Sonex strength

This is just a reprint of a post I put on the Sonex Builder’s Forum in January 2016.

I had the opportunity to test the strength of my Sonex recently so thought I’d give all you builders and flyers a warm fuzzy feeling. I was flying with a friend at close to MAUW on a warm day. The sun had been heating the ground up but cloud had moved in forcing us down to about 2000 ft AGL, so we were bumping along copping both turbulence from the clouds as well as thermals off the ground. Speed was backed off to under 110 knots when we passed over a pine plantation, much darker than the surrounding country, and hit a “rock”. There was quite a loud thump and we were thrown up against the seat belts, head-sets dislodged and ipad hit the canopy followed by relative calm and an uneventful landing, although I was pretty careful to avoid other pine plantations. Maximum recorded on the G-meter was +6.9, -0.5. I don’t recall ever seeing higher than 3g on it before.

Kerry Fores suggested the following check list:
• Spars and wing skins for permanent deformation.
• Tail surfaces, for deformation and security.
• Control hinges for security and cracks.
• Seat support structure and hinges.
• All structure for loose rivets.
• Engine mount for deformation or cracks
• Fuel tank support strap integrity
• Deformation of the upper longerons where the fuel tank support straps are attached.
I also checked the top longeron splice and tailplane attachment to longerons using a USB microscope and did a dye-penetrant check of the fuselage attachments to the engine mount and the result was:

No visible damage – I was pretty happy about that.

A couple of related points:
• My Sonex has aluminium tank straps – not the later type steel ones.
• All the rivets I used to attach piano hinge to the seat pan are 3/32 solids, which proved to be easily strong enough.
• My engine mount is in my opinion considerably stronger than the stock Sonex mount.

Sonex 894 engine and mount