Open Letter to USPA
Rick Lemons
The following is an open letter the the USPA on a safety issue too
long ignored.
In the November 1994 issue of Parachutist, USPA published an article
by Eric Bernstein called "Choosing The Right Spot". In the article,
Bernstein correctly points out the critical factor in separation
between exiting groups is ground speed and not air speed. In the
December 1994 issue, USPA published three letters stating that
Bernstein was wrong. As you may recall, this caused quite an argument
on the Internet. It seems that this issue splits skydivers right down
the middle. The arguments even got heated. One thing most skydivers
will agree on is that proper separation between groups is very
important (if you like living).
It is obvious that both sides of this argument can't be correct. So
far, the USPA has simply published the article, published the
rebuttal letters, and published some supporting letters. The number
of people that don't understand this subject is alarming. This is a
critical safety issue. Come on USPA--which side of this argument do
you take? You would think that the USPA would not publish the article
in the first place if it was incorrect. The USPA should not be in the
business of disseminating dangerous information to its constituents.
It has been nearly 5 months with no stand on this subject by the
USPA. We can't have half the skydiving population endangering the
other half with this kind of incorrect information. I want all those
who may exit behind me to know that Bernstein is right. In higher
head winds YOU MUST LEAVE EXTRA TIME. I want to live for another
skydive.
This will probably start another round of arguments with those that
believe you don't need extra time. That's not important. The point
is, you and I can argue this fact back and forth for years and
Bernstein's article and those that agree with him will never be able
to convince some of you. The point here is that in a situation like
this, the USPA should take a stance and publish the official response
to such a safety issue. The USPA can no longer ride the fence on this
one. If this is not something that the USPA should handle, who
should? If the USPA doesn't handle things like this, what do they
handle? Should the FAA regulate skydiving safety and control what
the USPA mission is? If the USPA won't take a stance on safety
issues, this may come to pass. Come on USPA. Take a stance.
Ben Daniels
on top of their regular lives, so we should write/call/email them so
that this issue stands out from all the other stuff that crosses their
desks.
>by Eric Bernstein called "Choosing The Right Spot". In the article,
>Bernstein correctly points out the critical factor in separation
>between exiting groups is ground speed and not air speed. In the
>who may exit behind me to know that Bernstein is right. In higher
>head winds YOU MUST LEAVE EXTRA TIME. I want to live for another
>skydive.
I find this easier to think about and do when I think in terms of
spatial separation instead of ground speed and time separation.
If I want 1,000 ft of separation on the bottom, then I need 1,000 ft
of separation on the top.
So I look out the door when the group in front of me goes and see where
they got out, then guage my climb out and exit so that I go when I'm
1,000 ft across the ground from where they got out.
I hope they also state it this way when they do take a stand.
Skratch
----------------
bdan...@csn.org
Bill Von Novak
>
> . . . (more about spotting in winds) . . .
>
>If I want 1,000 ft of separation on the bottom, then I need 1,000 ft
>of separation on the top.
>
>So I look out the door when the group in front of me goes and see where
>they got out, then guage my climb out and exit so that I go when I'm
>1,000 ft across the ground from where they got out.
this will work unless the jump run is crosswind or downwind or
the ground winds are from a different direction than the uppers. we
often have this problem since we're constrained to a east-west or west-
east jump run, due to the large foreign country a mile south of our LZ
(and the mountains to the east.)
-bill von novak D16479 AFF/SL JM95
Ben Daniels
>>If I want 1,000 ft of separation on the bottom, then I need 1,000 ft
>>of separation on the top.
>>
>>So I look out the door when the group in front of me goes and see where
>>they got out, then guage my climb out and exit so that I go when I'm
>>1,000 ft across the ground from where they got out.
>
> this will work unless the jump run is crosswind or downwind or
>the ground winds are from a different direction than the uppers. we
Don't try to confuse me :-) :-)
If my exit point is 1,000 ft from their exit point, is not my opening
point 1,000 ft from their opening point? Regardless of jumprun direction
and several layers of wind?
Is not my freefall trajectory exactly the same shape as theirs, just 1,000 ft
apart at all points top to bottom?
Skratch
----------------
bdan...@csn.org
Rick Lemons
In article <3oliam$e...@ncar.ucar.edu>,
> Don't try to confuse me :-) :-)
>
> If my exit point is 1,000 ft from their exit point, is not my
opening
> point 1,000 ft from their opening point? Regardless of jumprun
direction
> and several layers of wind?
>
> Is not my freefall trajectory exactly the same shape as theirs,
just 1,000 ft
> apart at all points top to bottom?
>
You are EXACTLY correct. This is the point we've been trying to make
all along.
Ben Daniels
>You are EXACTLY correct. This is the point we've been trying to make
>all along.
