Why different separation times?

[bfilarsky 0]

I've been trying to figure this one out for a while. Why are the separation times different for different winds aloft speeds? My best guess is due to the natural gradient, where the speed tends to decrease with a decrease in altitude, causing the jumper to be affected less by the wind than the aircraft. However, no one really seems to ever take into consideration winds at any altitude other than jump run.

The part that doesn't make sense about that, is that as far as I can figure, IF the wind was at a constant velocity from jump run all the way to the ground, it shouldn't make any difference what the speed is, as the jumper will always be affected equally with regards to the aircraft.

Can I get some insight into this?

[mnealtx 0]

Because it affects the speed over ground, which affects the separation between groups.

Higher speed over ground means less time needed between groups for proper separation. Conversely, a lower groundspeed equates to more time needed for proper separation.

Spreadsheet info by billvon, here.

Mike
I love you, Shannon and Jim.
POPS 9708 , SCR 14706

[LloydDobbler 2]

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The part that doesn't make sense about that, is that as far as I can figure, IF the wind was at a constant velocity from jump run all the way to the ground, it shouldn't make any difference what the speed is, as the jumper will always be affected equally with regards to the aircraft.

To put what Mike said a different way, what you need to worry about is where the groups will be with regards to the ground, not the aircraft.

If the ground speed is only 40 knots & people stick to standard 7-second exit separation, 5-way #1 will get just as much push back from the aircraft as 5-way#2...but they'll still be really close together, because the plane hasn't travelled much distance at all in between each group.

Signatures are the new black.

[SethInMI 145]

You are correct on both statements.

1. The basic assumption is that winds will decrease with decreasing altitude.

2. If the winds were constant from exit to opening, then wind speed would not matter.

This has been discussed here a lot; billvon put it really succinctly in another thread:

"Distance between groups will be given by aircraft ground speed plus wind speed at opening altitude, times seconds between groups."

And a plug for my freefall simulator:

FreeFall 3D Thread

It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

[mnealtx 0]

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You are correct on both statements.

1. The basic assumption is that winds will decrease with decreasing altitude.

2. If the winds were constant from exit to opening, then wind speed would not matter.

This has been discussed here alot billvon put it really succinctly in another thread I will try to track it down.

While the assumption that winds will decrease with lower altitude is correct (as a generality), the assumption that wind speed does not matter is wrong.

There are 3 major things that affect exit separation - groundspeed, group size and flying type (RW/FF).

Groundspeed affects the separation as the groups leave the plane. If your groundspeed is low, you have to wait longer for the necessary separation.

Larger groups will drift with the wind more than smaller groups, and RW groups will drift with the wind more than FF groups. In both cases, the 'slower' groups go first so that the wind drift will "carry them away" from the faster fallers.

All of that has to be adjusted for to ensure a safe distance between groups (2/10 mile minimum per billvon's post referenced in my first post).

Mike
I love you, Shannon and Jim.
POPS 9708 , SCR 14706

[SethInMI 145]

You are also (mostly) correct. I only addressed the question the OP was asking about a specific variable, wind speed, not about group size or flying technique (FS vs FF).

It is true that if the wind speed is constant w/altitude then it doesn't matter what it is, the only determinant is airplane airspeed. My edited post including billvon's statement is all you need to see this is the case. Airplane groundspeed is just Airplane Airspeed - Winds at Exit Altitude. If winds at exit = winds at opening, they cancel out, and the only variable left is airplane airspeed.

However, that is a very special case, and billvons statement covers all cases, as it takes in the three important variables, Airplane Airspeed, winds at Exit, and winds at opening.

I think that is all the OP wanted answered.

It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

[mnealtx 0]

The OP asked: "Why are the separation times different for different winds aloft speeds? "

I answered that question and referred him to information from Bill for a rule of thumb.

For the OP - check out this simulator. It not only shows why sufficient separation is needed, but why slower fallers should exit before faster fallers (which *does* tie into exit separation). Watch it, then click the link to reverse the exit order.

