1. EXIT SAFETY 2004 Spring Skydiving Expo Spotting, Winds, and Separation
John Kallend
Department of Mechanical, Materials and Aerospace Engineering
Illinois Institute of Technology
When you have a hammer, problems tend to look like nails. I’m an engineer and physicist and my inclination is to look for the physics inherent in a problem.

2. Large jump planes with many groups Multiple planes on jump run
ISSUES
Large jump planes with many groups
Multiple planes on jump run
Mixed disciplines with different fall rates
High exit altitudes (and higher winds, windshears)
GPS spotting
High performance canopies
The current state of skydiving is very different from what it was 25 years ago. Developments in equipment, aircraft, drop zone business practices and skydiving styles have brought new problems in their wake while reducing the importance of some old problems. In no particular order, (a) we are typically jumping from aircraft that can carry 20+ jumpers, so that many more groups may be in the air at the same time. (b) some DZs operate multiple aircraft during busy times, so that a go-around may be difficult to manage safely. (c) disciplines such as board flying, freeflying and speed skydiving mean that groups with significantly different fall rates may be on the same load. (d) exit altitudes from turbine aircraft may be as high as 15,000ft msl leading to longer freefalls, and greater exposure to higher winds typical of these altitudes. (e) The advent of GPS spotting by pilots has reduced the opportunities for students to practice spotting, and (f) faster, high performance canopies both allow us to get back from longer spots, but increase the distance flown in the few seconds immediately after opening when we are most vulnerable to a collision.

3. RESOURCES
for download of the 2-d Freefall Separation program, relevant links and a copy of these slides.
This presentation uses an interactive freefall simulation at several points. Three variants are available from my web site. David “Juggler” Blumenthal’s 3-d version is also on his own web site.

4. Out-landings in snake filled swamps Freefall collisions
AVOIDING THE BAD STUFF
Out-landings in snake filled swamps
Freefall collisions
Canopy collisions
The goal of making a good decision about when to exit the jump plane comes down to minimizing the hazards of: freefall collisions, canopy collisions, and off-DZ landings. The relative importance of these hazards will vary from DZ to DZ. In the mid-western USA, off- landings usually mean nothing more than a long walk, and collision avoidance is the primary safety issue. In some Florida DZs, snake and alligator infested lakes and swamps are serious off-landing hazards.
Each DZ has unique circumstances.
There is no single solution that fits all DZs on all occasions.

5. THE MESSAGE WILL BE: Check the spot and look for traffic below.
The “45 degree method” doesn’t work.
Belly fliers should go first.
Groundspeed methods are OK in most circumstances; learn the exception.
There’s no hurry

6. SPOTTING
Most DZs now use GPS for spotting.
It works very well, BUT

7. EACH SKYDIVER IS RESPONSIBLE
FOR HIS/HER OWN SAFETY.
Don’t be a “wind dummy”.
Look out the door!
Check that no planes etc. are below (GPS doesn’t see these). Other pilots are under no obligation to avoid DZs.
Do a visual check on groundspeed.
Be sure you can get back.

8. LZ SPOTTING: NO WIND Off OK OK OK OK OK Off Off
On a no-wind day, canopies opening within the shaded area make it back to the LZ. Outside this area will land off.
The boundaries of the shaded area extend about a mile beyond the LZ (depends on opening altitude). OK OK LZ OK
Modern canopies have excellent glide ratios, and from a nominal opening altitude can cover over mile in still air.
There exists an area of some 4 square miles from which a successful landing on the drop zone may be executed. Its exact size depends on the canopy type, opening altitude, and the skill of the jumper. OK OK
Off
Off

9. LZ SPOTTING: STEADY WIND Off OK OK OK OK Off Off
On a windy day, the area from which a canopy can reach the LZ is shifted upwind, but its size stays the about the same.
If making low hop’n’pop from a helicopter or balloon, you want to exit over this area. OK OK LZ
Wind
On a windy day the area from which a successful on-DZ landing may be made is shifted upwind. OK OK
Off
Off