Yes, this topic has been flattened to the point of concavity even though
there still seem to be some pockets of resistance.
But even the True Believers (tm) speak in terms of ground speed and time
between exits. My point is: How do you actually do that when you're
standing in the door deciding when to go?
Let's see ...
The air speed for this plane right now is ...
The uppers are coming from ...
So I take the cosine of this angle to get the component in the direction
Now I want this much spatial separation so I divide ground speed into ..
AHA! For this situation I should wait 8.75 seconds to exit ...
I think it's easier to just look out the door and see where they got out
and then pick another exit point 1,000 ft further along.
This has the added advantage that someone is looking out the door before
the jump.
I keep using 1,000 ft because it is a round number and seems about right
for the little loads that I prefer. I wouldn't go any less.
At the current drop zone I used the little 2,300 ft runway to eyeball
distances. You could measure landmarks with your car.
I also count between exits as a sanity check but it is very much secondary
to the visual picture of where they got out and where I'm going to get out.
Also I talk to young jumpers about this quite often.
So write to your favorite BOD members about this if you want USPA to take
action.
Skratch
----------------
bdan...@csn.org
Bill Von Novak
>
>
>In article <3oliam$e...@ncar.ucar.edu>,
><bdan...@athena.esig.ucar.edu> writes:
>> Don't try to confuse me :-) :-)
>>
>>If my exit point is 1,000 ft from their exit point, is not my opening
>>point 1,000 ft from their opening point? Regardless of jumprun direction
>>and several layers of wind?
>> Is not my freefall trajectory exactly the same shape as theirs,
>> just 1,000 ft apart at all points top to bottom?
>
>You are EXACTLY correct. This is the point we've been trying to make
>all along.
yeehah! here we go again!
the shape of the trajectory is the same, but you can end up
closer on opening. here's an example:
jumprun is at 6000 feet into the wind. wind 60 knots at 6000
feet dropping off to 20 knots at 2000 feet. (so the wind drops off at 10
knots per 1000 feet.) you give the group before you 10 seconds before
you exit. after you exit, you look down at the group who exited before
you. since they're about 1000 feet lower, they're in 10 knots less
wind than you are. you will slowly be blown over them. from your
perspective, they're moving towards you. if they get under you by
opening time it would be a bad thing.
if you take this to an extreme, the group exiting before you
could actually cross your trajectory and end up opening in _front_ of
you. what, you say? how the hell can they do that if all groups are
following the same trajectory? it has to do with moving frames of
reference. you're looking at your trajectory vs. their trajectory,
but in their case, someone's moving the graph paper as you watch, so
it doesn't _look_ the same to you.
i know everyone hates math but luke emery's formula works
really well for this. it boils down to the original suggestion - in
most common cases, leaving more time between groups on windy days works.
in most cases skratch's 1000-foot (or whatever distance) rule works also.
-bill von novak D16479 AFF/SL JM95
(now where's that fireproof jumpsuit?)
BRENT J FINLEY
: if you take this to an extreme, the group exiting before you
:could actually cross your trajectory and end up opening in _front_ of
:you. what, you say? how the hell can they do that if all groups are
:following the same trajectory? it has to do with moving frames of
:reference. you're looking at your trajectory vs. their trajectory,
:but in their case, someone's moving the graph paper as you watch, so
:it doesn't _look_ the same to you.
Bill, Bill, Bill....What?
If you are subject to the same forces over each trajectory, the horizantal
separation is the same at the same altitude. Someone might open under you,
_BUT_ by the time you get there, you will have achieved the same path as they
had done because you're going to get blown up/down the jumprun the same way
they did. 10 seconds apart. PERIOD! here are the steps to understanding
this...
-Draw the ground...
-draw a jump run...
-x out two exit points 1000 feet apart at altitude. A and B
-draw all the horizantal wind vectors of different speed at what ever altitude
til you are blue in the face... -draw some squiggly hypothesized trajectory of
Group A. -Take the same shaped path and start it from exit Point B.
Now even though the jump path may have some weird shapes and a 700 mph wind at
4000 feet will certainly blow group A under Group B, don't you suppose when
Group B hits it, they will blow back across Group A and wind up at the
original 1000 feet separation when they open? (This is for two groups with
the same vertical speed. If one group falls faster because they are doing sit
flying...the sideways acceleration due to winds will be less because they will
be subject to it for less time. Not likely to affect much, though...)
Now...After you open...you still have plenty of opportunity...
Here's the picture...Now...NO MORE TALK ON THIS OK!!!??? USPA print this and
we all can go on to explain it to the OTTER full of cessna jumpers that open
on my head when they come to boogies...
exit exit winds at
Jumprun -->----------A--------1000ft--------B-------------- altitude
12000 \ \ < 10 kts
11000 \ \ < 10 Kts
10000 | | 0 kts
9000 | | 0 kts
8000 / / < 30 kts
6000 / / < 30 kts
5000 / / < 30 kts
4000 / / < 30 kts
3000 / / < 30 kts
2000 | ---1000ft | Open altitude
1000
------------------------------------------------------------------------
exit separation at point of opening will be the same. They will not occur at
the same time, but they WILL be 1000 feet apart in space...
