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# Concise definition of exit separation

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So my project this weekend is to mount a video camera with a wide angle lens and a still camera with a zoom lens on the outside of an otter at a 45 degree angle and video several exits. This will be part of an attempt to prove once and for all that the 45 degree thing doesn't work. In order to not take away someone's favorite exit separation technique without replacing it with something else, I would like to come up with the most concise possible explanation of exit separation.

There are several ones that are good approximations. They certainly can be useful:

"Leave 1000 feet between groups by looking at the ground." This works under normal conditions i.e. jump run into the wind, opening altitude winds same direction but lower. But it doesn't work under all conditions.

"Leave X seconds between groups and then add Y seconds for every Z knots of wind at altitude." Same problem, and that will also vary depending on aircraft.

So while those will often work, I wanted to come up with a way to explain the more comprehensive solution in a manner that is readily understandable.

I think the most literally correct way to say it is that the distance you will get between the centerpoint of each group is a product of the difference in speed between the airplane and the wind at opening altitude and exit separation time. That sentence will confuse 90% of the people out there. So I need something simpler.

How about:

Figure out how fast the plane is moving in feet per second over the ground. Figure out how fast the winds at opening altitude are moving in feet per second. If the winds at opening altitude are from the same direction as the winds at exit altitude, add the two numbers. If not, subtract them. Multiply that number by the number of seconds you have between groups. That's your group separation at opening time. If you want it to be more than that, leave more time.

(For the purposes of this thread, let's pretend that there's no controversy over spaceballs landing in the same crater and all that; I'm just trying to see what language is the most understandable.)

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Probably fine on paper (I haven't taken a close look at it), but not practical for the real world (for most people). The 45 degree rule is used because it's simple. Doesn't work, but it's easy. I think the way to do it is with some kinda chart, posted in the plane by the door. On the ride up, don't know how much time to leave? Yell the winds to the guy by the door and ask him to check the exit timing. Make a chart for each type of plane. May not be perfect but it doesn't require math!
Dave

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Bill, I understand the theory behind exit separation and I agree with your points. But I'll be the first to admit that I can be lazy. While I'm capable of making complex calculations, I often look for an even *simpler* method than the ones you suggested, and I suspect other jumpers would do the same because of either laziness or inability.

Usually I'm at a big DZ and trust the separation suggested by the experienced jumpers (usually this is 8 seconds or so, and by that time it becomes very hard to see the group that left before me). Sometimes if the uppers are a little higher (it's obvious since the groundspeed is a bit lower when you spot) 12 seconds is suggested. Once the uppers were so high that the plane had a groundspeed of about 15mph. This was very obvious, it looked like we were sitting still when spotting. In this case a 20-25 second separation was the rule.

So I'm trying to think of a simple rule, like for example, a range of 8-25 seconds based on the groundspeed of the plane, which is kind of easily estimated if you're used to spotting. At least the difference between 90mph and 15mph was obvious to me that time.

I'm aware that could be too simple, since uppers aren't the only concern. How about a "worst case assumption" for winds at opening altitude (i.e. opposite direction, half strength of uppers for example). If you use that assumption all the time then you only create better separation when it's not true.

I'm just trying to think of a way we can make a very simple rule that doesn't require accounting for all the variables, that can be used by anyone without much effort...
www.WingsuitPhotos.com

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So my project this weekend is to mount a video camera with a wide angle lens and a still camera with a zoom lens on the outside of an otter at a 45 degree angle and video several exits. This will be part of an attempt to prove once and for all that the 45 degree thing doesn't work.

Anyone who would be swayed by your results would likely have picked up on some of the existing footage that clearly demonstrates the invalidity of the 45 degree concept, but having a source of information devoted to debunking the misconception could be useful.

Bad science has a life of its own.

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How about:

Figure out how fast the plane is moving in feet per second over the ground. Figure out how fast the winds at opening altitude are moving in feet per second. If the winds at opening altitude are from the same direction as the winds at exit altitude, add the two numbers. If not, subtract them. Multiply that number by the number of seconds you have between groups. That's your group separation at opening time. If you want it to be more than that, leave more time.

