Waiting Between Groups
by Tom Buchanan, S&TA

(UPDATED MAY 2004 TO CLARIFY THE FAILURE OF THE 45 DEGREE RULE)

'Back in the day' some things about skydiving were easier.  Spotting is one of them.  When the earth was still being formed we mostly jumped from little Cessnas, and generally learned to spot while making static line jumps just a few thousand feet above the hot lava.  Spotting was a skill learned by just about every jumper, and was practiced on almost every skydive.  These days we are fortunate to have larger airplanes that give us much more altitude, and spotting is generally left to the pilot with his fancy GPS, an advantage we didn't have when I was just a tadpole on a static line.

Most of us who survived the early years appreciate the modern advantage of big airplanes, higher altitudes, and the accuracy of pilot managed GPS.  Of course one of the things that we need to think about that hardly ever crossed our minds when history was being manufactured is the time needed to wait between groups exiting on the same pass.   This is a complex issue that seems to confuse even some of the most experienced jumpers.

I think we can all agree that midair collisions between skydivers are a bad thing.   In order to prevent collisions we need to allow space for each jumper or group to maneuver in freefall, then more space for tracking, and still more space as a buffer between groups on the same pass.  There is always a question about how much space, and how long to wait between groups.  We live and fly in a dynamic world that is constantly changing, presenting us with trillions of variables that define an ideal separation standard.  Well, maybe trillions is a slight overstatement, but there are plenty of variables and we need to consider at least some of them.

How Much Space
Let's imagine that two individual jumpers leave the airplane on the same pass and each falls straight down.  Neither jumper needs to track, so they require very little space between them.  If one of the solo jumpers is replaced by group of four relative workers, they will each break off and track for a distance, so more separation is needed.   Likewise, if one of the jumpers is a beginning head down flyer, we can expect quite a bit of sliding in freefall, and that requires more space for separation.  If we mix groups on a pass it is pretty easy to imagine an eight way flat group being followed by a four way head down group, with each group needing space to slide around a bit, and then more space for tracking.  Thus, we should be able to understand that big groups need more space than individuals, and groups that will be tracking or sliding around the sky also need more space.  With this in mind I like to think of separation standards as varying between roughly 500 feet and about 1,000 feet, with the greater separation used when groups are large or likely to move about the sky.  In extreme cases such as two very large or sliding groups, it may be necessary to increase separation to as much as 1,500 feet.

The best way to ensure groups are separated is to give each group a small part of the sky, space that can be visualized if we think of columns of air extending up from the surface to the jumprun.  Each jumper or group needs a private column of air, and those columns shouldn't overlap.  Some people like to watch the group in front of them to ensure separation by using a 45 degree angle, while others like waiting a few seconds between exiting groups.  Whatever method is used must be equally effective under different wind conditions, and must be adjustable to match different sized groups or freefall activity.

45 Degrees of Separation
The concept of watching a group leave and slide back under the tail to 45 degrees behind the airplane seems to work well with no wind, but it provides only a single separation standard that is difficult to quantify or vary for different types of groups that require more or less space.  If there is lots of wind, the 45 degree standard can provide considerably less separation than desired.  In fact, if we imagine an airplane flying directly into a wind so great that the plane isn't moving, it will still be possible to achieve the 45 degree standard as jumpers are initially blown back, but every jumper will be getting out over exactly the same place on the ground, they will be blown back along exactly the same path, and will open at the same place with no separation at all.   The separation doesn't suddenly disappear when the airplane stops covering ground in the wind, but rather deteriorates as the wind speed increases, and the 45 degree rule can't be expected to compensate.  Once we understand the 45 degree concept it becomes clear that it is really not effective at any speed, and it should not be relied upon.

Counting Seconds
Time equals distance.  This is such an important concept that I need to repeat it.   Time equals distance.  The more time an airplane spends moving over the ground, the more distance will be covered, and the more separation will be provided.   The amount of ground covered will increase if the airplane is moving fast, and decrease if the airplane is moving slow, but no matter how strong the wind is, for a given groundspeed time equals distance.  More time equals more distance over the ground, and thus greater separation.

Let's think for a moment about our normal parachute flight. If the parachute has 30mph of forward speed and there is no wind, we will be covering 30 miles of ground in an hour, or about 44 feet every second.  If there is a 20mph headwind, the parachute will only be covering 10 miles every hour, or about 15 feet every second.   It should be pretty obvious that a headwind reduces the distance we cover over the ground each second under canopy, and this is true of airplane flight too.

Our Twin Otters generally fly a jumprun at about 80knots, or just a bit over 90mph.   Sometimes pilots will fly jumprun a bit faster, sometimes a bit slower.   Assuming no wind, airspeed and groundspeed will be the same, and our Twin Otters will fly over the ground at about 90mph or 132 feet every second.  If there is a 20mph head wind, the Otter will be flying 70mph over the ground and will only cover 102 feet every second.  If the headwind is 40mph, not an unusual situation at 14,000 feet, the Otter will be covering just 73 feet per second.  In an extreme case of upper winds (at 14,000) of 60mph, the Otter will cover a scant 44 feet every second.   When we think of the effect of wind speed, the Otter behaves just like a parachute, with the speed over the ground decreasing as the headwind increases.