Brent
We now return you to your regularly scheduled rec.skydiving...
Rick Lemons
In article <3om1ef$6...@qualcomm.com>, <bil...@qualcomm.com> writes:
> you. what, you say? how the hell can they do that if all groups
are
> following the same trajectory? it has to do with moving frames of
> reference. you're looking at your trajectory vs. their trajectory,
> (now where's that fireproof jumpsuit?)
The frame of reference that is important here is the ground. It
doesn't move.
Rick Lemons
In article <950508223...@pves-server.apsc.com>,
> Bill, Bill, Bill....What?
>
> If you are subject to the same forces over each trajectory, the
horizantal
> separation is the same at the same altitude. Someone might open
under you,
> _BUT_ by the time you get there, you will have achieved the same
path as they
> had done because you're going to get blown up/down the jumprun the
same way
> they did. 10 seconds apart. PERIOD! here are the steps to
understanding
> this...
Brent,
You are correct (but you didn't need me to tell you this). The point
is, you can see how many people don't understand this. We need the
USPA to take a stance on this subject. It may not have an affect on
those that disagree, but we get new skydivers all the time. The USPA
should stop this incorrect belief before it grows any larger.
Bill Von Novak
> . . . they will wind up at the original 1000 foot separation when
>they open . . . exit separation at point of opening will be the same.
>in space.
just so i'm clear on this - you believe that winds have nothing
to do with clearance from another group at pull time?
>...Now...NO MORE TALK ON THIS OK!!!???
ok. i'm all talked out from last time anyway.
Bill Von Novak
>The frame of reference that is important here is the ground. It
>doesn't move.
the frame of reference here that is important is the one
stationary to either set of jumpers after they open, which is generally
the frame that matches wind speed at opening. you want to open clear of
the other set of jumpers, not clear of an imaginary point on the ground.
Jeff Scrutton
[ AHHHHHHHHH, PLEASE, NOT THE EXIT SPACING THREAD FROM HELL! ]
: is, you can see how many people don't understand this. We need the
: USPA to take a stance on this subject. It may not have an affect on
: those that disagree, but we get new skydivers all the time. The USPA
: should stop this incorrect belief before it grows any larger.
And USPA didn't? The idea is really simple, it's called _basic_ spotting
folks and is part of getting an A license right?
In five words: EXIT SPACING IS BASIC SPOTTING.
In many more words:
-------------------
If I'm on the wind drift load I'll guess a spot; a point on the
damn ground. Snap, crackle, pop: "Oh shit, uppers were cookin'
worse than I thought, wonder if we'll make it back. Next time
I'll take the spot to the end of the runway..."
An hour later I'm back in the door, looking down, turbo snot whip
flying around the cabin, I wait for the end of the runway and jump.
"Snap, crackle, pop: Wow, Pete's got a function, cool, dude owes
beer! Shit, Pete's got a round reserve... Spot looks good though..."
The "hour later" (TIME) has dick all to do with it; fifteen minutes,
ten seconds, five seconds, zero seconds - yes zero. Consider loads
from two planes on a formation jump run that exited at the same time?
The point:
If SPOTS were separated by _space_ then so were the OPENINGS.
To paraphrase Skratch again:
----------------------------
"In order to open 1000' from the guy that just jumped I should
get out on top of a place that's 1000' away from his spot."
Obviously (?) this does not account for people tracking or flying up
jump run but then we'd never do that right?
Blue skies.
--
-Jeff scru...@predator.ocean.dal.ca
Stephen Creagh
> In article <950508223...@pves-server.apsc.com>,
> <BFI...@apsc.com> writes:
> > Bill, Bill, Bill....What?
> You are correct (but you didn't need me to tell you this). The point
> USPA to take a stance on this subject. It may not have an affect on
> those that disagree, but we get new skydivers all the time. The USPA
> should stop this incorrect belief before it grows any larger.
>
convert them. I'm supposed to be a physicist (ha!) and my gut reaction
when I read that parachutist article was that this guy was wrong and
that airspeed is what's important. I didn't have to think about it,
really, it just seemed "obvious".
Airspeed *is* the determining factor if the wind speeds are
the same at exit and opening altitudes. I have a theory that doing lots
of physics problems conditions you to be very comfortable with this
assumption, what with always taking about inertial reference
frames and all. After reading more on the controversy a couple of weeks
ago, I did an informal poll in my office --- *everyone* I
asked started thinking in this way. (Okay, there was only me and this
other guy, I'm not Gallup.) I can't explain it completely, it just
seemed right.