Mike
I love you, Shannon and Jim.
POPS 9708 , SCR 14706

[kallend 1,623]

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I've been trying to figure this one out for a while. Why are the separation times different for different winds aloft speeds? My best guess is due to the natural gradient, where the speed tends to decrease with a decrease in altitude, causing the jumper to be affected less by the wind than the aircraft. However, no one really seems to ever take into consideration winds at any altitude other than jump run.

The part that doesn't make sense about that, is that as far as I can figure, IF the wind was at a constant velocity from jump run all the way to the ground, it shouldn't make any difference what the speed is, as the jumper will always be affected equally with regards to the aircraft.

Can I get some insight into this?

I can only suggest that you look up the winds aloft for a few places and see how often the wind is constant (speed AND direction) from 14,000ft all the way to the ground. That should answer your question.

...

The only sure way to survive a canopy collision is not to have one.

[Bertt 0]

To make this a little clearer, look at an extreme case. (I hope I'm saying this right - if not, someone please help.) Suppose the wind speed is 90 knots at all altitudes from 3000 - 18,000 feet. Suppose the airplane's airspeed is 90 knots on jump run. That means the airplane doesn't move over the ground. All the skydivers will follow the same path from exit to opening and could open right on top of each other. To make each skydiver or group follow a different path, the airplane needs to move. The slower the airplane moves over the ground, the longer you need between exits to separate the paths of the skydivers.
The other extreme case to consider would be a balloon jump where the winds are typically very light and the aircraft hardly moves at all. I'll leave that as an exercise for the class.

You don't have to outrun the bear.

[SethInMI 145]

I think bfilarsky has it right, and you are wrong here.

You forget about drift under canopy. To continue your example of a 90knot plane in a 90 knot headwind that continues down to opening alt, the plane may not be moving w/respect to the ground, but it will be moving w/respect to the jumpers that have opened, as they will be in a 90knot breeze. Just a few sec of delay between groups will be fine, and no one will have to worry about opening on top of eachother.

As John said, this case of constant winds at all altitudes is not a real world case, but I think it useful to show that one understands what the actual mechanics are.

It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

[mnealtx 0]

In the case you cite, proper group order and separation timing would become even MORE important to prevent groups from opening on top of each other due to differences in freefall drift.

Mike
I love you, Shannon and Jim.
POPS 9708 , SCR 14706

[billvon 2,396]

> Suppose the airplane's airspeed is 90 knots on jump run. That means the
>airplane doesn't move over the ground. All the skydivers will follow the same
>path from exit to opening and could open right on top of each other.

Correct - BUT - it doesn't matter. As soon as they open, they will see a 90kt wind pushing them away from their opening spot, and the next group will open in clear air.

Take three examples.

The first is yours. 90kt windspeed from the ground to 13,500 feet. Aircraft TAS of 90kts, so it's basically hovering. An observer on the ground watches. He sees them open all at the same place - but - the wind quickly pushes them away from the opening spot.

The second example is zero wind day. No winds, from top to bottom. The plane is flying at 90kts and people jump out. No problems, right? Plenty of separation. There's another camera ship at 3000 feet flying along at 90kts taking pictures of people opening. An observer on that plane is watching, and oddly, he sees exactly the same thing that the guy on the ground saw in your example! The plane doesn't move on jump run (from his perspective) everyone follows exactly the same path (from his perspective) and after people open they get "blown backwards" by a 90kt wind. Even though it looks to him like everyone is opening in the same spot, there are no problems with collisions or close calls.

The third example is the troubling one. Winds on jumprun are 90kts. Winds at 3000 feet are zero. Now the plane doesn't move on jump run, _and_ there are no winds to push the groups out of the way. In this case everyone opens on top of each other.

That's why it's the _difference_ in winds between opening altitude and jump altitude that matters.

