Main Page Skratch's Skydiving Stuff DJan's Evolving Theatre Other Skratch Stuff
Posted to rec.skydiving September, 1999.
This is based on my previous best effort (2nd try) to articulate my understanding of exit order and exit separation. This incorporates Bryan Burke's response to that effort.
I still (in 2004) consider this to be very good except for the over simplification of taking 10 seconds worth of canopy movement for all cases. Also see the note on the "45 degree rule".
I used to give this to students just off of AFF and take them through as much of it as I could. Now I hope to mesh this with the new ISP program. There are still *WAY* too many people who have not been taught how to leave safe separation.
Also note that this whole discussion assumes that people are falling straight down into the relative wind.
If anyone has significant horizontal motion, then the problem of safe separation is not solvable by this kind of mechanical algorithm. We must rise up one level to social solutions.
Such people must either know how to stay away from the other groups, or get out on a separate pass, or get some coaching so they can fall straight down.
Remember that we are always flying relative to the other groups, and the ground, as well as with each other.
Part 1 - The Physical Situation
Part 2 - Learning How To Do It
Separation From Other Jumpers While Opening Skratch D-981 1999-09-21
------------------------------------------- ------------- ----------
The Situation in a Netshell
---------------------------
The idea is to use
- the exit order
- the exit spacing
- and the breakup maneuver - tracking away from center
- flying canopies away from jumprun
to ensure that everybody is far enough apart at opening time.
A typical situation looks like this:
- The plane flies into the uppers
- Each group gets some forward throw
- Each group blows downwind with the uppers
- The winds decrease on the way down
- Within the groups each person tracks out from center
- Each person has some canopy motion while other groups
are still in freefall
The separation we are after is between opening canopies on the bottom end.
The only control inputs we have are:
- Separation of exits on the top end
- Tracking at breakup
- Flying canopies away from jumprun on the bottom end
**** It is important to remember that we are doing relative work
**** with the ground and everybody else on the load,
**** not just the people in our own group.
UPPERS ------ > < ------------------------------------ AIRPLANE
__X __X ____X ____X __X __X
/ / / / / /
| | | | | |
\ \ | | \ \
\ \ | | \ \
\ \ | | \ \
\ \ | | \ \
\ \ | | \ \
| | | | | |
__/ \__ __/ \__ | | | |
| | | |
__/ \__ __/ \__ __/ \__ __/ \__
AFF's, Tandems Fast Falling Slow Falling
High Pullers Freeflyers Boards, RW
# # # # # # # GROUND # # # # # # # # TARGET # # # # # # # # # # # GROUND # # #
First let's gather various times, speeds and distances together.
----------------------------------------------------------------
The Numbers in Excruciating Detail
----------------------------------
ft/sec = 5280/3600 * miles/hour = 1.467 * mph
30 mph = 44 fps 120 mph = 176 fps 170 mph = 249 fps
ft/sec = 6076/3600 * knots = 1.688 * knots
(Apologies to the rest of the world but America *still* hasn't
(started using the metric system.
------
90 knots indicated is about 120 mph true at 13,000 ft (airspeed)
is about 176 fps
Forward throw on a no wind day (from John Kallend's simulation)
airspeed | slow fall | fast fall | difference
---------|-----------|-----------|-----------
80 kts | 1215 ft | 1790 ft | 575 ft
90 kts | 1320 | 1960 | 640
100 kts | 1417 | 2121 | 704
------
RW groups - 120 - 130 mph (freefall speed)
- 120 mph = 176 fps typical
Head down - 150 - 180 mph
- 170 mph = 249 fps typical
Head down freefall drift = (176/249 = .707 = 7/10) * RW drift
Head downers drift only 7/10 as far in each layer of wind because
they are affected by that wind only 7/10 as long as RW groups.
------
Sabre loaded at 1.3 - 30 mph = 44 fps forward speed
- 1,200 ft/min = 20 fps downward speed
With canopies traveling this fast we should open at least 300 ft apart,
since that is only 3 or 4 seconds before collision if we are facing
each other.
| \ | / In order to be 300 ft from our
| \ | / nearest neighbor in our own
-----X----- -----X----- group we need to track
| / | \
| / | \ 212 ft for a 4 way and
track 212 ft track 392 ft 392 ft for an 8 way.
Now let's look at separation between opening canopies.
------------------------------------------------------
First is that horizontal separation is the goal.
------------------------------------------------
Vertical separation is nice when you have it, but with opening altitudes
going from 2,000 to 3,000 ft and sometimes more, 1,000 ft snivels, people
spacing out their altitude, and malfunctions, there is really no way to
guarantee it. 1,000 ft is only 5 or 6 seconds of freefall.
