Line Dump II

[Hooknswoop 19]

Line Dump
What is it? Line dump is when the pilot chute pulls hard enough on the bad and/or the line stows that hold the bag closed are loose or small enough that the canopy is yanked out of the bag before the lines are taunt. The canopy begins to inflate and then the lines go taunt, resulting in a hard opening. The opening would be very hard and cold result in injury and/or equipment damage (broken lines, torn fabric, etc)
For this to happen, the line stows that hold the deployment bag shut must be released before the line tension would release them normally. If the other, non-locking stows released early, the only difference should be a faster, not harder opening.
How loose would the locking stows have to be? That depends. Size of the deployment bag, distance between the locking stows, weight of the bag, amount of force the pilot chute delivers at your opening speed are all factors.
Taking some measurements on an old J3 main deployment bag, I found that the bag is 13 inches wide. This is how far apart the line stows are. The locking stows are 5 inches apart. If you make 2 inch line stows, you have 8 inches of line on the outside of 2 bands with 13 inches on line between the bands. So there is 5 inches more line between the band. Since there is more line between the stows of the stows, the line between the stows is heavier and if you accelerated the bag enough, the lines would be pulled out of the stows, creating line dump. The line stow bands would have to be loose or the bag accelerated at a pretty high rate to achieve this. The heavier the line and the farther apart the stows, the bigger the problem.
Why not make line stows that put an equal amount of line outside the stow as inside? The larger the stow, the larger the chance that one stow could find its way inside another stow, creating a bag-lock. Also, larger line stows tend to turn the bag. As a line is pulled out of the stow, the bag is tilted at an angle and is exposed to the relative wind and it is easy for the bag to spin. The same thing happens with very tight stows, the bag spends more time tilted at an angle, possibly being spun by the wind.
Now look at the old style J3 bag again. The locking stows are 5 inches apart. If you make 2 inch stows, there is 8 inches of line outside the stows and 5 inches between the stows. The line outside the stows is heavier and high acceleration of the deployment bag does not tend to create line dump. Acceleration tends to hold the stows in place until line tension pulls them out. Only if the locking stows were to break, allowing the canopy to come out of the bag could the locking stows line dump.
Looking at a J3 reserve free-bag, the locking stows are only 4 inches apart, so there is even less weight between the locking stows.
Looking at reserve packing instructions, I found the Dolphin manual says 2-3 inch locking stows, the Mirage manual says 2 inches for micro-line and up to 3 inches foe Dacron line, and the Reflex manual says 1 ½ inch locking stows.
I have used Tub Stoes for thousands of jumps and have never experienced a hard opening that I could attribute to line dump.
Mr. Booth, we could use your experience w/ this one.
Hook

[fred 0]

My rigger doesn't believe that line dump has ever been a problem. He believes that while it's possible that it could happen, it would not, in fact, lead or contribute to hard openings. The time is just not there for it to make a difference.
I don't know, and I'm very inexperienced, but my rigger makes a pretty convincing argument. Anybody daring enough to do 6-7 jumps without line stows to tell us the difference? I'd prefer a double-blind test. :)

[billvon 2,989]

>My rigger doesn't believe that line dump has ever been a problem. He believes
> that while it's possible that it could happen, it would not, in fact, lead or
> contribute to hard openings. The time is just not there for it to make
>a difference.
I can tell you from experience that canopies open a lot harder when they're out of the bag than when they are in the bag. I put a few jumps on a freepacked BASE canopy vs the same canopy in a bag, and the freepacked one definitely opened harder. The situations are not exactly the same but are similar.
-bill von

[gale 0]

Thanks for the explanation. I didn't really understand the situation and now I do.
Gale
Life's not worth living if you can't feel alive

[steve1 5]

Fred,
I'm no expert either, but I'd hate to make six or seven test jumps without locking line stows. If you were opening at terminal it wouldn't be any fun. In the olden days reserve parachutes were packed without a bag. The canopy would open and then you'd fall to the end of your lines. I've talked to people who have done this and again it was a very painful thing. When the army first started static line jumping they didn't have deployment bags on their rigs. If I remember right they had T-7 canopies. Again the openings were severe. I wonder if you could have misunderstood your rigger. I thought it was common knowledge why locking stows are so important. Steve

[riggerrob 643]

To continue with steve's comment about military round parachutes without d-bags: they were unreliable without d-bags. During the Second World War, British paratroopers had direct bags and they cheerfully jumped without reserves. American paratroopers didn't have d-bags, so they really needed reserves.
The other point is that military rounds are unreliable without some method of preventing the skirt from blowing under. Back in the 1960s, the British introduced anti-inversion nets, which vastly improved reliablity. More recently, several manufacturers of military S/L chutes have perfected sliders, which also prevent the skirt from blowing under and out the wrong side.
In the present day, BASE jumpers have found that squares without sliders suffer far too many (steering) line-overs. Their solution is a tail-gate which prevents the steering lines from wandering during deployment.

[steve1 5]

Rigger Rob,
I was wondering if you could explain why a T-10 canopy is shaped the way it is. If I remember right it has a parabolic shape. Does this reduce chances of malfunction or is it meant to produce softer openings? I was also wondering why the smokejumpers don't use this type of chute. I'm not sure what type of canopy the smoke jumpers use now. They have been experimenting with a large variety of them for years (even some squares). Also your input on sliders was very informative. I'm learning a lot. Interesting stuff. Steve

[cobaltdan 0]

quick note on line dump:
too many jumpers use large bands with modern micro line canopies. this is a poor practice. small bands are intended for use with micro lines. your packer may not like the small bands, but its your neck.
sincerely,
dan
atair