From: mandel@jumprun (Hector Mandel)
Subject: Re: You're dead -- unless you pull (was Re: Tiny Broadwick
X-Newsreader: Tin 1.1 PL4
Organization: University of Illinois at Urbana
Date: Mon, 28 Sep 1992 19:49:28 GMT

busey@milton.u.washington.edu (Thomas Busey) writes:
: I've never jumped, but as a statistics person I thought I'd throw my 
: e^-(x-mu)^2/2 sigma^2 / sqrt(2 pi sigma^2) cents worth into the fray.  When
: calculating the probability of a death, two things must happen:  the main
: chute must fail, and the reserve chute must also fail.  Otherwise either 
: chute will save the jumper's life.  Now the crucial thing here is whether
: these two events are independent.  They might not be independent, for example,
: if the jumper went unconcious or bad weather pevented both chutes from opening.
: If we consider that the two events are independent, then the probability of
: dying = prob (first chute doesn't open) * prob (second chute doesn't open). 
: Using the numbers given by previous posters, this is equal to :
: 1/500 * 1/1000 or 1/500000.  Considering that any time you don't die is 
: a successful jump, the probability of success is 499999/500000.  These are 
: pretty good odds.  
: 
: Of course in real life, the two events are never truly independent- the same
: person packs both chutes, and if the first chute fails it probably indicates

This is a crucial assumption.  It is wrong.  In real life, the same person
almost NEVER packs both chutes.  Main parachutes are packed by the jumper.
FAA regulations require that the reserve parachute be packed by an FAA
licensed rigger.

The other wrong assumption is that the probability of a reserve chute not 
opening is far less than 1/1000 (this is just my opinion -- as far as I know,
there is no good, reliable data on this probability for current technology).

The other wrong assumption is that the probability of a main not opening is
1/500.  This is popycock.  I don't know what the actual number is, but I do
know from personal observations that it is far lower than this.  (I have 
witnessed tens of thousands of jumps).

The other wrong assumption is, of course that parachutes (I'm talking about
square parachutes currently in use today) have an intrinsic malfunction mode
that will cause them not to open;  I do not think they do (well, not for 
practical purposes anyway ... if they have any, I believe their probability
is infinitesimal).


: probably ignore the debate.  If you consider that training and good equipment
: make jumpers a pretty homogenous group, and you are trained and have good 
: equipment, then I'd say that the above calculations apply to you.
: 

The calculations apply to no-one because they are based on wrong assumptions.
The bottom line is that most skydivers know they are not putting their life 
on the line in a random fashion with their fate left up to chance.  I believe
most skydivers know that they are in control of their fate.  The risk we take
is not a matter of random chance.  We cannot blame our equipment for our 
mistakes.  The mistakes skydivers make are not random events.  There is 
no such thing as "the average skydiver".  The risk we take is a calculated and
conscious risk almost entirely dependent on the degree of care we take in 
packing, following safety procedures and maintaining our equipment.  Safety in 
skydiving is almost entirely a matter of judgement.  




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