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See this note on the "45 degree rule".
Separation from other Jumpers at Opening Time Tue 2002-6-18 --------------------------------------------- ------------- This is an extract of a longer article "Separation Explained for Students". UPPERS ------ > < ------------------------------------ AIRPLANE |---1,500---|---2,000-------|---1,500---|---1,500---|---1,500---| X X X X X X __Tandem __AFF ____FF ____FF __RW __RW / . / . / . / . / . / . | . | . | . | . | . | . \ . \ . | . | . \ . \ . \. \. | . | . \. \. .\ .\ | . | . .\ .\ . \ . \ | . | . . \ . \ . \ . \ |. |. . \ . \ . | . | . . . | . | . __/ \__ . __/ \__ . . . | . | . . . . . | . | . . __/.\__ __/.\__ . __/ \__ . __/ \__ . . . . . . . . . . . . |---1,500---|---2,000-------|---1,500---|---1,500---|---1,500---| X X X X X X # # # # # # # GROUND # # # # # # # # TARGET # # # # # # # # # # # GROUND # # # Horizontal Separation at Opening is the Goal -------------------------------------------- Vertical separation is nice when you have it, but with opening altitudes going from 2,000 to 3,500 ft and sometimes more, 1,000 ft snivels, people spacing out their altitude, and malfunctions, there is really no way to guarantee vertical separation. The idea for horizontal separation is to use: - The exit order - Slow fallers, then fast fallers, then high pullers - Boards, RW, Freestyle, Freefly, AFF, Tandem, Wing Suits - The exit spacing - 1,500 ft across the ground for small groups - 2, 3, 4 people - 2,000 ft across the ground for 4, 5, 6, and up - The breakup maneuver - Tracking away from center - Flying canopies away from jumprun to ensure that everybody is far enough apart at opening time. A typical situation looks like this: - The plane flies into the uppers - Each group gets some forward throw - 1,200 to 2,000 ft - Each group blows downwind with the uppers - The winds decrease on the way down - Within the groups each person tracks out from center - Each person has some canopy motion while other groups are still in freefall The separation we are after is between opening canopies on the bottom end. The only control inputs we have are: - Separation of exits on the top end - Tracking at breakup - Flying canopies away from jumprun on the bottom end **** It is important to remember that we are doing relative work **** with the ground and everybody else on the load, **** not just the people in our own group. Judging Exit Separation at the Top ---------------------------------- First start learning the over all environment around the airport by - using aerial photos - looking out the windows and door on the way up - looking at the airport and surrounding country side while in freefall Ask jumpmasters, coaches and experienced jumpers what they use. - Runways - how long are they - good measuring sticks for covering 1,500 or 2,000 ft - which way do they point - helpful when people say things like "winds are twelve at two six zero" - Roads, freeways, rivers, shopping centers, race tracks, mountains - Anything that is large and really easy to recognize What you're after are easy to recognize landmarks and distances, particularly near the airport but also within the 5 or 10 miles that the plane is likely to be climbing in. I find it very comforting to glance out the door, see a familiar landmark, and know right where the airport is. When I'm at a new drop zone I like to make a jump or two just hanging out in freefall looking at things. Using a fraction of a runway as a measuring stick, mark out 1,500 ft intervals for typical jumpruns both on an aerial photo and while looking at the ground in freefall. Next watch a couple loads before going up so you have an idea of what to expect. - Which way is jump run - Is half the load getting out short and half long or are they starting straight up and stringing the whole load out up wind - Are people drifting in freefall - tandems are easiest to see - A lot of uppers means it will take longer to cover 1,500 - 2,000 ft - Are the ground winds the same as the uppers - Are canopies flying in all directions or are they barely holding When the plane starts turning on jumprun look out the window to see that the plane is indeed headed for the airport, and about how far away it is. I actually notice more what angle it is than some distance like 2 miles. When the group in front of you goes, look down. Looking straight down is a visual technique. You can't use your body sense because the floor of the plane is not level. You're often on your knees crouched over in an unfamiliar position, and if the plane is turning at all that throws you even further off. I divorce myself from the plane completely and just look out at the horizon. When my mind and eyes are level I drop my eyes straight down. Do this 3 times in a row and you will have a good idea what you are over. With a little practice you can become very accurate. If we are diving out I look out and down maybe 2 or 3 times while we are moving 1,500 ft across the ground. Just before we get there I start the count. If there is a climbout involved I have to guess how long it will take, and if we climb out much faster than I expected I look out and down while hanging there as a floater and wait until we have covered the distance across the ground before starting the count. Two other ways of judging the exit separation are the 45 degree rule and counting seconds