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jumplongisland

Exit Separation Chart

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Curious to if any DZs have an exit separation chart inside their aircraft? DZ i recently visited had a chart posted right next to the door with ground speed and corresponding group separation. Jump run comes up, pilot notes groundspeed and relays it to the back of the plane. Jumpers reference chart and away they exit. Curious whether or not many other DZs use this method? It made me feel much more comfortable having students exit after me when I know that they KNOW exactly how much time to give on that specific jump run. This is something i was thinking about bring up at my home dzs safety day. Discuss away!

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We have the chart posted in our Caravan. Definitely useful.

West Plains Skydiving, Ritzville WA., and the plane winters in Skydive Mesquite.
"We saved your gear. Now you can sell it when you get out of the hospital and upsize!!" "K-Dub"

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jumplongisland

...had a chart posted right next to the door with ground speed and corresponding group separation.



Without knowing what the wind speed and direction is at the deployment altitude, it is impossible to know what the acceptable exit separation time is.

It's not the wind speed at the exit altitude (which translates to ground speed) that's important. It's the difference in the wind speed (and direction) between the two that dictates safe separation.
Chuck Akers
D-10855
Houston, TX

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chuckakers

***...had a chart posted right next to the door with ground speed and corresponding group separation.



Without knowing what the wind speed and direction is at the deployment altitude, it is impossible to know what the acceptable exit separation time is.

It's not the wind speed at the exit altitude (which translates to ground speed) that's important. It's the difference in the wind speed (and direction) between the two that dictates safe separation.

I'm impressed. Somebody who gets it.

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"It's not the wind speed at the exit altitude (which translates to ground speed) that's important. It's the difference in the wind speed (and direction) between the two that dictates safe separation."



I'm not sure I follow that, Chuck. Please elaborate for us Newbies?

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Without knowing what the wind speed and direction is at the deployment altitude, it is impossible to know what the acceptable exit separation time is.

It's not the wind speed at the exit altitude (which translates to ground speed) that's important. It's the difference in the wind speed (and direction) between the two that dictates safe separation.



While technically true, unless the winds are really weird, the groundspeed chart is a pretty good approximation.

Do you have a better method that skydivers might actually use?

- Dan G

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DanG

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Without knowing what the wind speed and direction is at the deployment altitude, it is impossible to know what the acceptable exit separation time is.

It's not the wind speed at the exit altitude (which translates to ground speed) that's important. It's the difference in the wind speed (and direction) between the two that dictates safe separation.



While technically true, unless the winds are really weird, the groundspeed chart is a pretty good approximation.

Do you have a better method that skydivers might actually use?



Actually, Chuck's statement is also an approximation. It's not the difference between those 2 altitude, it's the overall wind speed and direction profile for the duration of the freefall, plus the effect of the opened canopies until everyone on the load is under canopy.

So, unless you want to start doing integrals, groundspeed is normally a close enough approximation.
Remster

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>I'm not sure I follow that, Chuck. Please elaborate for us Newbies?

What determines your separation is:
(groundspeed of the airplane + winds at opening altitude) * seconds

So if your groundspeed is 100 feet per second on jump run, and winds are 10 feet per second from the same direction at opening altitude, and you are waiting 10 seconds, you will get 1100 feet of separation between two of the same groups (like 4-ways.) Assuming they don't slide around, of course. 1100 feet is a good amount because then people break off and track towards each other, so that is reduced.

If the wind at opening is zero, then it's easier (groundspeed * seconds.) If the winds are _opposite_ at opening altitude (rare but it happens) then it's (groundspeed - winds at opening) * seconds.

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Skydive Snohomish, WA has a chart for minimum separation.

I find it especially handy for visiting jumpers who have ???? level of understanding of exit separation and it's importance. Further clarification can be discussed on the ground, but I'm glad there's a guideline in the plane that at least conveys "this is how we do it in this airplane".

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PiLFy

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"It's not the wind speed at the exit altitude (which translates to ground speed) that's important. It's the difference in the wind speed (and direction) between the two that dictates safe separation."



I'm not sure I follow that, Chuck. Please elaborate for us Newbies?



To explain it as simply as possible I will draw a scenario that isn't realistic but will illustrate the concept.

For our example we will assume that the wind at all altitudes is in the same direction and we will say the plane flies directly into the wind at a constant airspeed of 50 mph. We will also assume that everyone will exit and fall straight down at the same freefall speed and deploy at the same altitude, and that everyone is jumping round, non-steerable canopies to remove the variable of canopy speeds that can affect separation after opening.

If the wind at the exit altitude is 50 mph that would mean the plane has a ground speed of 0 mph. That could lead us to believe that no matter how much separation time is given it will never be enough because if the plane has a 0 mph ground speed it is generating no separation regardless of the time between exits. This is the fundamental misunderstanding I was referring to concerning the sign in the plane. That sign only considers the wind at exit altitude as the contributing factor in separation and that is only half the story.

What if the winds at deployment altitude are also 50 mph? In that case each group would indeed open in the same place (remember 0 mph ground speed on exit means everyone will open in the same spot), but upon deployment each group will continue to move with the wind at 50 mph. If each group gives the group ahead 10 seconds of separation for example, the group before will have drifted at 50 mph for that 10-second period after deploying and would not be in conflict. This scenario is true regardless of the speeds involved. 60/60, 40/40, or whatever. If the wind speeds are the same, the separation will be the same for any given amount of separation time.

This math works regardless of the speeds involved. If the plane goes 50 mph in a 0 mph wind condition, it would achieve a 50 mph ground speed for exit. If the winds at deployment altitude are also 0 mph, the separation would be exactly the same as in our first scenario where the plane had no ground speed at all. The only difference would be that the plane’s ground speed would create the separation rather than the winds at deployment altitude doing it.