I felt perfectly smug about this until I read the February
Parachutist when it arrived a couple of weeks ago (!), and realised
that this assumption is bad. Well at least I was convinced that
when winds are important --- ie when they're really howling --- all
the action is up high and then it's the ground speed that's important.
I might still put up a fight for airspeed when the winds are moderate
or light, but then it doesn't really matter anyway.
By the way, I haven't seen this anywhere, maybe it appeared
on the newsgroup already (I've only been following it a couple of
weeks), but I'll write it anyway.
horizontal seperation = t x [ v(airspeed) - windspeed(exit)
+ windspeed(opening) ]
where t is the time between exits v is the airspeed of the plane and
the rest is obvious. Airspeed rules if the wind speed is
the same at exit and opening altitudes. Groundspeed =
v(airspeed)-windspeed(exit) rules if the windspeed at opening
altitude is insignificant.
So, in a sense *everyone* is right. It's just that some are
more right than others.
Stephen A11379 (will get around to that B any decade now).
WGrulkey
you contact the planet. When in free fall or under canopy you perform
RELATIVE to the atmosphere. It doesn't care about the relationship to the
planet. Only when you (flying in the atmosphere) contact the planet
again, then does your speed over the ground have any effect at all on you.
Barry Chase D9545/AFF I'95/Tandem Instructor
*sigh*
--
_..-'( ba...@gate.net )`-.._
./'. '||\\. (\_/) .//||` .`\.
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./'.||'.|||| ||||||||||||. .|||||||||||| ||||.`||.`\.
/'|||'.|||||| ||||||||||||{ }|||||||||||| ||||||.`|||`\
'.|||'.||||||| ||||||||||||{ }|||||||||||| |||||||.`|||.`
'.||| ||||||||| |/' ``\||`` ''||/'' `\| ||||||||| |||.`
|/' \./' `\./ \!|\ /|!/ \./' `\./ `\|
V V V }' `\ /' `{ V V V
` ` ` V ' ' '
URL http://www.gate.net/~barry/
Johan Conroy
> jumprun is at 6000 feet into the wind. wind 60 knots at 6000
>feet dropping off to 20 knots at 2000 feet. (so the wind drops off at 10
>knots per 1000 feet.) you give the group before you 10 seconds before
>you exit. after you exit, you look down at the group who exited before
>you. since they're about 1000 feet lower, they're in 10 knots less
>wind than you are. you will slowly be blown over them. from your
>perspective, they're moving towards you. if they get under you by
>opening time it would be a bad thing.
True, they could open directly below you. However, you have to go through
the same layers of wind that they did. Since your exit point was 1000ft (or
whatever) from their exit point, by the time you
reach the altitude where they opened you'll once again be 1000ft from the
point where they've opened.
Whoa. On second thought, I don't think it's possible that they could get in
front of you (or even below you). Suppose the trajectories look like this:
Suppose round canopies for a while, ie. ignore canopy horizontal airspeed
# Group 2 * Group 1
< Jumprun
0s # * 6000 60kt
10s # * 5000 50kt
15s # * 4000 40kt
20s # * 3000 30kt
25s # * 2000 20kt Open here
45s # * 1000 10kt <- Trajectory becomes steeper, because
65s # * 0000 0kt horizontal airspeed is halved,
while rate of descent decreases
by about 85% after opening
When the first group is at 0s, the second group is at 10s. Taking 0s as the
time when the second group exits, separation through the dive is as follows
0s # * or # *
5s # * or # *
10s # * or # *
15s # * or # *
20s # * or # *
25s # * or # *
30s # * or # *
35s # * or # *
I hope the ascii drawing is clear. While it is true that the separation
becomes closer as you descent, it can never become less than the separation
between the two exit points. If you can show me a scenario where this could
happen, please do.
I'm afraid that I have to agree with Barry on this one.
Regards
Johan
> (now where's that fireproof jumpsuit?)
No flames, just figures.
Bill Von Novak
>I hope the ascii drawing is clear. While it is true that the separation
>becomes closer as you descent, it can never become less than the separation
>between the two exit points. If you can show me a scenario where this could
>happen, please do.
group group
2 1
<--------- jump run -> wind
* *
* *
* *
* *
* *
* *
* *
* * no wind
* *
* *
* *
* * <- wind
2 *
**** **** <- opening alt
**** **1*
1 and 2 are inserted at the opening altitude of group 2. note
the trajectories flatten out when canopies open (due to lower vertical
speed under canopy.)
although both trajectories look the same, the wind has blown
group 1 under group 2 by opening time. a slightly low pull by someone
in group 2 will put them in the canopies of group 1.
so the separation at any given altitude is always the same.
however, separation at any given TIME can change drastically depending on
winds.