Seatbelt Attach layup oops

[TMann]

I think the difference is that you didn't build the parachute yourself. Or maybe you did... in which case I would test it first.

Whew ........ that's great Steve. I've been looking for a volunteer!

[Marc Zeitlin]

Of course, this accident may have been entirely unsurvivable, but it is worth noting that the rear seat belts held, and both the rear passengers survived.

Thanks for the pointer. I had forgotten Tom's posting on this subject. It would be interesting to attempt to figure out if the seat belt attach points and shoulder harness attach points (per plans) would withstand the FAR's that you're testing to.

 

One possibility is that the attach points were destroyed by the crushing of the fuselage during the crash, but would work correctly in a crash that didn't harm the attach points directly. But assuming that Tom's info is correct, it sounds like the front ones failed, and that's obviously not good. On the other hand, plans built attach points have held up pretty well in many other accidents, so it's not like they're totally useless.

 

Find someone who's willing to test their plans built attach points at the same time that you test your new design - it would be an interesting comparison.

[Waiter]

I was involved in a LongEZ accident investigation several years ago in Baja Mexico. The aircraft went into the water, inverted, with no forward movement (inverted flat deepstall)

 

I'd have to dig out my photos and notes. I can't remember exactly how the shoulder attach points failed, I seem to recall the wood hard points were missing from the backrest and it looked like they were simply pulled out of the backrest structure.

 

However, I remember specifically how the lap belt attach points failed, as I was very impressed.

 

The metal piece that the nylon loops through failed. The nylon loop actually pulled the metal piece apart. The nylon didn't fail, the metal loop failed. The metal loop piece was still attached to the airframe.

 

(Keep in mind, the failure mode for the harness was vertical, there was almost zero horizontal component)

 

 

Waiter

[Steve Innova]

Find someone who's willing to test their plans built attach points at the same time that you test your new design - it would be an interesting comparison.

Actually, I'm in possession of a plans-built Cozy MkIV fuselage, in addition to my modified design. I've posted it for sale for some time, but now I'm leaning towards using it for destructive testing. If I could find local assistance, and instrumentation, it might even be possible to do dynamic testing.

[TMann]

So is it an assumption that the hardpoints for the shoulder harness were built to plans?

Falling below the glide slope is a significant deviation from 'the plan.' Are we to assume this was not the same problem with the hardpoints as well?

 

I would not jump to the conclusion that the plans methodolgy is falling short when so many variables are unknown.

[Kent Ashton]

I would not jump to the conclusion that the plans methodolgy is falling short when so many variables are unknown.

I agree. People are talking two different standards. I would bet the restraints meet FAR 23 standards, which are only designed to hold you in place at normal flight loads plus a little more.

 

If you expect restraint for the kinds of wrecks that have been discussed, seems to me you're talking chrome-moly roll cage.

[Steve Innova]

I agree. People are talking two different standards. I would bet the restraints meet FAR 23 standards, which are only designed to hold you in place at normal flight loads plus a little more.

 

If you expect restraint for the kinds of wrecks that have been discussed, seems to me you're talking chrome-moly roll cage.

Right now I'm only redesigning the shoulder harness attach points, but may revise the attachments for the lap belts as well.

 

To meet FAR 23 standards, the shoulder harness should be designed to meet the following static load requirements:

 

• FAR 23.785a specifies a design occupant weight of 215 lbs, with a 1.3x safety factor.
• FAR 23.561 specifies a static load (in the foward direction) of 9 Gs.
• FAR 27.785 (no mention of load distribution between shoulder/lap belts in FAR 23) recommends using 40% load for the shoulder harness load.
• I've heard recommendations to use a 2.0 safety factor for homebuilt composite construction. (I don't know if that replaces the FAR 23.785a 1.3x safety factor, or is in addition to it. I'll assume for the moment, that it's cumulative.)

That gives me static test load of 215lbs x 1.3 safety factor x 9 Gs x .4 x2.0 = 2012.4 lbs.