This math works the same way with the same differences in wind speed between exit and opening altitudes. If the difference in those speeds is 20 mph, it doesn’t matter if it’s a scenario of a 40 mph headwind on exit and a 20 mph wind speed at deployment altitude, or a 30 mph headwind on exit and a 10 mph wind speed at opening. If the headwind up top is 40 mph the plane would have less ground speed than if the winds were 30 mph, but the winds at opening altitude would also be faster causing the jumpers to drift more after deployment creating the same amount of separation as the 30/10 scenario.

Higher headwinds on jump run of course inhibit the plane’s ground speed, but that only matters when the winds at opening altitude are significantly less when the jumpers won’t drift as far under canopy between groups.

Some will contest this post based on real-world circumstances like variables in wind speed and direction during the freefall, etc., but I drew the scenario in the way that I did to simply illustrate the hard facts.
Chuck Akers
D-10855
Houston, TX

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DanG

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Without knowing what the wind speed and direction is at the deployment altitude, it is impossible to know what the acceptable exit separation time is.

It's not the wind speed at the exit altitude (which translates to ground speed) that's important. It's the difference in the wind speed (and direction) between the two that dictates safe separation.



While technically true, unless the winds are really weird, the groundspeed chart is a pretty good approximation.

Do you have a better method that skydivers might actually use?



No I don't and you are absolutely correct that using the upper wind speeds only does typically do the job. However it's important for jumpers to have a true understanding of why things work the way they do or they can find themselves in a bad situation and not know why.

Here on the gulf coast for example we often have winds in the winter that are opposing by 180 degrees and sometimes the wind speed differences are pretty big. Imagine using only the upper winds as the guide when the headwind at 14K is 70 mph and the deployment altitude winds are 35 mph in the opposite direction. We really do get that configuration here and failure to consider the deployment altitude winds in that scenario could lead to disaster.
Chuck Akers
D-10855
Houston, TX

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Thank You for the lengthy explanation, Chuck. I understand it a bit better, now. It looks like I need to spend some more time on understanding this dynamic. Jumping in relatively the same conditions most of the time, I've been getting away without spending too much time thinking about this.

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TommyBotten

Try reading up on Steven Geens instructor A thesis. It's elaborate, but at the same time not too hard to follow:

http://www.apf.asn.au/Members/Information/Exit-Seperation/default.aspx



Thank You. I'll take a look at it.

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PiLFy

Thank You for the lengthy explanation, Chuck. I understand it a bit better, now. It looks like I need to spend some more time on understanding this dynamic. Jumping in relatively the same conditions most of the time, I've been getting away without spending too much time thinking about this.



You're very welcome. As other folks have posted, the vast majority of the time proper separation can be calculated knowing only the upper winds. However I think it's important to have a true understanding rather than just following a stated procedure.

Hope everyone's input helped.
Chuck Akers
D-10855
Houston, TX

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chuckakers

Do you have a better method that skydivers might actually use?----

No I don't and you are absolutely correct that using the upper wind speeds only does typically do the job. However it's important for jumpers to have a true understanding of why things work the way they do or they can find themselves in a bad situation and not know why.



^^^^^ :D

Using groundspeed (typically very good) is based on certain assumptions. People should understand what those assumptions are so they know when the chart should NOT be used. (i.e., those days when the assumptions are wrong.....)

When the winds (altitude, opening, ground) are all or some at different directions from each other,,,,or the plane is doing a crosswind jumprun,,,,,,or if the winds decrease significantly on the way up......any time the wind profile is just plain goofy, etc....... A little thinking ahead can affect our safety.

...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

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TommyBotten

Try reading up on Steven Geens instructor A thesis. It's elaborate, but at the same time not too hard to follow:

http://www.apf.asn.au/Members/Information/Exit-Seperation/default.aspx




http://www.apf.asn.au/Members/Information/Exit-Seperation/default.aspx


Thanks again for this link. There's a lot to read, but I'm finding pearls of good information there. It's filling in gaps in my understanding of this. Enough to warrant a sticky, IMHO.

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It made me feel much more comfortable having students exit after me when I know that they KNOW exactly how much time to give on that specific jump run.

You're kidding yourself if you think it's only students you have to worry about. There are many more experienced skydivers than students who will just not give a shit or turn their brains off. At least students have recently had some oversight and training.
"I encourage all awesome dangerous behavior." - Jeffro Fincher

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Rick

I would like to see a clock mounted next to the chart some people have a different "5 second" count than others



Not too sure about that one. We already have issues with the "go on green" mentality thanks to GPS. I don't think we need any new excuses for exiting without checking the airspace below for clouds and traffic. Focusing on a clock inside the plane makes it impossible to focus on the important stuff outside the plane.

We would do better to simply teach people to count properly - out loud if necessary. Practice makes (nearly) perfect, so maybe some good ol' practice is in order.
Chuck Akers
D-10855
Houston, TX

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chuckakers

***I would like to see a clock mounted next to the chart some people have a different "5 second" count than others



Not too sure about that one. We already have issues with the "go on green" mentality thanks to GPS. I don't think we need any new excuses for exiting without checking the airspace below for clouds and traffic. Focusing on a clock inside the plane makes it impossible to focus on the important stuff outside the plane.

We would do better to simply teach people to count properly - out loud if necessary. Practice makes (nearly) perfect, so maybe some good ol' practice is in order.

did not think about the distraction.
we had a swoop du jour competition a few years ago your 3 team mates left then you had to wait 5 seconds before diving after them. You were responsible for counting the 5 seconds on your own. Fastest time to the formation wins. Watching the videos we had jumpers counting anywhere from 2 to 10 seconds before diving out.
You can't be drunk all day if you don't start early!

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