Sept 5, 1998

Thoughts on Exit Order

by Bryan Burke, former manager of Skydive Arizona,
currently on USPA Board Safety and Training Committee.

Until about 1992, there was very little debate about exit order. Regardless
of the aircraft type, it was usually largest group to smallest. The
exception might be a Cessna putting out a solo student, but for larger
planes like Beeches, DC-3s, and Otters the large to small progression made
sense. For one thing, putting the largest group out first allows a good
spotter to factor in climb-out time to make the most of the jump run. For
another, a lot of pilots like to keep some weight forward in the form of
the smaller groups as the biggest group is in the door. Finally, people
pulling high, like students or tandems, can get back from a longer spot
than people pulling at 2,000. In addition, they are the most likely to
balk on the exit. If an AFF student hesitates, for example, you can always
go around for another jump run. But if the AFF student is first, hesitation
will screw everyone else on the plane. You can't really go around, knowing
perfectly well that student is under a floaty canopy at 3,500 feet. So
traditionally, exit was largest to smallest. It made sense, it worked,
and it was safe.

In the early 90s some new types of skydiving came along: skysurfing and
freestyle. Back then these jumps were exceptions to the rule (remember,
for all practical purposes freeflying didn't really start until about
1994) and you might find a practitioner on five or six loads a day at a
busy drop zone. I suspect that the custom of launching them first started
in Deland, where many pioneers of these specialized skydives practiced.
It was also probably the most influential drop zone in the world at that
time, so what went on there would be accepted elsewhere.

Sending skysurfers out first makes sense from a convenience standpoint.
Regardless of the type of aircraft, it is difficult for a skysurfer to
put on his board in the middle of the exit line up. In addition, a lot of
skydivers didn't have the concept of horizontal separation thought out
carefully.

I say this because to this day you meet people who are horrified at the
thought that they will follow out a group which has someone pulling high,
such as one of the jumpers dumping out of the center. I'll digress here
and say that this concern is one of the most stupid ones I have ever heard
of. If some guy thinks safety is enhanced because everyone plans to pull
at between 2,500 and 2,000, he is a kook. For one thing, what if everyone
on the first group pulls at 2,500 and everyone on the next one at 2,000?
It has never been a problem because they are not in the same vertical
space. What if some guy gets dumped out of a funnel at 6,000 feet? It
isn't a problem because no one is in that vertical space. (Except, of
course, the next pass or the people in his group, but that's outside the
scope of this argument!)

The myth that vertical separation enhanced safety in any way was always
just a misguided notion. Vertical separation is erased in a second or two
when someone looses track of altitude, the fall rate varies, somebody
gropes for their pilot chute for a couple seconds, or a canopy has a
deployment problem. Counting on vertical separation was never a smart move
- we didn't do it before freestyle, and we sure shouldn't do it now.

In summary, at some time in the past someone thought, for reasons that
were completely unrelated to horizontal separation between groups, that
skysurfers and freestylists should leave first. In fact, it didn't really
matter, because skysurfers and freestylists do not fall significantly
faster than RW groups! If you doubt this, just time some videos or jump
with some of them. The only skysurfers that fall really fast are those on
beginner boards. Larger boards often have the slowest fall rate on the
load! Why do you think their camera flyers wear such slow falling suits?
Putting a board out first, even though they usually pull high, has never
been a safety issue. And a normal sized man or woman doing traditional
freestyle falls only slightly faster than a four-way team. Some fall
slower! In other words, the argument for putting out these folks first
because of fall rate was spurious - the fall rate was never that much
faster!

Then along came freeflying, with fall rates that really were significantly
higher. Because freeflyers were "different" from RW people they got lumped
in with skysurfers and freestylists - put 'em out first! And what had
never been a problem suddenly became a big one. Conventional RW groups
would look down to find canopies directly beneath them, even though they
thought they had left plenty of time at the climb out. What was going on?

For some reason, many very experienced skydivers will argue issues
endlessly without ever bothering to try and model them scientifically,
form a hypothesis, and then observe jumpers in the field to determine
whether or not the hypothesis is valid. So from about 1994 to the present,
many skydivers have been working with this hypothesis: "put freeflyers
out first because they fall faster. If they fall faster, they will open
first and be lower than the group following them. On the other hand, if
they leave after a slow group, they might open underneath them, resulting
in a collision." The most rudimentary testing would show this to be a bad
hypothesis.

Observation in the field soon showed that freeflyers leaving first were
often directly below RW people at pull time. It was only a matter of time
before the scant few hundred feet of vertical separation vanished due to
the many possible pull altitude variables and a collision occurred. In
fact, this appears to have caused a fatality in Australia about a year
and a half ago.

