Gear Leg Top Bolt Sheared
This incident occurred a couple of weeks ago. I wanted to give the factory an opportunity for analysis before I posted anything – hope you understand. I believe the factory will be issuing an Optional Service Bulletin in the near future to pull and inspect these bolts.
The facts that we know:
I’m about halfway through my Phase 1 testing – just completed Test Card 15 – 26 hours on the aircraft. I was performing this test at the “Heavy Aft” weight of 1220 pounds. On landing, it was probably closer to 1200 since I had burned off some fuel. Wasn’t my best landing – but certainly not my worst. Touched down tail first in a three-point landing position. As soon as the mains hit the runway my co-pilot side gear leg top bolt sheared – allowing the gear leg to spin 90 degrees. At this point I was now riding on the end of the axle bolt. Skidded a couple of hundred feet and stopped.
I’ve been working with the Sonex factory to identify the root cause and approve a fix. First thoughts from the factory were that it might have been a soft bolt. I expressed mailed them my bolts – unfortunately, no smoking gun – the bolts checked out good. A couple other observations. The titanium gear leg was in perfect condition – no bending at all. I know a few have bent their gear leg in the past – so the landing wasn’t all that bad. I was at gross weight – but Sonex pointed out to me that the Xenos lands at a higher weight (1275 pounds) – and there have been no issues with the landing gear. During my investigation, I also removed the top bolt from the pilot side gear leg. It was almost ready to break – probably only a few landings away. Pictures attached.
The FAA has already been out to see my aircraft and could not find any fault with the drilled holes, etc.
At this point, we are all scratching our heads to fully understand the root cause. Let’s face it – there are hundreds of these aircraft flying with no issues.
My personal thoughts:
The first thing that I noticed was that I had placed a washer under the head of the bolt. The drawing does not call for a washer in that location. This pulled the bolt back the thickness of the washer. There are two shear points on this bolt – one on each side of the gear leg collar. By pulling the bolt back a washer thickness, it appears that the collar on the nut end of the bolt is no longer in contact with the full shoulder diameter. As you know, these bolts all have rolled threads, and the bolt necks down a little just prior to the start of the threads. I believe in my case, the collar shear point closest to the nut was in this necked down area. That means that the bolt head end of the bolt was taking double the shear force normally seen on this bolt.
The only other item that might contribute to this issue would be the hole quality. As much as I tried to create a good hole – it might not have been perfect. We all know that a two fluted drill bit creates a triangular shaped hole – to some extent. Maybe this caught up with me.
Corrective Action:
I never want to slide down the runway on the end of my axle bolt again – so, I’m probably going a bit over the top. But it’s my experimental aircraft – so I can do that.
- I’m moving up to 5/16 bolts.
- I’m using NAS bolts vs. AN bolts – they have 25% more shear strength.
- I’m planning to under-drill the hole for these bolts (19/64) and finish with a 5/16 piloted reamer.
- I’ll match drill each hole with the mating collar.
- I’m planning to do this upgrade on both the top and bottom bolts.
Bill
Waiex N67WX
