Engine Motor Mount Cracked Sonex

[Michpenz]

Hi
I was carrying out my annual inspection when I found this crack.
It is right where the Service bulletin requires a visual inspection every time the cowls are removed.
I was shocked as 20 hours ago there appeared to be no crack and now it was 95% cracked.
The aircraft has done 500 hours with an Aerovee engine. Engine mount is original 3/4 x .035 material, not the 0.049 thickness in later mounts.
I have removed the mount and in the process of repairing. I plan to Re-powder coat white or light Grey for future crack detection.
Has anyone else experienced this or cracking in other areas?
Also has anyone carried out the Sonex alternative repair? I am Hoping it is straight forward, see the attachment.
Ref Service bulletins SNX-SB-002 and SNX-SB-005

[Kai]

Way back, my original thin walled mount cracked exactly as the pictures show after some 300 hrs operating from a rough grass field with a 85HP Jab. The SB from Sonex was already out by then- as I understood it, repair would require an excellent welder. Since 85HP on a rough grass field is only for those particularly interested, I decided to go for a 120HP Jab: I got hold of a new Jab 33A mount with the thicker wall tubing. So far so good.

I operated the aircraft for an additional 200 hrs with this setup with nom issues whatsoever- then Jab decided they would no longer support the older gen 1-3 engines with essential parts like cylinders, heads etc. Something had to be done, and I decided to go Rotax/ring mount: with a very slight mount change and some 12mm adapter bars for the original Sonex/Jab rubber isolators, this was easily accomplished.

The airplane has flown with this last setup for almost 150 hrs by now: so far still no issues. The thicker wall engine support tubing is holding up exactly as advertised.

[Michpenz]

Thanks Kai for your input. I am curious to see how many have had cracked mounts

[Squeaky]

This is cracked on right side it looks like while repair in bulletin shows left side. Am I missing something? Or does SB refer to both sides possibly cracked?

[Michpenz]

Hi Squeaky I think the drawing covers mounts for more than one engine type and it is showing LH repair as a typical repair which is applicable to both lower mount tubes. I will be carrying out the repair on both sides.

[peter anson]

Well done finding that. Unfortunately, cracked engine mounts are a fact of life with the earlier model Sonex, although most of the cases I have heard of have been with the Jab 2200. The picture below is a Solidworks FEA model of the lower engine mount with just half the weight of the engine (Jab 3300) on it.

That's just the 1G case. Engine torque reaction puts additional load on the left mount (Jabiru engine) or on the right mount (VW engine). If you want to read more about it, there's some additional info in the following article: https://www.ansoneng.com/sample-page/other-stuff-of-interest/fatigue-of-metal-aircraft/

Peter

[Skippydiesel]

Hi Peter,

Interesting stories you posted - thanks.

All this talk of engine mounts cracking has put the wind up me - My Sonex Legacy (approaching 140 hrs) powered by Rotax 912 ULS - The engine mount is (I think) for an AeroVee, with the Mk 2 Roatx adapter rails (Mk 3, a Ring Mount, only recently announced). I know later mounts had larger bolts to attach the adapter rails - this makes me wonder what thickness my Sonex mount is - how do I find out?

Should I be concerned?

[peter anson]

Hi Skippy, for some reason I thought yours had the bed mount. If the ring mount attaches to the standard Aerovee mount you need to inspect where the small red region is on the engine mount, especially the left side. That Solidworks model is for a 0.049" wall thickness tube so they aren't immune. I think the engine mounts made after about 2005 would be the thicker walled tube but I don't know of an easy way to check the one you have.

Peter

[Skippydiesel]

Hi Skippy, for some reason I thought yours had the bed mount. If the ring mount attaches to the standard Aerovee mount you need to inspect where the small red region is on the engine mount, especially the left side. That Solidworks model is for a 0.049" wall thickness tube so they aren't immune. I think the engine mounts made after about 2005 would be the thicker walled tube but I don't know of an easy way to check the one you have.

Peter

Yes it is a Bed Mount - the Mk2 Rotax adapter.

The Mk1 was parallel rails, with vibration isolators, mainly in shear.

The Mk2, also parallel rails, the isolators are in compression.

The Mk3, recently announced by Sonex, is a Ring Mount 1/2, from the airframe - You have to purchase the engine side from Ro Tax, better, a preloved one or if you have the talent/skill make your own.

[peter anson]

The problem is caused by the little overhang so that the tube is loaded in bending. If your engine mount doesn't have that, you don't have a problem.

Peter