10. LZ SPOTTING: STEADY WIND
From altitude, freefall drift will displace the exit area from which you can make it back to the LZ. You will need to exit inside the red line to make it back. The exact displacement depends on jumprun airspeed, fall rate, upper winds, and direction of jump run. The SIZE of the safe area to exit is unchanged LZ
Wind
There will be forward throw when you jump from an airplane that also needs to be taken into account, due to the speed of the airplane. This also shifts the area from which you may exit by an amount dependent on the speed of the plane and direction of the jump run.
However, in general the SIZE of the area in which a jumper may exit and execute a successful on DZ landing remains unchanged.
Drift

11. SPOTTING: GREEN LIGHT
Consider an area for a safe exit and a satisfactory spot that is 2.5 miles across.
With 100kt true airspeed and 40kt uppers, jumprun will take:
Downwind, 56 seconds
Upwind, 2 mins 10 sec.
(no need to rush). That’s 11 2-ways with 13 second separation)
Crosswind, 1 min 22 sec.
wind
Later we will see that the time between successive groups’ exits for safe separation depends on the winds, and increases with increasing headwind.
HOWEVER,
The time taken by the jump plane to cross the area corresponding to an acceptable spot also depends on airspeed, wind speed and jump run direction relative to the wind. With strong upper winds and an upwind jumprun, exits may be made over a time period exceeding 2 minutes and everyone can still land on the DZ, IF THEY SPOT PROPERLY!

12. SPOTTING
X marks the spot. First group out cannot wait for the perfect spot, or the last out will be HOSED. LZ X
WIND
Later we will see that the time between successive groups’ exits for safe separation depends on the winds, and increases with increasing headwind.
HOWEVER,
The time taken by the jump plane to cross the area corresponding to an acceptable spot also depends on airspeed, wind speed and jump run direction relative to the wind. With strong upper winds and an upwind jumprun, exits may be made over a time period exceeding 2 minutes and everyone can still land on the DZ, IF THEY SPOT PROPERLY!

13. GETTING BACK FROM A BAD SPOT
Learn the “accuracy trick”
Get “small” to reduce drag.
If upwind of the DZ, use rear risers or brakes to flatten the glide.
If downwind of the DZ, rear risers may still help in light winds. Otherwise use full flight. Front risers only help in very strong headwinds.

14. GETTING BACK FROM A BAD SPOT
Forward speed depends mostly on wing loading. Risers and brakes mostly affect descent rate.
Have a safe landing spot selected while still above 1000’agl
It is better to walk a mile than to be carried a few yards.

15. (Collisions are No Fun)
Separation
(Collisions are No Fun)
Having determined that you are in a position corresponding to an acceptable spot, how long should you leave between one group of jumpers and the next?

16. SEPARATION
“Now lets talk about separation from other jumpers. First of all, anyone who counts on vertical separation for safety is out of touch with reality. I see people in freefall at 1,500 feet and lower routinely, so just because someone plans to open at 2,500 doesn't mean you should bet your life on it. Everyone needs to open in their own column of air. Horizontal separation is the only guarantee of security.”
Bryan Burke
Skydive Arizona
this quote is from an article by Bryan Burke. I take it as axiomatic that we cannot count on vertical separation for collision avoidance. Anyone who doesn’t believe this should think hard about premature deployments, cutaways, and long snivels. I would like to draw particular attention to Bryan’s statement about everyone opening “in their own column of air” (my emphasis), since I will return to this concept.

17. Let’s take 300 feet as the absolute minimum acceptable separation.
HOW MUCH SEPARATION IS NEEDED?
A modern canopy flies at around 30mph = 44ft/sec
(some are notorious for off-heading openings)
It takes about 3 seconds to recognize a
collision hazard and take action
Two canopies on a head on course cover
around 300 feet in this time
So – how much separation is needed? Several people have made experimental measurements of canopy speed with the brakes set. Typical reported values are around 30mph or 44ft/sec. Some time ago the FAA researched pilots’ reaction times to potential mid-air collisions. They found that it typically takes 3 seconds minimum for a pilot to recognize a collision threat and initiate evasive action. I’m going to assume that this is also typical of a skydiver immediately after opening. WELL, three seconds at 44ft/sec means your canopy will have traveled nearly 150ft before you take action to avoid a collision. If you open on a head-on course with the “other guy” from your group who travels the same distance, this means that the two of you need to deploy 300ft apart to have a hope of avoiding each other. I shall therefore take 300ft as the desirable separation between members of a group. Everyone then can deploy in the center of their own “column of air” 150 ft in radius.
Let’s take 300 feet as the absolute minimum acceptable separation.

