, torqued and safety wired today) will do the job, although I intend to carefully inspect this part frequently. BTW I ordered the replacement AN bolt by the alpha numeric part designation on the Kitfox engine mount install spec sheet, from Pegasus Racing (a nearby supplier I have used for various parts, including Camloc pieces for the cowl).

Removing the broken threaded bolt from the engine mount turned out to be a blessing, in that I was able to remove it without totally removing the engine. By using an engine hoist, I loosened the 4 other mounting bolts and moved the engine far enough forward to make space to insert a right angle pick & persistently hammer on the side of the bolt to rotate in the “out” direction, occasionally viewing my project with a small dental mirror, aided by some Mouse Milk (a recommended concoction of what I gather is a solvent, mixed with penetrating oil). I’m a happy man.

Skot

[SWeidemann]

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