I just verified your results, once I figured out which two numbers you were adding.

For clarification, you add *Groundspeed* to *Headwinds at Opening Altitude* to get your effective speed for separation at opening altitude.

Making preliminary calculations part of your preflight procedure isn't a bad idea. You can get winds at opening altitude from Flight Service or a heads-up pilot, and groundspeed can be had from a GPS if one is installed in the jump aircraft.

As an aside, to get fps from kts you multiply by 5/3 => 60 kts is 100 fps and 90 knots is 150 fps (assuming I correctly recall that a nautical mile is around 6,000 feet - I'm too lazy to look it up).

Thanks Bill,

Winsor

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>I'm just trying to think of a way we can make a very simple rule that
> doesn't require accounting for all the variables, that can be used by
> anyone without much effort...

I think that sort of effort is akin to doing a 100-way without thinking too much. It's definitely possible, but for it to work you have to get at least one person to think a _lot_ about it, then they can just tell everyone else what to do.

Same here, I think. Only one person at the DZ has to know how to figure this out. Then they say "Eight seconds between groups, more if you have a bigger group" and that's simple enough for most people to understand. My two objectives would be to get the 'formula' to that one guy and try to stop all the other jumpers from using rules like the 45 rule that don't work.

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>> If the winds at opening altitude are from the same direction as the winds at exit altitude, add the two numbers. If not, subtract them. <<

Or, if the winds at opening altitude are from the same direction as the winds at exit altitude, ignore them. Otherwise, subtract them. Eliminates one calculation in most cases.

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www.jumpelvis.com

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why do the wind at opening altitude matter?

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Read the "opening high for bad spots" thread.

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www.jumpelvis.com

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I thought that was an other kind of discussion. Jumpers pulling higher compared to other jumpers without planning it and flying into line of flight.

On a normal jump you pull at the same height and fly off line of flight.

Two different things. Maybe there was a point somewhere in that thread that explained what i didn´t understand but I couldn´t find it.

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You're on the right track, this doesn't have to do with opening high (although it would be pertinent there). Look at Kallend's freefall trajectory modeling applet and observe what happens when the winds change directions before opening, compared to when they don't.
www.WingsuitPhotos.com

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Only one person at the DZ has to know how to figure this out.

Depends who that person is. Just picture a real life situation. A twin otter is flying over the DZ. The guy by the door asks an instructor how much time between exits. The instructor says 5 seconds. A guy with 30 jumps yells out "NO! I learned on dropzone.com that you need to add the blahh blahh blahhh and multiply by blah blah blah and if you do the calculation you need a separation of at least 12 seconds today!" Guess who anyone is going to listen to.

Dave

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>The instructor says 5 seconds. A guy with 30 jumps yells out "NO!
>I learned on dropzone.com . . .

Yep, that's the problem. Which is why I think the results from my experiment will get written up and sent to parachutist, since that instructor's more likely to read that.

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Yup, I've heard instructors give the 45 degree rule on the plane, and while I'm not totally adverse to arguing with someone with immensely more experience than me, the plane is not the place to do it IMO. I keep my mouth shut on the plane.

Along the same lines but slightly OT... I've heard very competent and experienced swoopers (not sure about instructors) talking about the wind hitting the topskin of their canopy and making them dive longer. It's hard to know when to speak up when you have 180 jumps and suck under canopy.
www.WingsuitPhotos.com

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Wait, there was more than one opening high-thread... maybe I got it right now.
I guess you just mean that there are two parts of the separation between two groups?:

Did I get this right

airplane speed=x feet/second
winds at exit altitude=y feet/second
winds at opening altitude=z feet/second
seconds between groups=t seconds

A. Freefall separation:
The distance from when I exit to the next person exits.
If flying with headwind on jump run I get x-y feet freefall separation each second the person after me waits.
A = (x-y)*t

B. Under canopy separation:
The distance I move towards or away from the other person under canopy while she is still in freefall.
If person after me waited t seconds, I have t seconds where I´ll make the distance A longer or shorter.
B= z*t, if the wind has the same direction
B=-z*t, if the wind has the opposite direction

A+B = horizontal distance between canopies when both of us have opened - if I flew my canopy perpendicular to the line of flight until she opened, if we didn´t track and if we had the same freefall time.