When I pointed this out, no one wanted to believe it! Was it really
possible that putting freeflyers out first reduced separation between
groups? Our own eyes tell us so. But wouldn't it be even more dangerous
to put them out after RW?

The idea that someone might leave after you, pass you in freefall, and
pull under you is scary only if they are in the same vertical space. And
why would they be there? There are only a couple ways to have skydivers
from different groups in the same vertical space. One is they don't leave
adequate separation between exits. (Hopefully the argument that ground
speed doesn't matter is forever dead!) The other is that some other factor,
such as varying horizontal drift between groups, puts people in each
other's space.

I knew this other factor existed. I also knew that unless I could convince
people that there was a good reason to go slow fall rate to fast in the
exit order, the status quo would remain. People didn't seem to grasp the
fact that it doesn't matter who is in freefall and who is under canopy in
the event of a collision. An RW person going through a freeflyer canopy
because they followed them out, or a freeflyer going through an RW canopy
because they went the other way - what the hell is the difference? They're
both likely to die. The whole point is to prevent collisions. Back to the
bottom line: only horizontal separation counts!

For several years now, I have amused myself analyzing myths in the sport
like fall rate, canopy speed and glide, airspeed vs. ground speed, etc.
I was able to go on any load I wanted to just to make observations about
whatever I was investigating, with no group performance to worry about.
GPS, variometers, freefall recorders, videotape and personal observation
became my tools. The first thing I did when considering a problem was
verify all the numbers people throw around, usually by timing a number of
representative videos and measuring distances as accurately as possible.
Then, I converted all times into seconds and distances into feet, getting
speeds in feet per second. I think this is important, because skydivers
simply don't work in miles and hours. We understand feet and seconds.

Another thing I did was observe loads from an impartial perspective - the
ground. Few skydivers want to "waste" any time watching countless loads,
but in Arizona it was a point of pride, and safety, to minimize bad spots,
traffic problems, etc. I haven't jumped at a boogie for years; instead,
I watch every load I can, adjusting the spot, advising the pilots about
canopies from the previous plane, and so on. I like to be where I can
watch all the planes and the entire jump run, deployment, and landing
area. So I have probably watched more jump runs from the ground than anyone
alive. When I am in the plane, I like to sit up front - watch the airspeed
and groundspeed, the distances on the GPS, the climb-outs. Under canopy,
I try and spot everyone else on the load, for reserves and just to see
how the spot is working out.

From the observer's perspective, things like freefall drift become
extremely obvious. And I got to test my theories the best way - altering
jump runs and exit orders. And if you don't consider freefall drift, you
certainly can't land a car within a couple hundred yards of target center
from a Skyvan at 13,000, which I can. It was pretty easy to adapt my drift
models for inanimate objects to skydivers. And freefall drift immediately
answered the problem of freeflyers and RW people mixing it up. So get out
your calculator, pencil, and a couple sheets of paper.

First, convert all mph speeds into feet per second. To do so, multiply
mph by 1.4667. In the field, rounding off to 1.5 makes calculations easy.
Thirty mph, for example, is about 45 fps.  (44.001 to be precise.) A ninety
(indicated) knot jump run at 13,000 is really about 120 mph true, or about
175 feet per second, airspeed. On a calm day, this is also the ground
speed. Add wind, and ground speed drops.

We'll run two models, one with no wind component, and the other a very
windy day - let's say 45 mph (66 fps) uppers, 35 mph mid levels, and 20
from 3,000 to ground so that it averages 30 mph, or 45 feet per second,
from exit to pull. Everyone is jumping a Sabre at 1.3 wing loading, which,
ignoring variations in suits, pilot chutes, etc. will do 30 mph with no
problem, or 45 fps. Such a canopy in straight, full glide flight will
descend at about 1,200 feet per minute or 20 fps.

RW groups fall at about 120 - 130 mph. We'll use 120 to emphasize the
separation problem. That's 176 fps. Head down freeflyers fall at 150 to
180. 170 is pretty typical. That's about 250 fps. We'll allow 10 seconds
between groups, and assume no drift as a result of body position or
formations sliding - although  this can be significant, especially with
novices in either discipline.

Scenario one: no wind, freefly group followed by RW group.  Separation at
exit: 175 fps x 10 = 1,750 feet horizontal separation.  Separation at
opening: 1,750 between opening points. But this is affected by opening
times.  Time between exits: 10 seconds Time between openings: 35 seconds.
(allowing 65 and 40 seconds for RW and FF, respectively.)