between groups. The 45 degree rule says: Watch the group in front of you fall away - when they get down and back about 45 degrees you are far enough apart and you can go. That is basically correct on a no wind day. With strong uppers though, the plane only moves a short distance across the ground while they are falling to that 45 degree place, and that short distance is how close you will be to them on the bottom end. We are fooled by this at first because our intuitions about jumping off of things developed for small jumps, close to the ground, with everything holding still. When I hop off of this chair, if something is right under me, I will hit it. If it is a couple feet over at a 45 degree angle I will miss it. Our untrained intuitions don't give us the right answers when the chair is moving around and there is 60 seconds worth of upper winds and fall rates and breakups and canopy motions and so on. Another way to judge separation between groups is to count seconds. At first this seems simpler. The difficulty is in how many seconds for each combination of airspeed, upper winds and winds at opening altitude. Let's look at a 90 knot jumprun ( = 176 fps true) and see how long it takes to cover 1,500 and 2,000 ft with some typical uppers. (ft/sec = 6076/3600 * knots = 1.688 * knots) uppers | uppers | true airspeed | 1,500 ft | 2,000 ft | knots | ft/sec | ft/sec | | | -------|--------|---------------|----------|----------| 0 | 0 | 176 | 8.5 sec | 11.4 sec | 10 | 16.9 | 159.1 | 9.4 | 12.6 | 20 | 37.8 | 138.2 | 10.9 | 14.5 | 30 | 50.6 | 125.4 | 12.0 | 15.9 | 40 | 67.5 | 108.5 | 13.8 | 18.4 | It's a nice table. It looks very scientific. There is probably some pattern - for each 10 knot change of airspeed with respect to the winds at opening altitude add so many seconds to the time between groups. But! ... - I don't know what the uppers and lowers actually are - They keep changing throughout the day - So I don't really know how many seconds between groups - It's all guess work - I can't remember it all anyway - it's too complicated And so I keep coming back to the idea that learning to look down and cover a known distance is not that hard, and it's the same distance, 1,500 or 2,000 ft, done the same way every time. And it has the added advantage that I'm looking out the door into the situation I'm about to jump into, instead of blindly trusting a little green light and counting some mysterious number of seconds. I actually count seconds between all the groups in front of me because separation is important and I like to know what people are doing with it. When the group in front of me goes, I start counting in the back of my mind. While looking at the ground watching the separation happen, I also glance at them falling toward the 45 degree place. But my real attention is on seeing that we are covering the distance across the ground. The 45 degree rule and counting reassure me, but looking at the ground is the only way to know for sure. Getting Opening Separation at the Bottom ---------------------------------------- The components of this skill are - keeping track of your lowness, your closeness to the ground - breaking high enough to track 200 - 400 ft out from center - tracking - looking around while tracking and knowing when you have gone far enough - grabbing rear risers upon opening (to turn away if you are close to someone (to turn away from jumprun - turning away from jumprun until you see the following group My best analogy for keeping track of lowness is the cop in the rear view mirror. When I'm driving down the freeway and I notice a cop a few hundred feet back, I don't fixate and start dwelling on him. I just keep doing what I'm doing. But I don't forget he's back there either, and I periodically check the rear view mirror to see where he is. I have trained myself not to go for too long in freefall without checking - my altimeter, someone else's, the ground, the horizon. Up at the top I'm pretty relaxed, but down closer to breakup altitude, when the cop has pulled in right behind me, I check every few seconds. I allow 1,500 vertical ft for breakup. That means we break at planned pull altitude plus 1,500 ft. When it's time to break - I give a wave off to the people I'm jumping with - turn 180 from the group (unless there is a good reason to go some other direction) - concentrate and do the best track I can (not only for separation but because tracking close to the (ground is really fun - look right, left, down, forward for other people tracking - flare into a big reverse arch pull position to slow down - throw the pilot chute and reach up for where the risers will be - look up when it opens and say thank you to my canopy - rear riser away if someone is close - turn away from jumprun and look for the other groups (in both directions but mainly for the following group at first) - After a few seconds when everything has settled down I turn toward the drop zone, collapse my slider and unstow my brakes - If someone has a cutaway I try to watch where their stuff lands and land by them if possible ---- This article has some fun training jumps for learning and practicing all this. http://www.iit.edu/~kallend/skydive/ has a simulation program written by physics professor John Kallend that is pretty helpful for learning to see how all these variables work together. Skr
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