 

It's also worth noting, that my seat-belts have static load design strength of 1500 lbs. For the four belts, thats 6000 lbs.

 

If you expect restraint for the kinds of wrecks that have been discussed, seems to me you're talking chrome-moly roll cage.

I happen to think that meeting these FAR specs should allow for some reasonable survivability. Although we don't have to follow these FARs in experimentals, I don't see how it can be a bad idea to match them. Lancair and some other kit companys have made a point of designing to FAR standards.

[Steve Innova]

I'm sure this will piss some people off, but I also did some basic calculations on the standard Cozy MkIV shoulder harness.

 

Rather than try to guess at the sheer strenth of the shoulder harness attach point layups, I calculated the load capacity of the entire shoulder brace (if that fails, you don't care of the hardpoints shear out, right?)

 

Here are some basic (and generous) assumptions:

• Rectangular cross section: 3.875" x 3.875" (it's actually a triangular cross-section, so actually less of the above, but I was generous and made it rectangular since my software doesn't calcualate those)
• 2 plies BID inside, 1 ply BID interior (Being generous, I used .013" / ply x3)
• 42" long
• Distributed load over 31.2", 5.4" from either end.
• Non buckling

These assumptions result in a calculated ultimate load capacity of 1422 lbs.

 

(I speculate, using correct triangular cross section, you end up with a lower #, but I've also ignored the role of the lower portion of the seatback below the shoulder brace, as well as the map pocket. (I don't think the map pocket's role is as simple as cutting the beam into 2 separate sections (which would significantly improve it's load capacity), but I don't have the ability to calculate that.)

 

Granted, these calcs are very limitted, since I don't have access to (or know how to use) FEA software. But if anyone can show me tests (or even any design calculations) that suggest that the standard Cozy MkIV meets these requirements, I'll buy you a drink. Make that 10.

 

The only way to know ultimate load for sure is to test it.

[Marc Zeitlin]

I'm sure this will piss some people off, but I also did some basic calculations on the standard Cozy MkIV shoulder harness.

The more you talk, the more you convince me. Not saying that I agree with everything you've written, but you're approaching the issue correctly and doing a good job.

[Cozy1015]

Hello guys,

 

Rather than redirect the 'Seatbelt Attach layup oops' thread, I'll spawn off another thread.

 

Regarding the FAR standards for restraint systems, if a person were to spend much effort to see that their restraint system met those standards, you might also want to consider how those standards appear to omit consideration for incidents like race-car driver Dale Earnhardt's final crash.

 

The reason I bring this up is that I believe that what happened to Dale and other race-car drivers has happened in more than one aircraft crash. I have to wonder when I look at news photo's of fatal aircraft crashes where the cockpit is not damaged much.

* I recall such a photo of a very lightly damaged RV that came to an abrupt stop. No visible damage to the cabin or tail but the two occupants died.

* A more recent example is the Turkish Boeing 737 in the Netherlands where the flight-crew died but the crew-cabin appears fairly intact (although an abrupt stop of vertical travel for the cockpit may also have factored in that one.)

* I recall a Bonanza that ditched close to the shore in Southern California in recent years in which the sole occupant died. In the news-photo I saw no damage to the immersed airplane so I wonder on that one too.

 

A solid shoulder-harness system with no 'give' would factor into these situations so I'm more uncomfortable with a shoulder-harness that does not give than one that does.

 

There is a second reason I dislike solid and tight shoulder-harnesses. I know a guy who pushed hard against his solid-harness while attempting to reach a fuel-selector valve, developed a sharp shoulder cramp and then blacked-out in flight. He had altitude and passengers who helped revive him. (Something similar happened once to a U.S.President when he choked on a pretzel, lost consciousness, and banged his head on a coffee table)

 

And then... At the shop where I recently took my metal-plane for brake-work, I picked up a 1-page article titled something like "Airbags for Airplanes are being considered". I haven't read it yet.

 

I don't dwell on or fear these things enough to keep me from flying, I also don't ignore it.

 

Tom / ttcse.com