If we think back to the earlier discussion, we know that the airplane must fly a specific distance over the ground to achieve adequate separation between groups, and it will take more time to cover that distance as the upper level headwind increases.  We also know that larger groups and jumpers that slide around the sky will require more separation, and thus more time and distance.  There are other variables that affect separation, such as differing wind speed or direction at lower altitudes, the percentage of wind actually classified as a head wind rather than a crosswind, the type of skydiving, the direction of tracking, the opening altitudes of each group, the speed of parachutes, and almost a trillion others.  Yet, the two most important variables for us to consider are the wind speed at altitude because it determines how fast the airplane is moving across the ground, and the size of the groups or type of jump activity taking place because it determines the amount of space needed between groups.

Too Much Time
In a perfect world without concern for the cost of operating expensive airplanes, we would each have our own pass, or we would at least wait for many thousands of feet of separation between groups.  Unfortunately, if we wait too long between groups we won't be able to get everybody out on one pass and the airplane will need to go around, thus burning fuel and costing the drop zone lots of money, and probably driving up jump costs.   Separation standards are a compromise that hopefully gives us plenty of space without stretching the jumprun too far.  I suggested earlier that separation could be as little as 500 feet for solo jumpers who will not be sliding, or might need to be as great as 1,500 feet for large groups or sliding jumpers.  If we imagine a one mile jumprun, there will be enough space to drop 11 solo jumpers if the airplane travels 500 feet over the ground between each jumper, 6 groups if the airplane travels 1,000 feet between each, and just 3 groups if the airplane travels 1,500 feet between each.  If there is a lot of headwind, it will take quite a while for the airplane to travel over that mile, and thus lots of time can be allowed between groups.  If there is just a slight headwind, or even a tailwind, the airplane will be covering that mile much faster and groups should exit with a shorter delay between them to achieve exactly the same separation.  Too much time equals too much distance.  The actual jumprun we use at The Ranch will often extend to almost two miles, so there is plenty of distance to get a full load out on one pass.  Keep in mind that the airplane is traveling over the ground at the same speed whether you are watching from the door, or starting your climb out, so you can build your anticipated climb out time into the waiting time.  It is up to each jumper and each group to determine how much distance is needed, and thus how much time needs to be allowed between exits.  The decisions made by each jumper and group will determine how many groups can exit on one pass.

As a point of reference, our runway at The Ranch is about ½ mile long, or roughly 2,500 feet.  500 feet of separation will allow roughly five groups to use the area defined by the length of the runway.

Suggestion of Thumb
We do not like to talk about "rules" at The Ranch, so as a "suggestion of thumb," when the winds at altitude are between 20mph and 50mph solo jumpers and small groups should be given between 5 and 10 seconds of time before the next small group exits.   Larger groups and sliding formations should be given between about 10 and 20 seconds of separation, and extremely large groups might need as much as 30 seconds.   The shorter times in each category are used when there is little or no wind, the longer times are used when the winds are stronger.  More time between groups will be needed if the wind is especially strong, and less time will be needed if the jumprun is downwind.

If you are really geeky and want a specific amount of time to wait, use the chart below.  The wind at altitude is listed on the left side of the chart.   Separation space is listed on the top, with the column for 500 feet of separation between groups used when two solo jumpers or small groups are following each other, 1,000 feet of separation used when the groups are in the mid size range, and 1,500 feet of separation when groups are really big.  The numbers inside the chart represent the time needed to achieve the listed separation standard under the given wind condition.   Keep in mind the chart is designed to deal with a very limited number of variables, and applies only to an airplane with a jumprun airspeed of 80knots/90mph.  The chart is an interesting look at how wind speed and distance of separation are related, but the simpler "suggestion of thumb" is much easier to use in a dynamic skydiving environment.

Determining the Wind
Of course each of us needs to know how strong the wind is so that we can figure out how much time to allow between groups.  Fortunately there are several ways of determining the winds at altitude, starting with a quick glance at the Winds Aloft Forecast posted each day at manifest and at the school.  The forecasted winds can be verified by asking the pilot how much wind he thinks he is encountering, a simple question for the pilot to resolve by looking at the ground speed displayed on his GPS, and matching that to his panel mounted airspeed indicator.  It is also pretty easy for an experienced skydiver to glance out the window on jumprun and offer a simple assessment such as "we aren't moving very fast over the ground, so there must be a lot of wind."   Likewise, when a group exits it should be possible for an experienced skydiver to look down and determine where they exited over the ground, and then watch the airplane travel over the ground to select a specific distance of separation based on the actual airplane travel.  In any event, each jumper should have an idea of what the winds are doing at jump altitude, and from that each jumper can decide how much time and distance to leave between groups.  Remember, time equals distance.

Even More Detail
Spotting and exit separation are interesting topics that can be analyzed from many perspectives and with consideration for a vast number of variables.  If you are interested in more information or greater detail, there is a fascinating abstract of a seminar conducted in 1999 by Winsor Naugler III and John Kallend at Quincy posted at:   http://www.koyn.com/clouddancer/articles/jmprun98.html .   There are also several computer simulator programs and briefings, including a fantastic PowerPoint presentation prepared by John Kallend at:  http://www.iit.edu/~kallend/skydive/ .

-Tom Buchanan S&TA