Seatbelts- will they hold in a crash?

Ferrying an aircraft with a mechanic friend, and as we were doing prep last night he changed out the original passenger belt for a newer one.

The older belt looked ok, typical Cessna belt from the 70's with no damage, just dirty. I asked why?

Seatbelt Failure was the determined cause is a number of fatalities in otherwise minor crashes one NTSB study determined. The failure was the STITCHING breaking, not the web or attach points. This was the main topic of an IA seminar, with a pretty pointed powerpoint, crash fatalities included.

Thread gets old, just like our pants, and degrades, even in the shade, and becomes brittle. I guess the same happens to sewing thread which will break in a sewing machine when getting old. Once the thread gets old and is subject to a strong force like and accident, it breaks allowing the person held to go forward.

Maybe this is not new to you guys, but the suggested life is about 10 years...

There are companies that take your hardware and put new web on that for a reasonable cost. That will be one of this summer's jobs for me to get done!

Just passing on information. I for one carry very valuable cargo often, so want good belts in my plane.;-)

Makes sense to me. Now that I know, mine will be changed before 10 years.

What kind of thread? Cotton? A synthetic? Dacron, like all the threads covering our planes? Is there a Service Bulletin or AD addressing this? Or just one NTSB report? Anyone familiar with the report or know how we can access it?

Jim

My understanding is that this was one NTSB finding in researching crash fatalities.

This was discussed at an IA seminar. No, I don't have much more information or the name of the report.

But in my experience, all threads (rope) gets weaker with age.

This does make sense that belts would fail because the thread let go but I have to wonder why with all the old cars on the road and all the auto accidents that belt failure hasn't been identified as a problem with cars. Could auto belts be sewn with better thread than aircraft belts?

hmmm... got a new pullback ram for my port power, just dying to find a use for... and access to a 5000lb hanging scale.... hmmm.... just need some donor belts....

aktango58 said:

My understanding is that this was one NTSB finding in researching crash fatalities.

This was discussed at an IA seminar. No, I don't have much more information or the name of the report.

But in my experience, all threads (rope) gets weaker with age.

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That certainly deserves consideration.
As far as your rope getting weaker with age, what did you expect?

I better get new blue jeans. Or somebody might see my rope condition.

My seat belts were sewn by the Air Force. They make those flimsy, hard-to-fasten American Standards look like Kiddie Car belts. On a Cub, I would be a lot more concerned with attachment points.

auto seat belts have an expiration as well, I remember seeing it while changing the seat in one of the kenworth's at work one day and if I remember correctly it was 10 years. after looking into it cat equipment does/did recommend changing the seat belts in they're equipment every 3 years!

Makes sense, fall protection harnesses expire after 5 years.

Cars have glass/plastic windows, attenuate UV....planes have plastic windows, unless treated, pass UV....

Just changed out the belts in my Pacer.....Dated 1969 and 1970 in the front, and 1956 in the back.

Mike, you still up for donor belts? if so, I'll send them to you.... PM your address.

One of my first acts in my new to me 180 was to replace the 48 year old seat belts with no shoulder harness with a BAS system.

Pacerfgoe said:

Just changed out the belts in my Pacer.....Dated 1969 and 1970 in the front, and 1956 in the back.
It
Mike, you still up for doner belts? if so, I'll send them to you.... PM your address.

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MCS repair Service
po box 672352
Chugiak AK 99567

I have some belts.. when I get time I would be happy to send them to you to destroy and thus learn... Maybe next winter..

I just saw you have donor belts... never mind..
I am curious if you would be so kind as to post your results though..
Thanks

I’ve actually seen some new Aircraft that have life limits for seat belts in the Airworthiness limitations.


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I don't remember the exact time but the nylon in a parachute if left in the sun for a week looses 50% of its strength. UV is a killer of Nylon and aircraft left out without at least a sun screen in the window will suffer. Fobjob hit it on the head.

46 Cub said:

I am curious if you would be so kind as to post your results though..
Thanks

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I'll try to video it....

No worthwhile UV protection from automotive side windows, and with no wings to provide shade I haven't seen any sun damage to worry about. None in old airplanes, either. If an outdoor airplane has UV concerns it should start with the tie down ropes. I used a set of black jacketed static climbing ropes for over 10 years and they looked great when I retired them. I still have them as backups. UV resistance in modern synthetic materials is night and day compared to 20 years ago.

I forget which belt company did the tests, but belts that were installed in an aircraft for 10 years could no longer meet the TSO requirements for strength. I forget if it was the stitching or the webbing that failed. Pretty impressive to see the video of a belt fail at something between 1000 and 1500 lb pull.


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Per car windows vs airplanes..I thought glass stopped most UV and plastic didn't..

I do think this is a bit of an esoteric discussion anyway, because if you do hit hard enough to test your belts, the difference will probably be the least of your problems... I'm thinking the airbag shoulder harness and ballistic helmet with room for A20s is my best chance for salvation but I haven't made that move yet either..starting with getting that compass out of the way..

dgapilot said:

I forget which belt company did the tests, but belts that were installed in an aircraft for 10 years could no longer meet the TSO requirements for strength. I forget if it was the stitching or the webbing that failed. Pretty impressive to see the video of a belt fail at something between 1000 and 1500 lb pull.

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The belts in aircraft are only rated to 1500# in the first place.

