> From: Skratch
> Date: Wednesday, September 09, 1998 2:46 PM
> 
> Bryan,
... I want to send a separation of groups things I posted a few weeks ago.
> 

Hello again;

Your paper on separation between groups is pretty interesting. I am glad
some people are thinking about this stuff. I agree that the sport is at a
watershed now that big planes are common, fast canopies are the rule, there
is a variety of fall rates, and we have a lot of people who come out to the
drop zone to NOT think. (It's playtime, isn't it?)

What you say is quite good. I am going to select a couple points from your
text and add some comments. Your complete paper is going in my "jump run"
file and I have already contacted the Chair of the Safety and Training
Committee about the fact that maybe we should have a SIM section on jump
runs, exit order, separation, etc.

> Separation Between Groups
> 
> In the early 70's OPEC changed the oil prices and within a few years
> multiple groups per pass was common. Since it has been going on for so
> long you would think there would be a clear procedure by now, but as is
> often the case circumstances got out ahead of generally accepted
> understanding and practice.

It was. In the DC-3 era every plane had a spotter, and often a loader. In
the pre-GPS days (drool.. Beechs and DC-3s) at Coolidge and Eloy this was
often my job, and boy do I miss it! And back then, everybody fell the same
rate and a PD 190 was the hottest canopy going. The only consideration was
whether or not you really knew what the winds were doing, and how long each
group was liable to take climbing out. I used to be able to routinely get
40 people out of a DC-3 on two passes AND land everyone on the DZ. (Well,
usually!) 
 
> Within the slow falling groups the groups could actually leave in any
> order as long as they are leaving proper horizontal separation.

Climb out time and weight and balance are the only reasons for big first
that I know of. 

> Also, perhaps the group with the longest climbout should go first in
> order to get the most groups out per pass.
> 
> This makes sense when the winds are in the same direction all the way
> down. In Colorado we often have uppers out of the west with a wind shear
> at 5,000 or 6,000 ft and lowers out of the east.
 
I think your wind shear is probably the exception to the rule. In cases
like this, I think the easiest thing to do is have a talented local come up
with a rough estimate of the total wind component between exit and opening,
and then, as you note later, remind the first ones out not to fly up the
line of flight. As you will see in my paper, putting fast out after slow
largely eliminates the latter concern because the second group (in a two
group scenario) opens BEFORE the first.

Regarding your numbers: I think most are pretty good; convert everything to
feet per second (oh, for the metric system!) since most people don't know
what a knot is. For some things, like airspeeds, I try and use true rather
than indicated. True jump run speeds are higher than most people think.

I think your canopy numbers are low. I bought a variometer/airspeed
indicator for just this reason. It is probably compromised by the fact that
the impeller is in my "bow wave" to some extent, but the straight and level
flight numbers for most canopies were higher than I thought. INDICATED
values were usually 30 to 35 mph in full glide flight for higher (1.2 or so
on up) loaded canopies. Few canopies fly less than 25 mph indicated in full
glide; most everything as fast as a Sabre at 1.2 is doing that with the
breaks still set!

On the other hand, descent rates are also higher. Typical "high
performance" canopies don't glide much better than a Para Commander, often
no better than two to one. Descent rates were usually in the 1,200 to 1,500
foot per minute range in full glide; with best brakes most canopies could
slow to 600 - 800 feet per minute. In a 90 or greater degree turn, toggle
or riser, any canopy would peg the 1,600 foot per minute max on the vario
and hit 40 to 50 mph on the airspeed indicator! I concluded, based on
timing rate through altitude, that some canopies are probably doing 2,500
fpm after a 360. No wonder the femurs break and aortas rupture! (about 30
mph vertical and another 30 horizontal!) 

When you factor in a 35 to 45 foot per second horizontal canopy speed, you
need to figure two canopies on a direct collision course will definitely
collide if they are less than 200 feet apart. That's if the pilots see the
developing problem immediately.