18. VIEW FROM ABOVE
150
300
If you keep your “column of air” from overlapping the other guy’s column of air, then you will be separated by at least 300’. The colored circles represent the columns of air, 150ft in radius.
300 ft separation means each jumper has a “column of air” 150ft in radius, with himself or herself at the center. These columns of air should not overlap.

19. SEPARATION WITHIN THE GROUP
In a group skydive, everyone needs to track efficiently after breakoff so that each member of the group has their own airspace when they deploy. How far to track depends on the size of the group. TRACKING IS A CRITICAL SURVIVAL SKILL IN GROUP SKYDIVES.
(The picture is of my 10-way team at the 2002 Nationals).

20. TRACKING A survival skill that is fun.
A breakoff track should be flat and fast.
Tony Hathaway

21. To obtain 300’ separation between jumpers in a 4-way, need to track 212 ft from the center so the individuals’ columns of air don’t overlap.
3 seconds after opening there could be canopies anywhere in an area 724 ft in diameter. Each group needs its own column of air.
Separation is achieved within the group by breakoff and tracking away from the center. Geometry shows that a track of 212ft minimum is needed by members of a 4-way to achieve 300ft separation – this when everyone tracks perfectly, with equal spacing. In actuality, accommodating imperfect spacing will require a longer track.
Since we don’t know a priori the directions in which group members will track, all we can safely say is that some 3 seconds after canopies open, members of the 4-way group with adequate separation between members can be anywhere within a circle 724 ft across (362ft in radius, the green circle in the figure). Of course, the area could be larger if members track farther. I shall call this the GROUP’S SPACE. Our aim, then, is to ensure adequate time or distance from one group to the next that one group’s space does not encroach on its neighbor’s group space. In addition to every individual needing his or her own column of air, each group also needs its own column of air.
724ft
The corresponding area for an 8-way is more than 1,000 ft. across. These numbers are minima.

22. How Far to Track, and What Size is the Group’s Air Column?
Group Size Tracking Distance (ft) Radius of Group’s Space (ft)
* (300) 495 (450)
* (345) (495)
* (438) (588)
This table indicates the minimum tracking distance and group space for groups of various sizes so that within each group a 300ft separation is achieved. Of interest is that once the group size reaches a 7-way or bigger, better separation is achieved if one member deploys in the center. Also note that a solo jumper does not need to track, but still needs his or her own group space.
Once again, these numbers represent a perfect scenario. In actuality, greater spacing than this will be needed.
Note: this table gives the absolute minimum distance to track and the absolute minimum radius of the group’s space for 300’ separation, assuming efficient tracking and equally spaced tracks. In reality, more space will be needed.
* For groups larger than a 6-way, the smaller values in parentheses are for when one jumper deploys in place and the others track.

23. Make sure the first wave doesn’t stop too soon.
A staged breakoff gives better separation for any group of 12 or more skydivers.
BUT
The “first wave” needs to go 300’ (or more) farther than the next wave. SO
Make sure the second wave waits long enough before turning and tracking.
For 12-ways and larger, a staged breakoff can give better separation. This requires greater discipline by the skydivers, however.
AND
Make sure the first wave doesn’t stop too soon.

24. SEPARATION BETWEEN GROUPS
This section deals with how to determination exit separation

25. Centers of groups should be separated by > sum of their radii to minimize chance of collisions between groups.
R1 + R2 R1 R2
Group 2
Group 1
The distance needed between centers of adjacent groups so that their group spaces don’t overlap is given by the sum of their radii. As groups get larger, more separation is needed between them. Again, this is a minimum that will allow each skydiver his/her own column of air 300ft across. In an actual situation, more spacing should be allowed.
Example: Group 1 is 8-way, radius 541; group 2 is 4-way, radius 362. Separation should be > (= 903ft).
Since these are absolute minima, you should space more conservatively. Plan on, say, 1,200ft to allow for long tracks, sliding, etc.

26. How much spacing is needed?
Group Sizes 1 2 4 6 8 10
6-800
800
1000
1100
1200
1300
1350
1500
1450
1600
Solos and small groups have a greater tendency to slide or track off the “tube” so need more space than you might think.
Larger groups break higher and track farther, in general.