Now, the first group out has to leave a little short of the dz or the
plane needs to make several passes, which we all know infuriates DZOs and
pilots. So as soon as they open, they turn for the dz. The line of flight
of the jump run will usually be about right over the top of the landing
area.

Distance a typical canopy covers in 35 seconds: 1,575 feet.

So, in this no wind scenario, and not allowing for any tracking for either
group, the 1,750 feet of horizontal separation at exit has diminished to
a mere 175 feet. The only consolation is that if both groups pull at 2,500
feet and no one has a deployment problem, the freefly group will be at
1,800 feet while the RW group is at 2,500. After all, what are the chances
of a USPA member EVER being in freefall at 1,800 feet?

You can throw in variable such as the heading canopies open on, time to
clear brakes, get oriented, and so on to imagine you won't be that close,
but then you have to also assume that at least one of the guys on that
4-way is tracking down the line of flight towards the freeflyer. Even a
lame tracker can do 30 mph; most can hit 50 in two or three seconds. So
if he averages 30 mph for three seconds, (imagine he tracks for five, but
he spends a third of the time speeding up and a third slowing down) he
has gone - 135 feet! Now we are down to 40 feet of horizontal separation
and 700 feet of vertical. That 700 will go by in (700/176) four seconds
- but you know what? This guy probably last looked at his alti at 4,000
feet, went for one more point, turned to track, looked for his buddies,
and has no idea what his altitude is. He doesn't need to know, either. He
knows the clock is ticking, and he needs to pull soon, but if the spot is
good or he has a traffic problem with one of his team mates, he also knows
he can hang out for a couple seconds. What he may not know is there is a
pair of freeflyers directly below him.

Scenario two: same conditions, RW goes first.  Separation at exit: 1,750
Separation at opening: same in space, not in time Time between openings:
fifteen seconds (RW opens 65 seconds after leaving the plane. FF waits
ten, has 40 second freefall, opens fifteen seconds prior to RW.)

In that  fifteen seconds, the same canopy will cover 675 feet. Here we
are assuming that one of the freeflyers opens and flies directly away from
the landing area, down the line of flight, for fifteen seconds. Remaining
horizontal separation: 1,075.

Difference in horizontal separation between Scenario one and two: 900
feet.
                            (typo - 15 sec * 20 fps = 300 ft   skr)
Difference in vertical separation: the freeflyers are 200 feet lower than
the RW people.

But when we add variables, it gets even better! The freeflyers are going
to get open and head for the dz, which is almost below them! They can
easily increase canopy separation. The 4-way has to get open and fly
straight back to the dz, towards a group that is already below them and
clearing the airspace further!

Scenario 3: FF first, average wind in freefall is 45 fps. Everyone knows
it is windy, so the RW group makes sure a few extra seconds elapses between
the FF exit and theirs. They leave 15 seconds after the freeflyers.

In that 15 seconds, the plane goes 1,635 feet.

The freeflyers, in 40 seconds of freefall, drift 1,800 feet down wind.

This puts them 3,435 feet down wind of where the RW group leaves.

The RW group, in 65 seconds of freefall, drifts 2,925 feet downwind.

   (typo - 40 sec * 20 fps = 800 ft vertical   skr)
510 horizontal feet and 600 vertical feet now separate the opening points.
The 20 mph canopy level winds mean the freefly canopies have a 15 fps
ground speed. In the forty seconds they have under canopy before the RW
group opens, they cover 600 feet. Net separation: negative 90 horizontal
feet and 600 vertical feet. To emphasize how ephemeral that separation
is, if one of the freeflyers pulls at 2,800 and one of the RW people pulls
at 2,200, it is completely gone.

But let's say the RW group only waits 10 seconds. The plane only goes
1,090 feet. That puts them, on exit, 2,890 feet upwind of where the
freeflyers will open. But they drift back 2,925. The RW opening point is
now 35 feet downwind and 600 feet above the freeflyers.  Factoring in
canopy time, the freefly group will actually be about 600 feet upwind!
Hmm, maybe it is safer to put the FF out first - as long as none of them
pull three or four seconds early, or as long as their fall rate doesn't
slow down in the middle of the dive! Make them sit flyers with a 150 mph
fall rate (about 50 seconds of freefall,) and see how the situation
changes!

Scenario 4: winds average 45 fps. RW goes first. 10 seconds between exits.
In that time, the plane goes 1,090 feet. The RW group adds on 2,925 feet
of drift. The FF group leaves the plane 4,015 feet from the RW opening
point, looses 1,800 to drift, and opens 2,215 feet upwind, 200 feet below,
and ten seconds before the RW group. They will never get closer under
canopy.    (typo - 15 sec before and 300 ft below  skr)

Plug in any numbers you want. Putting fast fallers out before slow fallers
always results in lost horizontal separation.

