Cessna Shoulder Belts

Cessna Shoulder Belts

<HTML>George, I read your reply on 10/31/02 regarding shoulder haarnesses and the rear wing spar. I unzipped the headliner on my 1958/59 Cessna 175 and in line with the pilot and co pilot seats in the spar is what looks like # 10 riv nuts. A 10 -24 fine thread fits in there, I didnot have an AN 3 on hand but giure that is the right size. Does this qualify me for installing with a log-book entry as you discussed? And where do I get the parts ? Air Craft Spruce sells a kit, but I cant tell what they use for attachments. A # 10 riv nut seems pretty small to mount the shoulder restraint to the spar ????? What are your recommendations on this ? Thanks, Tim Bailey</HTML>

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Re: Cessna Shoulder Belts

<HTML>A AN-3 bolt would do, along with a AN960 flat washer.  But a cleaner installation would be a stainless MS 27039C machine screw of the needed length along with a stainless AN970C washer.  (I'm estimating a MS27039C-1-10 which is about 3/4 inch long.)  (Aircraft Spruce 877/477-7823)
By the way, when dealing with AN3 or #10 screws, it's not a 24-pitch thread in there.  It's a 32-pitch thread.
  As for the req'd strength, Cessna already determined the required strength when they selected the size rivnut.  A #10 MS27039 screw is equivalent to NAS 220/Mil-S-6050 and has 125,000 to 145,000 psi tensile strength!  The shear-strength of that screw will be the least of your problems if those levels are reached in a crash.
It will take the signature of an A&P in the logs after you finish this job.
"Installed shoulder harness in accordance with ACE–00–23.561–
01] utilising _________ parts and acceptable methods, materials, and techniques." might be a suitable entry.</HTML>

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Re: Cessna Shoulder Belts

<HTML>Thanks George for the info. Tim Bailey</HTML>

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Re: Cessna Shoulder Belts

<HTML>BTW, most triangular end-fittings on harnesses require a bushing for the bolt/screw to go through.  This lets it bottom out on the bushing while letting the end-fitting have freedom to swivel.  If your end fitting isn't a near perfect match for the #10 screw, then use a bushing to provide for this.  You can make one out of steel tubing or steel bushing material from Aircraft Spruce.</HTML>

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Re: Cessna Shoulder Belts

<HTML>I ran some numbers on the bolt you are talking about.

Using a minimum pitch diameter for a #10-32 UNF 2A of 0.1658 inch and a tensile strength of 125,000psi requires a force of about 2,700 pounds.  The tensile strenght is the maximum load that a material can support without fracture when being stretched, divided by the original cross-sectional area of the material. You wouldn't want to get even close to that force on the screw.

The impulse force of a 200# man being restrained during a constant deacceleration from say 80 mph to 0 in say a 0.5 second would result in a force on the order of 1500 pounds. 
Divide this force by a three point restraint and you get 500 pounds on each attachment (divided by the number of screws).  Seems like a good safety margin for the screw.  The weak link may be in the material to which it is attached and its geometry.

my two cents</HTML>

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Re: Cessna Shoulder Belts

<HTML>Cessna had to certify the system to withstand 9 G's.  That rivnut/screw set up meets that requirement.  Remember that the main work is done by the lap belt.  The upper torso is nowhere near 200 lbs.</HTML>

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Re: Cessna Shoulder Belts

<HTML>George, I was estimating to get some kind of idea what the numbers might look like and thought it would be interesting.  9 g's that's 1800 lbs total force. Pretty startling, huh?  And if most in absorbed in the lap belt, in an accident of this magnitude with that force across your hips and waist.

Where do you get the technical specifications for Cessna equipment or is this an FAA requirement?  Is there a library on the Webb?</HTML>

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Re: Cessna Shoulder Belts

<HTML>I believe the specs are from CAR 3, the certification standards under which most of the Cessna singles were originally certified.

Excerpts:

3.391 Personnel and Cargo Accommodations - Safety belt or harness provisions.
All seats and berths and supporting structure shall be designed for a passenger weight of 170 pounds (190 pounds with parachute for the acrobatic and utility categories) and the maximum load factors corresponding to all specified flight and ground load conditions including the emergency conditions of paragraph 3.386.

3.386 (a)  Ultimate accelerations to which occupants are assumed to be subjected shall be as follows:

Upward-  Normal and Utility 3.0g, Acrobatic 4.5g
Forward- Normal and Utility 9.0g, Acrobatic 9.0g
Sideward - Normal and Utility 1.5g, Acrobatic 1.5g</HTML>

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Re: Cessna Shoulder Belts

<HTML>As Will Smith would say, "Yeah.  What HE said!" ;Þ

(Thanks, Stan.  I didn't want to have to look that up.)</HTML>

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Re: Cessna Shoulder Belts

<HTML>Thanks,

I'm searching for the reference on the webb</HTML>

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Re: Cessna Shoulder Belts

<HTML>I have CAR Part 3 as a 419 KB PDF file (70 pages!) if you'd like me to e-mail it to you.</HTML>

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<HTML>I couldn't confirm the specs are from CAR 3, the certification standards. What is a CAR?

What I did find was from  code of federal regualtions which pretty much agree with what you stated.  Excuse my ignorance here....

Cessna products must meet or exceed government regulations... I assume the airworthiness certificate based on standards in 14 CFR Part 23  and not Cessna's specifications.

There's plenty of information here, more than what I would like to repeat here. I'm almost afraid to ask - so what I'm interested in is when repairs are done, what do they have to meet, the original cessna specifications or demonstrate in some other way that they meet 14CFR 23.  As a practical matter I know the answer should be to return to original configuration because few brave hearted people would attempt to redesign these planes.  Anyone have the scoop...</HTML>

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Re: Cessna Shoulder Belts

<HTML>CAR stands for Civil Air Regulations.

The current Federal Aviation Regulations, or FARs, are re-codification of the CARs.  FAR Part 23 is the re-codification of CAR Part 3.

I really don't know if repairs on airplanes originally certificated under CAR Part 3 need to comply with the later FAR Part 23 or not.  Maybe somebody else knows.</HTML>

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Re: Cessna Shoulder Belts

<HTML>One other thing I forgot to mention:  The FARs are under Title 14, Code of Federal Regulations (CFR).</HTML>

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Re: Cessna Shoulder Belts

<HTML>The FAR Part 23 sections that are the equivalent of the CAR Part 3 sections 3.391 and 3.386 are 23.785 and 23.562.

Here's the link:<http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/MainFrame?OpenFrameSet></HTML>

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Re: Cessna Shoulder Belts

<HTML>Stan, ...Aircraft originally certificated under CAR 3 do not have to meet the requirements of later regs unless a new application for type certificate is made.  In other words, Airplanes already produced do not have to meet later standards.  An example is the very subject being discussed in this thread.  Shoulder harnesses were not originally required and therefore ANY type of shoulder restraint is better than the level of safety originally req'd for older aircraft, so the FAA will allow non-TSO'd and non-aviation harnesses to be installed as long as no structural alterations are made.  (You could almost tie a rope around your shoulders and loop it thru the closed rear seat belts, and it's better than nothing and so it's OK.)</HTML>

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Re: Cessna Shoulder Belts

<HTML>Thanks, George!

I thought that was the case, but since I wasn't sure, I figured I'd defer to someone who knew.  :-)</HTML>

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