27. SO HOW DO WE ACHIEVE THIS? People do the strangest things
(like tracking up the line of flight) SO
We won’t try to predict what they will do
The simulations that follow assume no deliberate or inadvertant actions by the skydiver that cause a deviation from a ballistic trajectory.
We now need to use a freefall trajectory simulation program to investigate these and other effects. This can be downloaded from
Since it is an interactive program, you can put in typical jump plane speeds from your DZ, (70kts IAS might be typical of an Otter) and check out the effects of fall rate, windspeeds, etc. I suggest that you try various combinations of upper and lower windspeeds to check out the conclusions.

28. Computer model will use virtual “Spaceballs” with fall rate adjusted for the discipline, to remove human factor. Freefly spaceball falls 11,000ft in 50 seconds. RW spaceball takes 65 seconds to fall the same vertical distance. Spaceballs define the “perfect” trajectory, no backsliding or tracking.

29. Question: Do these methods work?
ACHIEVING SEPARATION
1. Watch angle from vertical to previous group.
2. Look down and watch until 1,200 ft have passed. (Skratch’s method).
3. Count to 5, then jump (Otter covers about 750 ft in this time on jump run at 13,000ft, no wind)
4. Wait for time taken to cover 1,200ft across ground (groundspeed method)
In practice, what do skydivers do to ensure separation between groups. Several methods are in use: One is to look down when the group before you exits, and wait until the spot vertically below has moved 1,200ft across the ground before your group exits. Another is just to count to 5 and then go (the problem with this is fairly obvious – 1,200 ft takes 8 seconds in no wind at 70 kt IAS). A third is to use the groundspeed of the plane to calculate how long it will take to cover 1,200ft across the ground, and exit when that time has elapsed. Yet another is to wait until the angle between the vertical and the previous group to exit reaches some value (like 45 degrees). Do these methods work, and if they have shortcomings, what are they?
Question: Do these methods work?

30. First we’ll take a look at the “angle” method.
“Wait until the group in front of you makes an angle of 45 degrees behind the plane, then exit.”
Many people have been taught to do this. It doesn’t work.

31. Computer model Freefall does the math.
Uh-Oh
Who can accurately judge 45 degrees anyway?
By setting the time between exits to zero and both jumpers to the same fall-rate, the program will plot the angles between the jumper and the plane every two seconds. The plot of angle vs. time graph is at the bottom of the window.
There is very little change in angle, it never reaches 45 degrees, and is independent of wind speed.
THIS METHOD DOES NOT WORK. Depending on it for separation is not safe.
You might as well count the windows in the plane as an indicator for exit spacing, it is just as futile!
Can you judge 45 degrees ?
If you can, does the method work anyway ?
Computer model Freefall does the math.

32. Conclusion 1 Video by Bill von Novak
The angle made between the vertical and the previous group varies very little after the first two or three seconds. It does not depend on wind speed. It rarely reaches 45 degrees.
There is no physical or mathematical basis for this method. DON’T USE IT.
Note that the angle from the vertical doesn’t vary very much anyway, and doesn’t change in response to wind speed changes. The angle method is clearly ineffective. (you could also deduce this by looking at the trail of skydivers behind a plane on a big-way. This trail is at an almost constant angle.)

33. How well can you judge the vertical from a moving aircraft?
2. Look at the ground.
From 13,000 feet, a  2.75 degree error in judging the vertical leads to a 1,200 foot error on the ground!
How well can you judge the vertical from a moving aircraft?
I have tested my ability to judge the vertical, at home in a quiet room, standing on a chair that was not moving. I can do no better than plus or minus 1 degree. An error of plus or minus 2.75 degrees corresponds to a 1,200ft error on the ground when viewed from a plane at 13,000ft agl. I am not at all confident that I could achieve this accuracy from a noisy, moving airplane with other jumpers moving around. Your mileage may vary.

34. 3. GROUNDSPEED METHOD
Ask the pilot the groundspeed and you do the math. 1 knot is roughly 1.5 ft/second, or 100kt = 150ft/sec
Example: groundspeed = 100ft/sec and you want 800ft separation. Then you wait 800/100= 8 seconds between exits. Does this work?
So, what relevance does the ground have anyway – we don’t deploy our canopies on the ground. The concept of groundspeed can lead to completely erroneous conclusions under some circumstances. What we will do is imagine our jumpers equipped with strobe lights, photographed from the ground to indicate the path they take. Click on the “Simulation” link to run the simulation program.