Second is exit order - Boards, RW, Freestyle, Freefly, AFF, Tandem, Wing Suits.
-------------------------------------------------------------------------------
Freeflyers get more forward throw and less freefall drift and should
therefore follow the slower falling board jumpers and RW groups.
The idea that the fast fallers should go first doesn't work because
- they would get more forward throw *toward* the slow fallers
instead of away from
- the slow fallers get more freefall drift and would blow back
over the fast fallers
- even though the fast fallers would get to opening altitude 30 or 40
seconds sooner, they would have to face the target or land way short,
which means they would have 30 or 40 seconds to fly up under the
following groups while losing only 600 to 800 ft of vertical
By following the slow fallers
- they get to opening altitude first
- they are over the target
- it is the RW groups on one end and the students on the other
who must face the target *and* both the RW groups and the
students will be higher than the fast fallers
Third is how much exit separation will keep the groups far enough apart.
------------------------------------------------------------------------
The big variable is the canopy motion of group 1 while group 2 is still
in freefall. Without that the problem would be simple.
For 2 RW groups or 2 Freefly groups the exit separation at the top
would just be
212-392 ft -- group 1 tracking
212-392 ft -- group 2 tracking
300 ft -- between the two closest jumpers from groups 1 and 2
-------
724-1084 ft of exit separation with no canopy motion
(The distance between the last RW group and the first Freefly
(group would be greater than this because of forward throw
(and freefall drift.
If there were 10 seconds between exits, a group 1 canopy could move
440 ft towards group 2 with no winds
778 ft towards group 2 with 20 knot lowers opposite the uppers.
(20 knots * 10 sec = 338 ft)
So for exit separation we have
1164 ft = 724 + 440 ft for small groups and no wind shear
1862 ft = 1084 + 778 ft for medium groups with 20 knot wind shear
UPPERS ------ > < ------------------------------------ AIRPLANE
Group 2 Group 1
|< ------- 1164 to 1862 ft -------- >|
|< ------- Exit Separation -------- >|
| | The situation
| | at the moment
| < -- freefall -- > | Group 2 opens
| |
/ \ / \
/ \ < -- track -- > / \
open open open open
< ----------- ----------- >
canopy canopy
motion motion
Conclusion - An Easier to Remember Summary
------------------------------------------
I have trouble remembering all this even just sitting here writing
about it, so when I'm out at the drop zone standing in the door
leaving space between me and the previous group,
I round things off to 1,000
1,500
2,000 ft depending on the combinations.
Also, if I consider people sliding around a little in freefall,
margin for error, and other possibilities, 1,000 ft seems kind
of close for exit spacing, except maybe for individuals and 2
ways where both people can track away from jump run, so I use
1,500 and 2,000 ft as my generic numbers.
With a 90 knot jump run on a no wind day
1,500 ft is 8.5 seconds between groups
2,000 ft is 11.4 seconds between groups
so this is in line with the common practice of 8 - 12 seconds
between groups (on a no wind day).
I come from the "Look at the Ground" school of thought so
- When the group in front of me goes
- I look down
- Cover 1,000 - 1,500 - 2,000 ft across the ground
- Go
- And fly my canopy away from jumprun right after opening
UPPERS ------ > < ------------------------------------ AIRPLANE
|---1,500---|---2,000-------|---1,500---|---1,500---|---1,500---|
X X X X X X
__Tandem __AFF ____FF ____FF __RW __RW
/ . / . / . / . / . / .
| . | . | . | . | . | .
\ . \ . | . | . \ . \ .
\. \. | . | . \. \.
.\ .\ | . | . .\ .\
. \ . \ | . | . . \ . \
. \ . \ |. |. . \ . \
. | . | . . . | . |
. __/ \__ . __/ \__ . . . | . |
. . . . . | . |
. . __/.\__ __/.\__ . __/ \__ . __/ \__
. . . . . .
. . . . . .
|---1,500---|---2,000-------|---1,500---|---1,500---|---1,500---|
X X X X X X
# # # # # # # GROUND # # # # # # # # TARGET # # # # # # # # # # # GROUND # # #
Teaching / Learning Separation Between Opening Canopies
-------------------------------------------------------
The two skills to be learned are
- Judging exit separation at the top
- Getting opening separation at the bottom
The way to learn and practice these skills is to
- First examine the components of each skill
- Then design dives which exercise and focus these components
Judging Exit Separation at the Top
----------------------------------
First start learning the over all environment around the airport by
- using aerial photos
- looking out the windows and door on the way up
- looking at the airport and surrounding country side while in freefall
Ask jumpmasters, coaches and experienced jumpers what they use.