CharlieN said:

The belts in aircraft are only rated to 1500# in the first place.

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At least one belt mfg has tested this, and found they shredded and failed under 1400 #. The company’s stance was that the TSO’d belt mfg’s skimp on fabric since they only need to get to that low threshold.

Regarding belts and harnesses, The FAA is not unlike the government as a whole, where big money plays a part in selection and ensuring the success of existing mfg’s approved. Lots of assurances that other players won’t be able to break into the game.


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I'd wager that with an applied load of 1400# and higher the majority of retention belt failures in small airplanes would occur at the attach point tabs and hardware, not at the belts. Especially in original Cub airframes.

stewartb said:

I'd wager that with an applied load of 1400# and higher the majority of retention belt failures in small airplanes would occur at the attach point tabs and hardware, not at the belts. Especially in original Cub airframes.

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This I truly agree with. Even if the belts hold the seat structure in many planes is not engineered to take the forces applied in a crash. Be it seat rail failures or just the structure that was never considered to take these loads 50 or 80 years ago that is still in use today.
Last year riding in the back of a Mooney 201 going out to OSH, the shoulder belt just slide off my shoulder consistently. The installation had no consideration for how the belt would ride on a human body.
In any forward crash the major loads are on the shoulder belts, the belts need to go straight back from one's shoulders primarily to reduce the upward load on the attachment to the lap belt. Issue here is the shoulder belts draw the lap belt above one's pelvis. You now have a major amount of the deceleration loads on your soft organs.
The plane I am building will have 5 point belts in it installed just as in all the road race cars I build. There is plenty of information out there supporting how and why they are done as they are.
There has been now testing done for harnesses installed in light aircraft, just hanging belts from a spar tube is not something that should be considered a safe bet in a crash, especially if the structure tears apart. I personally would want the webbing to fail if a wing tip impact twists the top of the cabin off.

What is the shear strength of An-4 and -5 hardware? I assume it's at least 1-2000 lbs for each bolt(?), but Charlie probably knows specific numbers. I would agree with tabs in particular and possibly tubes breaking/shearing.

Charlie,
Regarding a wing impact twisting off the top of the cabin, I'm pretty familiar with that scenario. I believe todays design capabilities aircraft _should_ have a carefully thought out stress analysis regarding attachment locations and hard points, although I expect most aircraft we fly has very little consideration. In my case my race car belt worked, even though top of the fuselage left. I would hope the same results for anyone in a similar incident.

I don't see the belt as something that should be part of the energy absorption equation. I'd like to think that the extremities will dissipate the forces, and the harness and cage remain intact. There was a tremendous amount of testing by that fellow on the rocket sled regarding what type of forces a human body and withstand if properly restrained. I'm sure that was well over the 1400 lb rated load of basic aircraft belts.

Edit : I eluded to this in the thread I started in March, but what's the thought process regarding a 5 point harness as somewhat of a "integral" part of the structure during a crash? I say this because one of the serious benefits (as I perceive and believe) of a 5 point harness is tranferring any load on the straps down to the floor. In essence you are connecting the top fuselage to the bottom fuselage. If the top of the cabin is ripped off, the load is not transferred to the lap belt trying to rip your internals out of your head, but now is trying to pull up on the fuselage at the bottom. Again just my opinion, but I see this as being a very valuable piece of the design. I'd just as soon let the belts help hold the "cage" together for me, instead of pulling on me personally.

probably on one of the 3 info pages here
http://www.aircraftspruce.com/catalog/mepages/aluminfo.php?clickkey=249653

The AN-4 shear is theoretically 2766# with a -5 being 4408#. Even derating for single shear tabs either one is plenty for these belt loads.
I can not say I am comfortable with these bolts since I am used to 5,000# webbing in the belts I work with and utilize 3/8 and 7/16 fasteners. More a visual thing to me than a true engineering aspect but I need to build to regulations so that is what I use.

The true benefit of the 5 or 6 point harness is these sub straps keep the lap belt from climbing over your pelvis, "submarining". In a forward deceleration the major loads involved are the path of shoulder belts down through the sub strap(s). in race cars 2" lap belts with 3" wide shoulder belts are becoming common. Add to that the shoulder belts commonly reduce to 2" wide passing one's neck going aft. This is mandatory with some head & neck support systems.

The webbing in all the belts do stretch allot under high load. The long length of shoulder belts going up the the spar structure with the shallow angles from your chest to the anchor point will allow one's torso to travel forward quite some distance. One is lucky if the lower body rises off the set since that reduces submarining. Issue being if there are downward forces as one would expect the lap belt is prone to rise if no held down.

Now everything I wrote above relates to belts with a central buckle point over your pelvis. A side located buckle with a single diagonal chest support, Well those are why our cars have airbags, very little high load situations are safe with them.

Myself, I will have a 5 or 6 point belt system with the shoulder belts going straight back to their anchor points. This is fine for me with a side by side seater and easy enough in the back of a tandem. Clearly the front seat, if you carry a passenger, things are a bit inconvenient and compromises are involved.

Mounting points, any tab should be welded to a doubler. A tab of any thickness welded crossgrain to a thinwall tube will be prone to ripping out. Many options on how to address this.

mike mcs repair said:

MCS repair Service
po box 672352
Chugiak AK 99567

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Put them in the mail today, should be there in about 10 days....