Higher reserve pull force on dirty rig (& reserve loop)?

[erdnarob 1]

Do you thing the rig owner could have flipped on his back and slided when landing on a sandy ground

Learn from others mistakes, you will never live long enough to make them all.

[pchapman 261]

While drpeguy's comments haven't been debated much here, I'm starting to think plain old loop length is a big part of the pull force issue, like he said. Lubrication of the loop might just make a smaller contribution to the issue, whether well siliconed, not siliconed lately, or gritty with rocky dust. How much? 10%? 20%? Who knows.

I was playing around with one modern rig over the last couple days, giving it a really tight loop, although only slightly shorter than it has been before, say 1/8". The rig was closed using a positive leverage device within the normal range of force, although the rig had to be worked along the way and the last flap was tough to close. Thus it was on the very tight side but likely nothing outside most riggers' experience.

I could record various different pull forces for that rig. I have started the pin moving at 18 lbs, and one could well have stopped there. After all, riggers aren't required to actually pop the reserve to check pull force. But after the initial movement, the pull force to move the pin was more like 22lb.

I bet the 18 vs. 22 difference was due to the very slight taper at the top end of the pin, before it gets really wide. That tapered area does go onto the grommet and to some degree allows the loop to 'slide downhill' when the pin starts to extract.

That opens the possibility of having a legal 22 lb pull force noted, but an actual pull force to fully extract the pin some pounds higher.

Going from sufficient silicone on the loop to almost too much silicone didn't have much effect, even if it were smeared onto the pin and grommet. At high loop tension, the metal on metal grinding forces stayed high and pull force didn't go down much.

Leaving a rig for an hour or overnight is a commonly accepted way to allow things to compress and make it easier to close a rig or get a correct pull force.

Sometimes leaving things a while lets pull force go up though. After noting a legal but high force to move the pin, e.g. in the 18-22 range, I left the rig overnight. Then I might still get a legal force to start the pin moving, but to almost extract the pin took 30-35 lbs. Hmm! The pull forces rather resembled those of the dusty rig that started all this thinking about pull force issues. The force is probably hardly noticeable for most people when moving an arm & ripcord handle with some momentum to the end of the ripcord slack, but still doesn't meet the rules about pull force.

So then I kneaded the reserve container for 15 seconds, just like one might do repeatedly while closing flap after flap to close a rig. Voila, pull force was back to 20-22 to almost pop the pin. Leave it half an hour and the force starts to creep up again.

So when a rig is closed, it can be totally legal, but pull 30-35 lbs later on. (Even without desert dust, and with lots of silicone on the loop). That would suggest the loop is "too tight" in some sense, but this is not detected other than by rigger feel for the loop length, or additional pull force tests beyond the legal one.

I haven't checked the FAR's, but one would hope that when making the rules, they took into account that actual pull forces might be greater. Hopefully it is not illegal to jump a rig with higher pull forces, as long as it was closed legally.

This was just some playing around with one particular rig and isn't real engineering, but brings up some pull force issues I don't see discussed, at least here on dz.


(@erdnarob: I don't know what the owner of the rig in the original post was doing. But yes I suspect they weren't always landing on their feet, or were otherwise a little casual about handling their rig in the desert.)

[Boogers 0]

pchapman

I bet the 18 vs. 22 difference was due to the very slight taper at the top end of the pin, before it gets really wide. That tapered area does go onto the grommet and to some degree allows the loop to 'slide downhill' when the pin starts to extract.

I've had riggers tell me that the shoulder of the pin should not be sitting on top of the grommet, because that leaves an airspace underneath the pin, such that just throwing the rig down onto the carpet for packing, or any other sharp whack, could cause the pin to bend. They say to lay only the pin itself across both sides of the grommet, so that there's no high spot which is subject to bending. For that reason, my pin is always primed with the shoulder back off the grommet.

Some people would look at that and say it's not fully seated. Others would say it's the way it's supposed to be.

Perhaps a subject for discussion...

[riggerrob 558]

RedundantRigger

***Parachutes de France issued that Service Bulletin because of too much "cutting oil" remaining after the housings had been formed/rolled.

I stand corrected! Do you have it by any chance?

.................................................................................

Sorry, but I lost my copy of the Parachutes de France Service Bulletin.
In essence, it says to remove enough hand tacks that you can lay the cutaway housings straight.
Then clean them with a gun-cleaning rod (.22 caliber for Americans or 5.56mm for everyone else) and a gun-cleaning solvent like Hoppes' ... or some fancy de-greasing fluid.
Pull cotton gun-cleaning patches through the housings until they emerge clean.
Re-stitch the hand-tackings.
Inspect.
Return to service.

This cleaning process is second-nature to gun-owners and old soldiers.