Did I get it right?? Or did I still miss something?

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What is opening altitude? Is it the 2,000 foot mark where I open, or the 5,500 foot mark where other folks open? How the heck do we figure it all out?

My preference is to simplify and use exit level winds because everybody is exiting at the same altitude, they are *almost* always a bit faster than lower winds, and it's easy to calculate using the uppers. That all provides an extra margin of safety.

In this world, simple may not be best, but simple can be applied easily and if it is a reasonable approximation of OK I'll take it...that is as long as it's not the 45 degree rule.

Tom Buchanan
Instructor (AFF, SL, IAD, Tandem)
S&TA
Author JUMP! Skydiving Made Fun and Easy
Tom Buchanan
Instructor Emeritus
Comm Pilot MSEL,G
Author: JUMP! Skydiving Made Fun and Easy

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lol...

Bill, I can't even do math on the GROUND, and you want me to do math at ALTITUDE??

j/k

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>If flying with headwind on jump run I get x-y feet freefall separation
> each second the person after me waits.

This would be a good question to ask on the other thread.

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Bill

Can I put two scenarios to you, just to make sure I understand what you're talking about.

#1
Winds at height = 20 kn.
Airspeed = 70 kn.
Acft is flying directly into wind -> groundspeed = 50 kn
Winds at opening height (say 3k) = 10 kn in the same directions as the uppers.

groundspeed + opening height headwind = effective speed
50 + 10 = 60 kn
(60 kn = 100 fps)

seperation = effective speed * exit timing

seperation = 100 * 10
=1000'

(10 seconds for 1000')

#2
Winds at height = 20 kn.
Airspeed = 70 kn.
Acft is flying directly into wind -> groundspeed = 50 kn
Winds at opening height (say 3k) = 10 kn in the opposite direction to the uppers.

groundspeed + opening height headwind = effective speed
50 + -10 = 40 kn
(40 kn ~ 70 fps)

seperation = effective speed * exit timing

seperation = 70 * 15
=1050'

(15 seconds for 1000')
--
Arching is overrated - Marlies

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My preference is to simplify and use exit level winds because everybody is exiting at the same altitude, they are *almost* always a bit faster than lower winds, and it's easy to calculate using the uppers. That all provides an extra margin of safety.

You should say "That *almost* all provides an extra margin of safety". If i understand properly your simplification, for all the cases where the winds at opening altitude are in the same general direction as the jumprun, you actually get less separation than expected.

Billvon's "concise definition of exit separation" uses the difference of winds between exit and opening altitude. The formula is accurate and reliable... its application is not, because it relies on a time, computed by somebody, for a given jumprun, then communicated to the whole load. Since there is no visual reference to cross check, i foresee situations where the whole pass would use a completely wrong exit timing, because of improper computation, improper communication, or both.

Hence, i'm not sure if the gain in accuracy usually results in more safety. However, it would be a great formula to warn you when your usual simplifications become unsafe. On these very few days, you could change your procedures to keep your extra margin of safety (and get rid of the worrying *almost*).

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What is opening altitude?

Take the maximum difference of winds between exit altitude and the ground. Then everybody is safe even with high and low openings.
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Come
Skydive Asia

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I am not sure how this will affect the thread here but here goes. Now how do different drift rates in freefall figure into it? Head down, sit fly, hybrid, small belly groups, larger belly groups, etc. This may be the fatal variable in creating this grand unified theory of exit separation. If the wind thing could be simplified could this factor be?

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How about this, we get all the engineers. They put their heads together, and after the loud dull thud, come up with a formula that takes all the different variables, aircraft speed and direction, different wind layers of speed direction and altitude, different fall rates etc.