35. How Groundspeed Can Mislead (Unless you deploy on the ground)
Case 1. Groundspeed = 40kt Wind at 3000ft = 30kt Exit delay = 8 seconds
Does having groundspeed and leaving some time between exits necessarily lead to separation at opening altitude? The surprising answer is “NO”. The variables will be already set up for this scenario.
Case 1 – the drift of canopies right after opening leads to MORE separation
Click on the link Freefall Simulation

36. Case 2: Same groundspeed = 40kt
Wind at 3000ft = 30 kt tailwind
Exit delay same (8 seconds)
Use Freefall Simulation again
Observations:
Groundspeed calculation works if winds are in same direction at all altitudes
2. Separation also depends on wind at opening altitude, if in opposite direction to uppers, BEWARE.
Case 2 is the same as Case 1 except the wind at 3000ft is a tailwind. This happens fairly often near the ocean, great lakes, or mountains.
In this case the open canopies drift towards the following jumpers, reducing separation.

37. CONCLUSION 2 It’s not the speed over the ground that counts.
It’s the speed relative to the air at opening altitude.
Usually groundspeed methods work because the winds at 2,500’ are light and/or in the same direction as the upper winds.
So, what relevance does the ground have anyway – we don’t deploy our canopies on the ground. The important variable is actually the speed of the jump plane relative to the air at opening altitude. Fortunately, in most circumstances this is pretty close to the ground speed or the error it causes works in our favor. Occasionally it works against us and reduces separation.

38. the lower winds are opposite the uppers, IF THAT IS THE CASE THEN
CONCLUSION 2 (cont.)
To achieve separation between groups that have the same fall rate, methods based on groundspeed work and have margin for error
UNLESS
the lower winds are opposite the uppers,
IF THAT IS THE CASE THEN
extra spacing is needed. ADD the lower wind speed to the jumprun headwind, or SUBTRACT the lower wind speed from the groundspeed.
Why, then, do the common groundspeed (or ground-distance) based methods work? Basically, because the windspeed at opening altitude is usually small compared to the airspeed of the jumpship, and can be fairly safely ignored at most times.
If the lower level winds are opposite the uppers, then separation at opening time is reduced and then extra spacing between groups is needed.

39. Ask the pilot, or Call 1-800-WX BRIEF, or Point your browser at:
HOW TO GET WINDS ALOFT
Ask the pilot, or
Call WX BRIEF, or
Point your browser at:
aviationweather.gov/products/nws/fdwinds/

40. TO GET 1000FT OF SEPARATION Jumprun indicated airspeed= 80kt
Headwind(kt)
Delay between exits (sec). 6 10
6.5 20
7.5 30
8.5 40 50 12
This assumes lower winds are light and/or in same direction as uppers.
Note that the delay times are non linear with respect to the windspeed.

41. Freefall drift due to upper winds
Next we’ll look at:
Fall Rate Differences
Forward throw
Freefall drift due to upper winds
Let’s now use the interactive software to look at fall-rate and wind effects.

42. For Forward Throw
Click on Freefall.exe

43. CONCLUSION 3
In no-wind conditions, a freeflier will have a forward throw down the line of flight of about 1,800 ft.
A belly flier will have a forward throw of around 1,200 ft.
Now compare the fast and slow fallers for conditions of no-wind and no exit delay. The freeflying Spaceball has considerably more throw down the line of flight than the RW Spaceball. Try it for different jumpship airspeeds. In all cases, the freeflyer goes farther than the RW by a distance roughly equal to the distance covered by the jumpship in 3.6 seconds. Next try putting the freeflyer out first, with a 5 second delay. The paths of the two balls intersect. How much delay is needed to get an adequate horizontal separation? Finally, put the RW ball out first. This clearly leads to better horizontal separation.

