From: Bryan Burke
Subject: Danger & Training - Separation
Date: 6 Feb 1995 11:32:23 -0500

ANOTHER SEMI LONG POST, THIS ONE ABOUT THE PROBLEM OF SEPARATION AT
DEPLOYMENT. SINCE IT IS CLOSELY RELATED TO THE OTHER TWO POSTS, I DECIDED
WHAT THE HELL, I'LL PUT IT ON. I PROMISE NO MORE FOR AT LEAST A WEEK!

It is gratifying to see people put in so much time on this forum to
discuss safety issues. Hopefully the exit separation issue has been
resolved in everyone's mind - it is imperative that we leave more room
between exits on windy days. 

For all the math and physics people out there who want to play with
another topic, here is something else run simulations on. I consider the
problem of separation on deployment to be as serious a safety issue as any
in this day and age. In the past year we have had several post deployment
collisions here. In California, two very current skydivers who often came
to Eloy were killed last year in separate post deployment canopy
collisions at their home dzs. I miss them and hope their tragic lesson
will inform others.

In both California cases, and in one here that nearly had the same result,
the situation was simple. All parties involved had high performance
canopies at wing loadings of 1.3 or higher. After break off from 4-ways
with video the individuals tracked and dumped in the usual way. Off
heading openings pointed them at each other and collisions resulted. 

For about the last three years I have been trying to persuade everyone who
will listen that 3,500 is too low to break off anything bigger than a
4-way and even that is too low if we enter all normally encountered
variables. Consider...

One: there is no way the average good skydiver can achieve a horizontal
speed of more than about 60 feet per second without considerable time to
build speed. Take into account turning away from the formation,
acceleration time and deceleration time, and wave off. Assuming the turn
to track starts at 3,500 and the pilot chute is being thrown by 2,000
(again, way too low! but that's a separate issue) that gives a maximum of
8 seconds working time for separation. Of that perhaps three is being used
for really efficient tracking. Let's say the typical skydiver breaking off
at 3,500 and opening at 2,000 can cover at most 200 horizontal feet.

Two: consider that fast canopies are probably doing about 25 feet a second
before the brakes are off. (see Peter Chapman's post on canopy speeds from
13 January, and my estimate becomes conservative) I have studied a video
where two canopies deploy with at least 150 feet of horizontal separation.
The collision takes place three seconds from the time the center sections
are inflated, before either party could correct by steering. Both
skydivers were current experts with thousands of jumps. 

Mad John of California, a math teacher, drew up some simple examples for
me a while ago. He divided a circle into four equal sections on the
assumption that the cameraman would dump in place and each of the four
skydivers would achieve perfect division of the available space, track at
maximum efficiency, and then deploy. Assume straight ahead openings and
you have no problem. Change the model so the canopies (all high
performance) can open in any direction, and you have to draw a circle
around each deployment spot big enough so that the skydiver has at least
four seconds to clear brakes, deal with line twists, etc. That translates
into 200 feet of separation to keep the circles from overlapping.

So far so good, but just barely. What if each one has a possible 20 degree
error when they pick their tracking line? What if you make it 8 skydivers
instead of 4? Or they break off low and don't have much time to track?

I recently read a fascinating book called "Extinction: Bad Luck or Bad
Genes?" The author was intrigued by various statistical models to explain
extinction. Check out those called "the gambler's ruin" and "the field of
bullets." If you break off 8-ways at 3,500 and most of the people on them
have fast canopies, sooner or later you are going to have collisions.
(This assumes all are dumping with less than a couple hundred feet of
vertical separation - account for snivels and fast openings -  we really
need to think spheres instead of circles) I am a big believer in vertical
as well as horizontal separation, but it needs to be planned, not random. 

At this time I am convinced only blind luck and the big sky theory are
keeping some of you alive - the same luck keeping alive those who are
having trouble understanding the Skratch Principle of exit separation.  We
all like to think things will always work out for the best, but
probability says each jump could be your worst - low break off from a
funnel, off heading deployment with line twists! Think about it...

Bryan Burke, Danger and Training Advisor at Skydive Arizona
"Minds are like parachutes...sometimes they just don't work!"

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