How to install seat belts

How to keep the Cessna 170 flying and airworthy.

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Re: How to install seat belts

Postby Bruce Fenstermacher » Tue Aug 13, 2019 11:33 am

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.

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.

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 This has been superseded to an MS27039-10.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.
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Re: How to install seat belts

Postby GAHorn » Tue Aug 13, 2019 2:29 pm

Cessna engineers intend that nutplate for shoulder harnesses installations. If you can trust their wings/struts ... You can probably trust their nutplates. :wink:
(Don't forget to also believe their warning against slips with flaps extended in your B-model also.)
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Re: How to install seat belts

Postby ghostflyer » Tue Aug 13, 2019 11:57 pm

While we are all focusing on the single bolt and it’s associated nut plate ,I think we should also be considering the surrounding structure and how it’s attachments to the fuselage . Over the years and different owners and workshops interventions many attach points are not what Cessna had originally intended. Especially the shoulder harness addition on the side support. I have just completed a course on the inflatable seat belts and they certainly have there place in the cockpit. While I have a full harness system it’s tempting to retro fit them to my aircraft . If you look at the new cessnas coming off the production line ,their seat belt attachment systems are are little more complex that a single bolt system .
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Re: How to install seat belts

Postby cessna170bdriver » Wed Aug 14, 2019 4:31 am

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.

STC’d shoulder harnesses should come with any required brackets, hardware, and installation instructions. My BAS harnesses came with powder coated steel brackets, each of which utilizes the existing anchor nut in the rear spar carry through, as well as two new structural screws through the cabin roof.
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Re: How to install seat belts

Postby iowa » Wed Sep 11, 2019 12:11 pm

would installing new seat belts
fall under
Aircraft Preventive Maintenance
and s.t. that the pilot could do
with a log entry?
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Re: How to install seat belts

Postby n2582d » Wed Sep 11, 2019 3:02 pm

FAR 43 Appendix A (c) “Preventive maintenance. Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations:
... (14) Replacing safety belts.”

Maintenance Aspects of Owning Your Own Aircraft
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Re: How to install seat belts

Postby goodair » Wed Jan 15, 2020 1:23 am

This is worth a read for anyone considering adding harness to an aircraft that was born without. This was supplied to me by the great folks at BAS.

BUMMER, the file is too large. Here is a link: ... _21-34.pdf
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