- Runways - how long are they - good measuring sticks for covering
1,500 or 2,000 ft
- which way do they point - helpful when people say things
like "winds are twelve at two six zero"
- Roads, freeways, rivers, shopping centers, race tracks, mountains
- Anything that is large and really easy to recognize
What you're after are easy to recognize landmarks and distances,
particularly near the airport but also within the 5 or 10 miles that
the plane is likely to be climbing in. I find it very comforting to
glance out the door, see a familiar landmark, and know right where
the airport is. When I'm at a new drop zone I like to make a jump or
two just hanging out in freefall looking at things.
Using a fraction of a runway as a measuring stick, mark out 1,500 ft
intervals for typical jumpruns both on an aerial photo and while
looking at the ground in freefall.
Next watch a couple loads before going up so you have an idea of what
to expect.
- Which way is jump run
- Is half the load getting out short and half long or are they
starting straight up and stringing the whole load out up wind
- Are people drifting in freefall - tandems are easiest to see
- A lot of uppers means it will take longer to cover 1,500 - 2,000 ft
- Are the ground winds the same as the uppers
- Are canopies flying in all directions or are they barely holding
Ask people who just landed where they got out and how much freefall
drift there was. Not everybody will know but you will soon learn who
notices things like that.
When the plane starts turning on jumprun look out the window to see
that the plane is indeed headed for the airport, and about how far
away it is. I actually notice more what angle it is than some distance
like 2 miles.
When the group in front of you goes, look down.
Looking straight down is a visual technique. You can't use your body
sense because the floor of the plane is not level. You're often on
your knees crouched over in an unfamiliar position, and if the plane
is turning at all that throws you even further off.
I divorce myself from the plane completely and just look out at the
horizon. When my mind and eyes are level I drop my eyes straight down.
Do this 3 times in a row and you will have a good idea what you are over.
With a little practice you can become very accurate.
If we are diving out I look out and down maybe 2 or 3 times while we
are moving 1,500 ft across the ground. Just before we get there I start
the count. If there is a climbout involved I have to guess how long it
will take, and if we climb out much faster than I expected I look out
and down while hanging there as a floater and wait until we have covered
the distance across the ground before starting the count.
Sometimes the uppers are stronger than I realized and I don't catch it
until my out and down check outside the plane. It is hard to hit the
pause button at that moment, but I make the effort because separation
at opening is so important.
For actual spotting you have to be able to look straight down but for
covering a known distance across the ground it is enough to just be
able to look down in a consistent way.
Two other ways of judging the exit separation are the 45 degree rule
and counting seconds between groups.
The 45 degree rule says:
Watch the group in front of you fall away - when they get down and back
about 45 degrees you are far enough apart and you can go.
That is basically correct on a no wind day. With strong uppers though,
the plane only moves a short distance across the ground while they are
falling to that 45 degree place, and that short distance is how close
you will be to them on the bottom end.
We are fooled by this at first because our intuitions about jumping
off of things developed for small jumps, close to the ground, with
everything holding still. When I hop off of this chair, if something
is right under me, I will hit it. If it is a couple feet over at a 45
degree angle I will miss it.
Our untrained intuitions don't give us the right answers when the chair
is moving around and there is 60 seconds worth of upper winds and fall
rates and breakups and canopy motions and so on.
One way I retrain my intuition is to practice standing in the door
with my primary focus being my relation to the ground and my motion
across it. With high uppers I can see that I am not moving very much
and the previous group is being swept away by the upper wind. When
I step out I too will be swept away by that same wind and will end
up pretty close on top of them.
With no uppers, I am covering distance across the ground and basically
leaving the previous group where they got out (except for forward throw)
and moving away from them.
Later on you can actually follow them out on a high wind day when they
get to that 45 degree place and see for yourself that you end up pretty
close on top of them. I've watched other groups in freefall a number of
times. It is very helpful in learning to see the effect of upper winds
and exit separation. Once I had the physical data I practiced seeing
things this way in my mind many times. It's cheaper there, but without
the physical data it's also incredibly easy to mislead myself.
It is also quite an experience being blown across the surface of a planet,
but large scale RW is another topic for another day.
Of course you will need to track away from them a little early on the
bottom end to get out of their way, so don't do this as a student.
Be a good citizen and get the separation before you jump out.
Another way to judge separation between groups is to count seconds.
At first this seems simpler. The difficulty is in how many seconds
for each combination of airspeed, upper winds and winds at opening
altitude.
Let's look at a 90 knot jumprun ( = 176 fps true) and see how long
it takes to cover 1,500 and 2,000 ft with some typical uppers.