44. What About Winds? CONCLUSION 3 (cont)
For any jumprun airspeed, Freefliers will have a forward throw down the line of flight that is greater than that of belly fliers by a distance approximately equal to the distance covered by the jump plane in 4 seconds. For 80kts IAS this is around 600 feet.
Now compare the fast and slow fallers for conditions of no-wind and no exit delay. The freeflying Spaceball has considerably more throw down the line of flight than the RW Spaceball. Try it for different jumpship airspeeds. In all cases, the freeflyer goes farther than the RW by a distance roughly equal to the distance covered by the jumpship in 3.6 seconds. Next try putting the freeflyer out first, with a 5 second delay. The paths of the two balls intersect. How much delay is needed to get an adequate horizontal separation? Finally, put the RW ball out first. This clearly leads to better horizontal separation.
What About Winds?

45. WIND DRIFT
Even in freefall you “blow along” with the winds, which may be quite strong at altitude. Freefliers spend less time in the upper winds.
Compare wind drift for freefliers and bellyfliers: Freefall.exe for winds.

46. CONCLUSION 4
In headwind, freefliers have a steeper trajectory than belly fliers.
For each knot of average upper wind, a belly flier will drift 20 feet farther than a freeflier (so a 40kt average wind will result in 800 feet more drift for the slow faller).
This adds to the forward throw difference if the jumprun is into the wind
Now run the simulation with some headwind (say 40kt uppers). Observe that the track of the freeflying ball is steeper than the RW ball. The RW ball spends more time in the wind and drifts farther. If the freeflier exits first, the wind drift effect is to further reduce separation.
If the wind is strong enough, the freeflier and belly flyer can be on intersecting paths. This would seriously spoil someone’s day if the freeflier had a premature deployment.
If the freeflier exits last, wind drift increases separation.

47. CROSSWIND AND DOWNWIND JUMPRUNS
Forward throw is unaffected.
Wind drift effects on separation go away on crosswind jumpruns
Wind drift effects are reversed on downwind jump runs.

48. CONCLUSION 5 BUT Separation can be achieved with any exit order.
If the freefliers go first, adjacent groups CONVERGE. This is not fail-safe!
If RW groups go first, groups DIVERGE, a fail-safe situation. It is easier and takes less thought to achieve horizontal separation if freefliers exit after RW groups.
These results show that it is easier to achieve horizontal separation between freeflyers and formation skydivers if the freeflyers exit last. If freeflyers go last, the paths of the freeflyers and flatfliers diverge, clearly a fail-safe situation. Under circumstances where freeflyers go first for other reasons, then their paths converge and additional delay has to be inserted, as shown in the slide. If the RW group is large and will require, say, 15 seconds to climb out, then this delay may be quite easy to achieve. So, an exit order with freeflyers first can be made to work but because the paths now converge, there is less room for error in deciding the delay.
Finally – there are other factors to be considered. Climb-out time suggests that large groups should exit before small groups in the same discipline. This is the norm at most DZs. Students, tandems, and others intending to open high should exit last.

49. IF FREEFLIERS EXIT BEFORE RW GROUPS
extra spacing is needed to achieve adequate horizontal separation, maybe tens of seconds.
But it is not always a bad idea…
For example, if a freefly 2-way is followed by a large RW group (like a 16-way) that will take a lot of time to climb out, it may be easy to get the required separation.

50. IF FREEFLIERS EXIT FIRST
Use your usual method to calculate spacing.
Add 4 seconds to account for extra forward throw.
Add 2 seconds for every 10kt of upper winds
Example: Want 1000ft spacing, no anomalous winds, groundspeed =70kts, uppers=30kts. Normal spacing = 9 seconds.
Now require (3 x 2) = 19 seconds between exits.
When freeflyers go first, additional delay is needed as shown, and there is less margin for error.

51. THE MESSAGE WAS: Check the spot and look for traffic below.
The “45 degree method” doesn’t work.
Belly fliers should go first.
Groundspeed based methods are OK in most circumstances; learn the exception.
There’s no hurry

52. ACKNOWLEDGEMENTS Discussions with: Winsor Naugler III Skratch Garrison
Tamara Koyn
Tim Wagner
Articles:
Bryan Burke
Bill von Novak
Video: Bill von Novak
By no means does this presentation represent all my own ideas. I had many discussions, both live and by , with others who have thought long and hard about the issues of spotting and separation. In particular I wish to mention Winsor Naugler who started me thinking about this topic.