> How much horizontal separation do we want between groups?
> Now this is the most unclear part of this whole question.
> 
>   1 - How far do people need to track to be at least say 300 ft
>       from people in their own group? Is 300 ft enough? Are you
>       willing to open face to face with someone who is 300 ft away?
>       (Line Twists?)

Three hundred is probably a good minimum to shoot for. 

>       So to open 300 horizontal feet from people in my own group
>       I need to track 200-400 ft. (Thank you Mad John.)

> Groups                 | 4 & 4      | 4 & 8     | 8 & 8
> -----------------------|------------|-----------|----------
> Tracking               | 200+200    | 200+400   | 400+400
> Opening surge          | 100+100    | 100+100   | 100+100
> Group 1 canopy motion  |   300      |   300     |   300
> Buffer between groups  |   300      |   300     |   300
> -----------------------|------------|-----------|----------
> Separation of Groups     1200         1400        1600
> 
> This looks pretty plausible. The numbers are about right from experience
> with a little more separation between larger groups.
> 
I think your numbers reflect field experience very well - with slow
canopies. Assuming an Otter or King Air on jump run (175 fps true) and zero
ambient winds in the equation, the 1200 foot separation means seven seconds
between groups, and the 1,600 is nine. On a calm day using a 90 knot
indicated jump run, climb out time for groups normally suffices, and five
seconds between solos. A four way usually takes 8 - 12 seconds to climb out
and go, or 1,400 to 2,100 feet. Bigger than that, climb out time is always
enough on calm days, unless a small (say, 3) group follows out a big (8
plus) because of the bigger tracking spread of the large group and the easy
climb out of the small.

However, first, put in higher numbers for canopy speed. Now, about tracking
speed...
Tracking speed is harder to measure but I did what I could. At Eloy, the
ground is marked off in 1/4 mile sections by roads and ditches - a big
grid. On slow days, I would get out last or first, with the plane flying
along one of the lines. To eliminate airplane throw, I would turn 90 and
track for the whole jump. Using my skycorder I could get my seconds in
freefall and visually gauge my distance within maybe 100 yards. (Yeah, big
error factor, but as we will see, the best we can do at this time.) 

In 60 seconds, I could track a mile and a half on calm days (I might
practice tracking on windy days, but I only went for numbers when the
consensus among the pilots was that there was little to no wind.) Anyway, I
am not built like a tracker, at 5 foot 9 and 200 pounds - that's why I
started practicing! But I can average 90 mph. horizontal on a long track. 

This is with plenty of time to build speed. On a normal break-off, most of
the time is spent just getting up speed. So I think we can assume the
AVERAGE speed in a five second track is in the neighborhood of thirty to
forty miles per hour, or 45 fps times 5 - far enough for separation on a
4-way, not enough on big ways, which is why they must break off higher. 

(Your point on varying tracking rates within a dive is a good one. I often
tell people that the object of break-off is not to see how far you can
track but to see how far you can get from other people. Sometimes, that
means just hanging out while people track away from you, or pulling at
3,000 instead of 2,000. Incidentally, the slowest fall rate I ever recorded
was 106 mph by Skycorder while tracking roughly 90 mph horizontal on a long
tracking dive. And many people can out-track me!)

> Today's canopies travel several hundred feet in 10-15 seconds, and
> if the lower winds are 20-30 mph and opposite the uppers they travel
> even further (thinking now about the first groups getting out short
> and turning toward the target).

Again, the opposite lowers are a pretty unusual case. Even so, if group 2
beats group 1 to opening altitude, ta da, problem solved!

>     The groups getting out first (short of the target) should
>     leave more space between groups and fly off of the jump run
>     line until they can see the following group. It is kind of
>     fun watching the next group break up and open anyway.