(ft/sec = 6076/3600 * knots = 1.688 * knots)
uppers | uppers | true airspeed | 1,500 ft | 2,000 ft |
knots | ft/sec | ft/sec | | |
-------|--------|---------------|----------|----------|
0 | 0 | 176 | 8.5 sec | 11.4 sec |
10 | 16.9 | 159.1 | 9.4 | 12.6 |
20 | 37.8 | 138.2 | 10.9 | 14.5 |
30 | 50.6 | 125.4 | 12.0 | 15.9 |
40 | 67.5 | 108.5 | 13.8 | 18.4 |
It's a nice table. It looks very scientific. There is probably
some pattern - for each 10 knot change of airspeed with respect
to the winds at opening altitude add so many seconds to the
time between groups. But! ...
- I don't know what the uppers and lowers actually are
- They keep changing throughout the day
- So I don't really know how many seconds between groups
- It's all guess work
- I can't remember it all anyway - it's too complicated
And so I keep coming back to the idea that learning to look down and
cover a known distance is not that hard, and it's the same distance,
1,500 or 2,000 ft, done the same way every time.
And it has the added advantage that I'm looking out the door into the
situation I'm about to jump into, instead of blindly trusting a little
green light and counting some mysterious number of seconds.
I actually count seconds between all the groups in front of me because
separation is important and I like to know what people are doing with it.
When the group in front of me goes, I start counting in the back of my
mind. While looking at the ground watching the separation happen, I also
glance at them falling toward the 45 degree place.
But my real attention is on seeing that we are covering the distance
across the ground.
The 45 degree rule and counting reassure me, but looking at the ground
is the only way to know for sure.
Getting Opening Separation at the Bottom
----------------------------------------
The components of this skill are
- keeping track of your lowness, your closeness to the ground
- breaking high enough to track 200 - 400 ft out from center
- tracking
- looking around while tracking and knowing when you have gone
far enough
- grabbing rear risers upon opening
(to turn away if you are close to someone
(to turn away from jumprun
- turning away from jumprun until you see the following group
My best analogy for keeping track of lowness is the cop in the rear view
mirror. When I'm driving down the freeway and I notice a cop a few hundred
feet back, I don't fixate and start dwelling on him. I just keep doing what
I'm doing. But I don't forget he's back there either, and I periodically
check the rear view mirror to see where he is.
I have trained myself not to go for too long in freefall without checking -
my altimeter, someone else's, the ground, the horizon. Up at the top I'm
pretty relaxed, but down closer to breakup altitude, when the cop has
pulled in right behind me, I check every few seconds.
I allow 1,500 vertical ft for breakup. That means we break at planned
pull altitude plus 1,500 ft.
Tracking is an important skill and a fun activity in its own right.
It was invented by Loy Brydon in the late 50's, who got the idea from
watching ski jumpers, and the basic idea has changed very little since
then. Tracking Across the Planet has a description of how to track.
When it's time to break
- I give a wave off to the people I'm jumping with
- turn 180 from the group
(unless there is a good reason to go some other direction)
- concentrate and do the best track I can
(not only for separation but because tracking close to the
(ground is really fun
- look right, left, down, forward for other people tracking
- flare into a big reverse arch pull position to slow down
- throw the pilot chute and reach up for where the risers will be
- look up when it opens and say thank you to my canopy
- rear riser away if someone is close
- turn away from jumprun and look for the other groups
(in both directions but mainly for the following group at first)
- After a few seconds when everything has settled down I turn
toward the drop zone, collapse my slider and unstow my brakes
- If someone has a cutaway I try to watch where their stuff lands
and land by them if possible
Some Dives Which Focus on These Skills
--------------------------------------
Training Jumps for Separation at the Bottom
-------------------------------------------
| One nice jump is to do the breakup maneuver several times.
\ The tracking parts will all be in the same direction away from
| jump run. So for an east to west jumprun start by facing north
\ for example. Imagine you're on the outer edge of a record
| breaking 100 way something or other.
\
At an easy to see altitude like 11,000 ft, wave off, turn south, track
for 7 seconds, flare, go into a pull position, check altitude. See if
that took about 1,500 ft.
Hold the pull position a few seconds, throw an imaginary pilot chute,
and put your hands up where the risers will be on opening.
Then face north and start over.
On the real pull, grab the risers, face away from jump run, and find
the following group.
The idea is to practice the breakup sequence as a maneuver in its own
right and get a feel for how long the tracking part should last.
Another nice dive is to do the breakup maneuver with a partner who falls
straight down so you can turn around and see how far you got.
