Service Bulletin SB 14-8-29

Completed 9/6/2014
approx. 1070 hours

This fall thanks to my buddy Vic spotting some light rusty cracks in his engine mount, Van's found one area of potential failure for some of the RV-10's out there.  It should be noted that there are now well over 700 RV-10's flying, and many have accumulated over 1,000 hours and countless landings, so the fleet is aging and has gained a lot of experience over the years.  That's why it was a bit unusual to find something that snuck up on everyone.  The reason in, the failure is in an area that wouldn't normally be closely inspected by most builders for many many years, unless there were a highly obvious defect.  Thanks to Vic's keen eyes though, this one didn't slip through the cracks.  Following the forums, I've found that there are so far about 3 people that this has affected, so it's a rare issue, but an important one.  It also has only affected people who flew out of rough grass strips on a normal basis.

The situation is this:

On the engine mount is a metal plate where a stack of 4 elastomeric rubber donunts compress up against.  These donuts also have a bonded on brass plate (it could be aluminum but I don't think so), on them, that lays flat against the engine mount, and the bottom of the shaft attaches to the nose gear leg. When you hit a bump, the forces are transferred up that shaft to compress those donuts against the engine mount, as a shock absorber.

Above that metal plate is a circular well surrounding the shaft that sticks up through the mount.  On top of that shaft sits a big ol' Abraham Lincoln style top-hat piece of metal with a bolt that goes through it and the shaft to hold it on.  Under that top hat sits 1, 2 or 3 washers, to take up any looseness in the rubber forces the donuts to stay compressed against the engine mount tightly, even under no weight load.  Over time, these donuts compress...initially a lot, then less over time.  As they compress, you have to keep adding washers to keep that area tight, or the gear leg can bounce with no compression on the donuts, causing even more force to be applied to the face of the plate in the engine mount.  If you let it get loose, you will be at a higher risk of failing that plate in the mount.

The builders with cracks, had nose gear that had been pushing up so hard with those elastomers, due to rough runway ops, that the plate on the engine mount was flexed upward until cracks radiated out from the center one case the metal was almost falling out on it's own.

Here are some pictures of the cracking found, and of elastomers that had compressed quite a bit

One Builder's Cracks Elastomer Compression

Cracked Elastomer Plate Another Builder's Cracks

Now the issue is bad enough with just cracking, but, if the pieces of metal get lost from the engine mount plate, that top hat can pull through, and potentially cause the nose wheel to swing back under the airframe, which would make for a very noisy landing!

So, along comes SB 14-8-29 to fix the issue.

This SB basically has 2 parts to for people with cracks, and one for people without.  New engine mounts aren't subject to the SB, so they did fix this for future builders.  The crack-fix version has some welding involved and will take a lot more time.  The no-crack version, which most people will need, is much simpler.

I was lucky that even with 1070 hours, and a bunch (maybe 50 or so) grass strip landings, with many at near gross wt, I didn't have the problem. I've also always kept weight off the nosewheel on landing, and during taxi, so that I'm sure helps too.

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These are pictures from mine.  The first photo shows the elastomer plate.  This was a poor situation since day one...those plates interfered with the mount tubes. I should have ground them back but didn't...I let the plates deform, but it still scrapes the mount a little.  Bad.  You can see the areas where the plate touched the mount.  When I disassembled the top-hat, the shaft was oily from previous engine oil leaks.  The shaft underneath the donuts was not oily.  You do NOT want oil or grease there...the donuts need to grip the shaft to provide increased resistance.  My plate was also discolored due to oil leaks, and although the paint was still somewhat on it, it was wearing thin.  For many people this paint may be worn off already.
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There is one area I still have future concerns for.  If you look at the shaft after lifting up one elastomer, there is some slight thinning of the shaft, right at the top of where the first elastomer is.  This is the area that passes through the hole in the engine mount.  There is of course no bushing or protection or wear surface that can be replaced, so over time this shaft will wear and need to be repaired or replaced.  I can't think of a good way to do a field upgrade to that area to keep it from wearing.  With 1000 hours on the shaft though, I'm thinking that perhaps at every engine overhaul it will be time for a new shaft.

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I measured the elastomers, just for future reference.  I had the "no cracks" fix, so I installed the doubler plate, which is about TWICE as thick as the original plate.  For that fix, the doubler just lays up against the existing plate.  They send a roughly accurate cut plate that goes around the tubes, but I did have to grind a little extra away to clear some welds so that it would lay perfectly flat.  Van's does not spec anything to hold the plate in position or keep it from rotating, either.  Notice the hole is a bit larger than the original hole...this helps make it easier to align.  I'm not sure if it's better or worse from a wear perspective on the tube passing through the hole.  Better that there is less area to wear against, or worse because the wear will be more focused on a small area...   After grinding the plate to fit, deburring and painting it, I chose to put a very thin layer of RTV between the plate and mount.  Proseal would perhaps be a better option but would be hard to remove later if necessary.  It should not be necessary though.  After putting the RTV'd plate in, I jacked up the tail of the airplane to give it plenty of down pressure to squeeze out any excess RTV so there was absolute minimal gap, to keep this plate tight against the other.  The RTV is there to keep moisture out and keep it from spinning, is all.  Then it was time to lubricate the washers, and tophat and top of the shaft (especially around that wear area) and re-install.  When re-installing it will take upforce on the tail, to get the bolt through, if you have proper preload on the donuts.  If it does not take lots of force, you need to add more washers and try again.  That's less future problem to worry about.
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