[bfilarsky 0]

Thanks Billvon and SethinMI - I appreciate the help! Makes a lot more sense now.

[bfilarsky 0]

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I can only suggest that you look up the winds aloft for a few places and see how often the wind is constant (speed AND direction) from 14,000ft all the way to the ground. That should answer your question.

While that will almost never happen due to ground friction, the winds can stay surprisingly constant in velocity for several reporting layers, which would be a significant portion of a skydive. On an odd day, I don't see it totally unreasonable that winds are relatively constant from aircraft to opening altitude.

[klafollette 0]

if there's 90 kt winds aloft, I'll watch you guys from the ground while enjoying a beer, and will let you know what the separation looks like. This aught to be entertaining.

[billvon 2,396]

>if there's 90 kt winds aloft, I'll watch you guys from the ground while enjoying
>a beer, and will let you know what the separation looks like.

I'll just borrow Luigi's VX39. At a loading of 5.1, I probably won't be backing up _that_ fast.

[kallend 1,623]

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I can only suggest that you look up the winds aloft for a few places and see how often the wind is constant (speed AND direction) from 14,000ft all the way to the ground. That should answer your question.

While that will almost never happen due to ground friction, the winds can stay surprisingly constant in velocity for several reporting layers, which would be a significant portion of a skydive. On an odd day, I don't see it totally unreasonable that winds are relatively constant from aircraft to opening altitude.

Would you bet your life on it's being an "odd day"?

You profile shows you in San Diego - a place where the lower winds can be in the opposite direction altogether from the uppers due to the effects of the ocean and the mountains.

...

The only sure way to survive a canopy collision is not to have one.

[bfilarsky 0]

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Would you bet your life on it's being an "odd day"?

You profile shows you in San Diego - a place where the lower winds can be in the opposite direction altogether from the uppers due to the effects of the ocean and the mountains.

No, I wouldn't bet my life. Its not hard to figure out what the winds are doing on the way to altitude. All it takes is a GPS and an airspeed indicator.

If you reread my original post, you'll see I'm not trying to change anything or suggest we don't do different separation times, I'm simply trying to understand the physics behind it. To do that, often you need to simplify cases in order to understand those, and then get better understanding of the real life case.

I'm a commercial pilot with over 1,000 hours, so I can assure you I have a better understanding of how the winds work than your average almost 100 jump wonder like myself.

Just to clarify once again, I'm just looking for better understanding of the reasons behind the procedures in place - I'm not trying to change them or look for loopholes. Billvon's explanation answered my question quite well, and shows me that the rules in place are quite appropriate and well thought out.

Blue Skies!
Brian Filarsky

[topdocker 0]

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>if there's 90 kt winds aloft, I'll watch you guys from the ground while enjoying
>a beer, and will let you know what the separation looks like.

I'll just borrow Luigi's VX39. At a loading of 5.1, I probably won't be backing up _that_ fast.

You'll be backing up eventually, when the ambulance gets to the trauma center, and they have to unload you!!!!

Jump more, post less!

[billvon 2,396]

>You'll be backing up eventually, when the ambulance gets to the trauma
>center, and they have to unload you!

No, see, due to my several almost stand up landings under a Xaos-27 89, I am more than ready to jump this canopy, and in fact I will be safer than all the people who jump huge canopies who get blown backwards. It's just a fact that people who are confident under their canopy are safer than people under bigger canopies!

[mnealtx 0]

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>You'll be backing up eventually, when the ambulance gets to the trauma
>center, and they have to unload you!

No, see, due to my several almost stand up landings under a Xaos-27 89, I am more than ready to jump this canopy, and in fact I will be safer than all the people who jump huge canopies who get blown backwards. It's just a fact that people who are confident under their canopy are safer than people under bigger canopies!

You forgot to mention your "mad skillz", though - you alluded to it, but didn't use the actual words. Gotta take points off for that, sorry.

Mike
I love you, Shannon and Jim.
POPS 9708 , SCR 14706