One factor here is that the pull is the most stressful part of the jump
and it can be hard to really focus on doing all the parts of the breakup
right when it is jammed right up against the pull.
So take a coach or an experienced friend, fill the top part of the dive
with assorted entertainment, then break at 5,500, turn, track, stop at
4,000, turn and see how far you got. This way you have 1,000 ft to just
hang out and take it all in before pulling.
| |
| | < --5,500 start the breakup maneuver 1,000 feet higher
| \
| \ < --4,000 stop the breakup, turn and look
| |
| | < --3,000 pull
If you didn't get very far, your experienced friend can turn and slide
out a ways while you are watching.
I do this high break fairly often on regular jumps where we all turn
and track and then stop and look back at each other for a while before
pulling. It seems to make the freefall last longer, and it is nice to
feel the transition from up high relative work to down low switching
to canopy mode. Plus it's fun watching other people open.
Training Jumps for Separation at the Top
----------------------------------------
Exercises on the top end are a little harder to arrange these days
with large planes and everybody getting out on one or two passes
all at 12,500.
Tracking can be done but it should all be in the same direction away
from the jump run. Tracking back and forth with other groups in the
air is a bad idea. Don't try to get back in line where your column
of air used to be. It is too easy to make a mistake.
Causing a freefall/canopy collision is a serious social blunder,
and if you live through it people will be very irritated at you.
Better is to take a 9,000 ft pass on the way up so you can practice
spotting the plane and then have the sky to yourself for tracking.
This may not be possible in certain assembly line environments.
But if it is, arrange with a coach or experienced friend to have the
plane turn in on jump run 3 or 4 miles out so you have time to watch
the jump run develop. Practice the looking out and down technique
and tell your coach when you are over or even with various landmarks.
Give corrections - 5 left or right - to guide the plane to a pre chosen
landmark such as the end of a runway. When you get there give it your
best guess and go. Look down. What did you really get out over? Can
you see your forward throw?
Tracking in formation with an experienced friend works pretty well, too.
Another useful and fun jump is to look at things. Like an eagle stepping
off a cliff and swooping into the valley below, I step off the Otter floor
and swoop into the pea gravel several miles below. The airplane, the freefall,
the canopy, the ground, it's all one entity.
And on the way I look at the separation from the group in front of me,
watch them in freefall, look at the airport, find the hanger, imagine
people in there packing, find my car in the parking lot, find the
port-o-potties, imagine someone deep in thought, life goes on, make
friends with the ground, watch cars pull up to stop signs and pull away,
people going about their lives, watch the group in front of me break up
and open, feel the transition from freefall to canopy, mingle with the
crowd, fly a nice pattern, touch down in the peas, life goes on.
Another possibility is the Cessna Spotting Seminars. In this we take four
people and one experienced jumper and go up to 3,000 or 4,000 ft and make
individual passes.
For the first person the plane comes across on a jumprun, still climbing,
and the student then guides it around a rectangular pattern - 90 left,
90 left, 90 left, and 90 left onto (the real) jumprun. Then gives 5 left
or right to some easy to recognize place - the peas, the end of a runway.
The place depends on the winds. Once the plane is at the correct altitude
it just maintains altitude. Also we don't give a cut, each person just goes.
When that person goes, the next one takes over and guides the plane
around the pattern again, and so on. I just follow the last one out.
Other Stuff
-----------
This whole discussion assumes that people are falling straight down
into the relative wind. An RW group sliding 5 ft/sec for 60 seconds
covers 300 ft. A new head downer covering 20-30 ft/sec for 60 seconds
will cover 1,200 - 1,800 ft. That means people learning head down
need to orient themselves perpendicular to the jump run.
The general idea in all of this is learning to look out the door with
some intelligence.
It is better to look first, and then go or not go, than to discover part
way down that we're too far out and have people suddenly start tracking
or pulling high. That is too uncoordinated and dangerous.
At places like Eloy and Quincy it is OK to get out too short or too long
because you can find a safe place to land and get picked up. But if the
drop zone is really tight, either with trees or with laws, you have to
ask for a go around.
In either case do your best to leave proper spacing between groups.
"Don't hose the last groups out" translates to "Just kill the people
in the first ones instead".
Summary - The Three Main Points
-------------------------------
1. Correct exit order
Slow fallers first
Fast fallers second
AFF's and Tandems last
2. Correct exit separation
Look at the Ground
Get the horizontal separation
3. Upon opening
Turn 90 degrees from jumprun
Find the following group
Some other good reading:
- Bryan Burke's writings and
Bill Von Novak's writings at the URL at the bottom of the page.
- John Kallend's freefall simulation program
http://www.iit.edu/~kallend/skydive/
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