From: Bill Von Novak
Newsgroups: rec.skydiving
Subject: separation cheat sheet
Date: Mon, 02 Aug 1999 06:40:07 GMT

	there's been a lot of discussion about exit separation lately.
a lot of math, theories, arguments, claims of support from experienced
people.  all interesting, but i think there's a part that's lacking -
what should a 'typical' skydiver do when they get in the plane to make
sure there's enough separation?

	now, i'm a nerd.  i actually do sit in the plane and figure out
jump run airspeed vs. opening windspeed in feet per second, and try to
come up with a number that gives about 1000 feet between groups.  then i
add a second or two for good measure.  then i add more time if there's a
big way, or if any of the freeflyers are really slidy.  and that works
out ok for me, because i have a good feel for what the winds really are,
and i know what the people at brown are generally going to do.  and that
works out ok for the other people on the load, because they see me
sitting back there by the door, and they assume that i'm figuring it
out.

	but not every DZ has handy nerds that go on the load.  so what
does a normal, non-math-obsessed jumper do when he's trying to figure it
out?  there's no perfect answer, but there are several rules of thumb
you can use.

	first, before you get on the plane, ask!  the previous load will
probably have sone clue about what happened on the last load.  even if
it's "we gave five seconds and no one died" that's at least something.
ask them how close they were to the previous or next group.  be warned,
though, that skydivers are notorious for underestimating how long it
takes them to exit, and often don't count climb out time in the total.

	also, ask where the spot is.  this info is often not passed on
(since so many places have GPS spotting) but it can help you determine
where the winds are coming from and how strong they are.  this is useful
info in any case.

	if you're on the ground, watch a load that's in the air.  do
they seem to be moving in freefall?  (assuming you can see them of
course.)  i once saved myself from an off-DZ landing by noticing that a
tandem (easily seen with its bright pink drouge) was really hauling
across the sky.  the forecast was for 20 kts, but they were closer to
50.  a good rule of thumb - if an RW group moves half a mile in
freefall, they were jumping in a 30 mph wind.  this is average, so it
might be 60 mph at altitude and 0 at opening.

	also, note what the ground winds are doing.  as a general rule,
winds at 2000 feet are close to surface wind direction, but stronger.
some places have crazy surface winds and this doesn't work.

	while waiting for the plane, note what group is where.  is there
a big way going out first?  is a first time freeflyer right after you?
they can cover distance like crazy.  perhaps you might remind him to
face 90 degrees from line of flight so an accidental track will not send
him careening into you.

	in the plane, the pilot will generally have a winds-aloft
forecast.  sometimes they get them for all altitudes i.e. 3000, 6000,
9000 and 12000.  these can be useful, but are wrong at least 30% of the
time.  as you're climbing, you can look out the window to see what the
wind is doing.  is the plane moving sideways?  if yes, there's a pretty
strong crosswind.  plane hardly moving?  there's a strong headwind.

	so now you're on jumprun at 12K, no handy nerd in sight.  you
have a good idea what the winds are doing at 3K and at 12K.  what to do?

	one excellent rule is the skratch rule.  this rule says to look
out the door and see where you are over the ground when the group ahead
of you leaves.  wait until the plane moves 1000 feet over the ground,
then go.

	there are a lot of advantages to this rule.  one, you don't need
to know what the winds are doing, other than in a general sense.  two,
it requires no math.  three, it gets you to look at the ground, and
that's a good habit to be in.

	there are some limitations, though.  it only works if the winds
and the jump run are 'normal' - i.e into the wind, with the wind the
same direction (to within 90 degrees) all the way down.  if the winds at
opening are strong, that's fine, you'll still get separation.  (in fact
you'll get more.)  if they are the _opposite_direction_ this rule will
not work.  this is a rare case though.

	an easier approach is to just wait ten seconds.  this works if
the following are true:

	1. the plane's flying into the wind, and the winds are 'normal'

	2. the plane is doing at least 80 knots (pretty normal for an
otter)

	3. the uppers are 20 kts or less

	for each 10 kts of wind add another two seconds.

	this method gives you at least 1000 feet between groups,
assuming light winds at opening altitude.  if the winds at opening are
stronger, you will get more separation.

	in both cases, use common sense.  if people break off higher (as
happens in big ways) they will track farther.  jumpers tracking from
4500 to 2000 can cover nearly 1000 feet if they're good.  that means you
have to add another 1000 feet to your wait if you are following such a
group.  newbie freeflyers can cover a lot of ground unexpectedly.  if
you have any other doubts, add even more time.  you may hose the group
behind you, but that's better than a collision.  if you notice that you
have plenty of separation when you open you can always reduce the time
you wait.

	and whatever you do, don't give in to the group behind you
screaming "go!"  this is (unfortunately) pretty common.

	i've made a lot of assumptions in the above two methods.  i
tried to pick methods and make assumptions such that you will get good
separation in most cases.  it's important to remember that there are
exceptions, and the above methods should be considered a starting point
only.

	the very best way to gauge separation is by direct observation.
 watch people from the ground, note their exit timing and opening
separation, and that will tell you what the story is with no math and no
arguing over boats in a river.  the second best way is to do the math,
or use winsor's smoke-trail-and-lights thing.  the two techniques i
mentioned are not as good.  i figure an approximation is better than
nothing, though, and may be useful to some jumpers.

-bill von

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