Here is what folks don't get. The shear strength of that single bolt is so high that should it shear from the load developed by your body on impact, you will be dead. This is because the organs in your body will be ripped out of place from the sudden stop of impact before that bolt is overloaded.IcarusSalinas wrote:Is there a reinforced anchor bracket available for mounting front seat shoulder harnesses on my 1952 C170B? The original single overhead anchor (nut plate) in the wing spar seems inadequate.
But yes, Wag Aero, Hooker and BAS have STCs to install a mount. In the case of Wag Aero and Hooker the results are a single point for seat belt attachment. The BAS system results in a mount for their inertia real system. And of course you could always remove the original nut plate and replace it with the next size up or two.
After thoughts.
My thoughts in my first paragraph were formed over 30 years ago while as a Army Warrant Officer Candidate. We had a class on accidents and accident survival. Of course the class was given by an Army Aviator who survived being burnt over most of his body. It made an impact. One of many details was the exact amount of force it takes for your organs to be ripped from there place and the fact that those forces are different depending on the direction the force was applied, wish I could remember these forces. But I digress.
I did a little sleuthing and figuring. Cessna calls for a NAS229-10 to hold the shoulder harness into the nut plate in the carry through spar. This has been superseded to an MS27039-10. This is a Pan head 10-32 structural screw. Info I found stated this screw has a double shear limit of 4250 lb or 2125 lb in single shear, which is the case of our Cessna shoulder harnesses.
If you are 200 lbs, that means your body could have as much as 10.625 forward g-load till that bolt could see a 2125 lb load. Could that kill you? Yes, depending on how fast and how long the force is applied and at what angle to your body. If the same screw was increased in diameter to 1/4-28 the shear goes to 3680 lb or 18.4 g-load from the 200 lb pound person.
Clearly putting any shoulder harness system in with the standard MS 10-32 screw is better than nothing. I've heard too many times folks being killed from the impact of their head on an object from an otherwise survivable accident. Think what your head is going to hit on a forward impact if you have no upper body forward restraint at all. It's not pretty.