As noted in the thing I sent you, flying off jump run will, I hope, soon
become a standard survival trick. 
> 
> Individuals could be closer together perhaps, but if someone is sliding
> 3 ft/sec over the course of a jump, that's a couple hundred feet, so I
> wouldn't go any closer than 800 ft which is

Another number that I think is a bit low. Have you ever videoed bad 4-ways?
I had a lot of fun for a couple years doing RW camps where I organized the
dive and then videoed it. From the video spot you get a great perspective
on fall rate changes and sliding. Someone hanging off the side of a
compressed accordion, for example, can drop the fall rate below a 4th,
separate individual's ability to float up and get an honest 15 fps
horizontal motion going. Novice freeflyers are probably often doing 30 to
50 fps learning to go head down. When working on my Pure Vertical Speed
project, which put me at freefall speeds in the 300 mph range, I would
drift about 1,500 horizontal feet on a calm day strictly from body induced
deflection, or lift. It was this stuff that first got me thinking about
fast and slow, flying off the line of flight, etc. (It also got me to think
it would be fun to have a race, fastest guy from 13,000 to ground without
busting the 2,000 foot deployment - or any bones. I'll bet with a little
practice and the right canopy I could do it in less than 75 seconds!)
  
> 3 - Separation by Spotting (Spatial)
> 
> The reason for all the effort at separating opening points is that
> 
>     To separate opening points by 800, 1200 or 1600 ft,
>     just separate exit points by 800, 1200 or 1600 ft.
> 
> Simple.
> 
> That's all there is to it.
> 
> Also, assuming both groups are falling straight down (relative to the
air),
> separation of exit points is the only thing we have control of. That and
> the groups getting out short not flying up the jump run line until they
> can see the following group.

That pretty well sums it up. The question is, how do you know 800, 1,200,
or 1,600 feet have gone by? I figure I've spotted a thousand passes, from
Cessnas at 3,000 to DC-3 formation loads at 16,000. Most people will never
get a tenth of that experience. They sure don't spend any time on the way
up looking out the door or window or at the GPS, figuring ground markers,
winds, etc.

>     At first long ago in the 60's ... to figure out how far we were
tracking. (Seems like this could be done now by tracking with a GPS.)

A guy I know is a GPS specialist for McDonnell Douglas (or whatever they
are now) flight testing. He says that in a few years they will have GPS
accuracy down to centimeters. BUT... using the stuff for skydiving is a
long way off. Even the best of small units need a stable antennae and a
little while to initialize. You would have to make an antennae suit and
stand outside the plane for a minute before jumping. But we can dream...

> I remember a couple years ago a loader for one of the Casas at Quincy
> was emphasizing to people on every load that 3-5 seconds was all you
> need between groups.

                  (A paragraph of personal remarks deleted.)

> Sport Death is still around. The Reaper Lurks. 

People don't spot him as easily, now that he traded in that old hooded robe
and clumsy reaper for a designer jumpsuit, full face helmet, and a
stiletto.

> At some point I'm going to send a version of this to USPA. Perhaps they
> will create a guide line or something.

Got it.
 
> Now that we have typical separations of opening points we can see
> how many groups might reasonably get out on a pass.
> 
The number that can safely exit on one pass is inversely proportional to
the number of people on the load who think they know it all. This is
because such people 1) want to use an exit order that does not take
advantage of the difference between fast and slow fall rates and 2) They
all want to jump an Icarus Xtreme. That is one reason why the price of
jumps is going up. The more 3,000-jumps-in-three-years wonders there are,
the more  jumps will cost. 

(Corollary: in the event there is some truly wise skydiver on the load, no
one will be at the plane in time to discuss exit considerations, since they
were too busy dirt diving (RW) or running up from the parking lot at the
last minute (freefly.) The wise one will just keep quiet most of the time,
anyway.)

I guess I'll sign off now. Let's keep in touch about this. I can't seem to
get rec.skydiving - or any newsgroup. (Blessing, or curse?) If you think my
stuff will generate some interest, please post it for me and let me know
how it goes!

Bryan

Top of this page
Main Page      Skratch's Skydiving Stuff      DJan's Evolving Theatre      Other Skratch Stuff

Copyright ©