Once the formula is laid out, one of us codes it into a program, shouldn't be to hard to make versions for the web, pc and palm. Then, we make it a kind of standard operating procedure to have the results posted in the loading area. Make the pilot responsible for updating the variables for the formula.

edit: The formula needed should work for all conditions, crosswind, downwind, no wind etc.....the only possible exception is when there is a big way in the plane and they need to be given extra time. That could probably be added but lets leave that for version 2.0

Any takers?

Methane Freefly - got stink?

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How about this, we get all the engineers. They put their heads together, and after the loud dull thud, come up with a formula that takes all the different variables, aircraft speed and direction, different wind layers of speed direction and altitude, different fall rates etc.
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Unfortunately all this data is too imprecise to plug into a consistent, reliable formula:

(1) For starters the "winds aloft" info is obtained from ascension balloons released by the NWS at various sites (not at your DZ), and even THEY say their data is inexact at a given point in time. Also winds are variable throughout the day -- they only release two balloons to measure the winds twice a day from each specified weather station. (Check out http://aviationweather.gov/products/nws/fdwinds/dynamic/boston_fd1.shtml).

(2) Another point: taking into account the imperfection of ascension balloon technology, there is rarely a great deal of difference in wind speed and direction at altitudes between 3,000' and 12,000'. If you study the NWS wind charts regularly, you will come to realize this is so.

(3) Various RW 4-ways can fall at significantly different rates: 110 mph or 122 mph, a difference of 17'/sec (the faster grp will be covering 600+ more feet of FF than the slower before opening). The fall rate difference can be even greater for bigger ways depending upon the spread of participants.

(4) Not everyone tracks as well as the next guy. How far from the center of the formation do you calculate an "average" track from a 4-way? 8-way? etc. to plug into the formula?

I suggest a formula with the "KISS" approach and err on the side of safety -- something that will apply to all situations. Here's a starting suggestion:
If the ground speed of the aircraft is 60 knots or more (>101'/sec), count 10 seconds between groups of one or two jumpers; for groups of three or more, add one additional second of space for each jumper (4-way would need 14 seconds); plus for each 10 knots slower than 60 knots ground speed, add 2 seconds of time (for 50 knots, you'd space solos 12 seconds; 4-way would need 16 seconds).

This formula would be used for any skydivers...flat, FF, sit, hydbrid, etc.

It's conservative, but safe. Can anyone suggest any good mods. of this formula?

Our jump pilots should probably be making a second pass more often than they do now.
Dave

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Get a GPS-enabled cell phone, attach a teeny drogue chute to create a bit of drag, drop it from exit altitude, and monitor its position via phone (might need a little code here). When the vertical coordinate stops changing (i.e. it hit the ground), you should have all the data you need to figure out correct separation.

Now you just need to find the phone.

Er... does GPS even report altitude? Better strap a Neptune to that puppy.

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This is not a reply just something to add.

I think the best solution is that someone that really understand the seperation issues uses the tools and presentation material from Kallend (He has already said it's ree to use). This person tries to educate everyone on the dropzone as to what really counts for exit seperation.

Before the first jump of the day, a few of the people in the know work out a reasonable exit seperation based on weather reports. This is revised once the pilot has assesed the wind using GPS. This is further revised during the day as the weather and winds dictate.

I was asked only last weekend by a jumper how long should he wait after I jumped. I had to bite my lip to stop my self laughing.
You see we were jumping from a C182 at 5000ft. The winds at 5000ft were about 20kts, dropping to 8 kts on the ground. We were all exiting solo.
I told the guy to leave 7 seconds

The way I see it, once you understand what is going on, unless you are operating large turbines, you can always err on the side of saftey. You have lots of space to play with. If you are operating large turbines, then you should have a bunch of people capable of informing the less knowledgable about the exit timing.

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It's all about time folks. You can come up with 45 degree rules or 1000 foot rules however, it all comes down to time.

Time is the factor of airspeed, wind speed, ground speed, drift, and opening altitudes that vary.

Don't make it harder than it is. When in doubt, be it a 182 or a super otter, add a few seconds. Spotting is about "time in the cone" and so is separation.

Kallend has the figures calculated into his program, try using it for a graphic display of "time".

Blues,

J.E.
James 4:8

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