People have started to figure this out on a gut level. Besides the safety
factor, it's costing everyone jump dollars to put out fast fallers first,
because it strings out the exit as people leave more and more time between
groups. A second pass in an Otter costs everyone on the load a dollar and
a half. (Otters need to make $600 an hour to make a profit. That's $10
per minute. A second pass adds three minutes. Not only is that thirty
bucks down the toilet for the dz , but if we do just eight second passes
per day an entire load (24 minutes of flying time) has been lost. A load
is worth about $300 in dz income. So those eight second passes really cost
the dz about $560 for the day.

(Incidentally, faster canopies will result in the same type of cost
increase, as groups need more and more airspace to avoid canopy collisions.
Furthermore, highly loaded canopies out there have shitty glide ratios,
as low as two to one, which also means the jump runs can't be as long if
everyone is to get back.)

No matter how you analyze the problem, putting fast fallers out before
slow fallers is a bad idea, in terms of safety and in terms of drop zone
operations. That's why at Skydive Arizona we adopted the following general
policy:

Skysurfers first if they want that slot, but they must expect to be short
on occasion. The other determining factor is exit point; you don't want
cut-away boards coming down over sensitive spots! If they can put the
board on anywhere in the plane, skysurfers can just be another solo. (DZ
economic tip: benches allow skysurfers more flexibility and cut down
loading time, adding to efficiency. Combined with around-the-body belts,
as opposed to through-the- harness, they are also more crashworthy than
floor seating, and make emergency exits easier - good risk management.)

From there on, largest to smallest groups, roughly. If a FF group is the
largest on the load, they should send one or two small RW groups first,
and then go, cautioning the following jumpers to give a little extra
separation.

Solos and two ways need to know a couple of things. One, they need to be
aware of the spot and pull a little high (3,000 or so, but not higher if
two or more planes are flying) if they are a little long. Two, they need
to know the winds aloft and give adequate separation if they are following
a freefly group - or a big way that will have a huge deployment area as
people track longer. Three, they should consider tracking perpendicular
to the line of flight for at least five seconds of the jump. This should
be coordinated with other solos so they are tracking in opposite
directions.

Students and tandems should still go last. For one thing, they usually
pull high. If they pull low, they need immediate discipline! They can also
get back from longer spots. As long as they pull no lower than 3,000, and
any freeflyers in front do not pull above 3,000, there shouldn't be a
problem. This is one case where we need to depend a little on vertical
separation, until we can train everyone in the following procedure.

At pull time EVERYONE should immediately fly off of the line of flight!
The faster your canopy is, the more important this becomes. That way, if
you are a freeflyer followed by a solo student and they only wait four
seconds and drift above you, since you will be open a good 20 to 30 seconds
before them you can get well out from under. (20 seconds under a Stilleto
will get you a thousand feet off the line of flight.)

Realistically, the problem of solos, students, and freeflyers at deployment
can only be resolved by a high degree of awareness of the above
considerations or by much more separation at exit and the resulting higher
cost of jumping. Also, freeflyers need to be extremely aware of the hazards
novice freeflyers present. A novice freeflyer (and that can be someone
with thousands of jumps) learning a head down position on a solo is
basically an unguided high speed missile being fired through the jump run
if their orientation is along the line of flight. Body position induced
drift can be as much as 50 or 60 feet per second horizontally. They need
to be very aware of this and deliberately pick a heading perpendicular to
the line of flight.

I also think that in the plane or at the loading area, when people are
sorting out the order, solos and two ways need to be flexible. Maybe they
didn't want to track off the line of flight for a few seconds - but if
your plan will compromise safety, it needs to change: just like we won't
let people pull at 5,000 to try a new canopy if we have three airplanes
going.

We need to recognize that in the next decade we are going to see ever
increasing variety in freefall rates and canopy speeds. Exit planning will
become crucial to safety, as will knowledge of the line of flight. Line
of flight awareness should be requirement before anyone is allowed to jump
unsupervised from anything bigger than a C-182.

Again, cost will be a factor. If skydivers can learn the principles stated
above, and find the initiative to go out to the loading area a minute
early to determine exit order, drop zones won't need to provide a loader
for every plane. If drop zones need to start writing fall rates and pull
altitudes on every load slip and stationing a loader at the plane to ensure
a safe exit order, the cost will be passed on to the jumpers.

Incidentally, one other way to almost eliminate the problem is to fly
crosswind, semi-parabolic jump runs. But I think it will be easier on the
pilot staff to try the first approach!

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