Avoiding Deployment Collisions
Last month, “Safety Check” addressed the issues of finding clear airspace for your main canopy deployment and avoiding the other jumpers within your group. This month, “Safety Check” addresses the issues surrounding separation between groups.
For the many skydivers who jump from Cessna 182s or Cessna 206s at their local drop zones, group separation is not much of an issue. If the airplane carries two 2-way or 3-way groups, by the time the second group climbs out and exits, the airplane usually covers enough distance that group separation is not a problem. However, larger airplanes usually mean more groups on board. On top of that, if the jumpers are performing many different disciplines, the group dynamics may be very complicated because they may be falling at very different speeds and not necessarily straight down. (Wingsuit, tracking and angle flyers cover a lot of real estate before breaking off for deployment.) Each jumper in all the various groups must plan and execute the jump properly to ensure that everyone has clear airspace for deployment.
First let’s look at separation between groups that fall straight down: formation skydivers and freeflyers. The ground speed of the airplane is a large factor in determining the time that should elapse between exiting groups, as is the size of each group and the type of skydiving the groups are performing. The stronger the upper winds, the slower the ground speed of the airplane, and the more time required between groups for the airplane to cover enough distance to allow for adequate separation. How much time? A little math can help clarify this issue.
To establish a minimum distance between groups, let’s assume that it takes a skydiver under canopy three seconds to observe a pending collision and make a corrective action. In three seconds, a modern sport parachute will cover roughly 150 feet of horizontal distance. So, two skydivers flying their canopies directly at each other need at least 300 feet of separation to recognize the potential for a collision and avoid it. As an example, for two 4-way groups exiting the same airplane, the jumpers within each group need to track a safe distance from each other and also have adequate distance from any jumpers in the next group.
The speed of the airplane measured across the ground determines how much time you will need between groups to achieve adequate separation. Lots of factors come into play to determine the actual ground speed—the upper winds, exit altitude and temperature, and the airplane’s indicated airspeed vs. true airspeed—but that is mostly for your pilot to worry about. For the jumper, knowing the ground speed of the airplane is the determining factor for how much time to wait between groups. The plane’s GPS provides ground speed data, and the pilot can relay that speed to the jumpers during jump run. Old-school spotters are good at looking out of the door and estimating the speed across the ground, but that’s another topic of discussion!
Here are some sample speeds and distances:
120-knot ground speed equals 200 feet per second.
90-knot ground speed equals 150 feet per second.
60-knot ground speed equals 100 feet per second.
In addition to needing to provide enough distance for jumpers to react and avoid a collision, you need to account for the distances the jumpers in each group track to separate from each other at deployment. This means you should leave a separation of about 900 feet between two small groups just to achieve the distance necessary to barely avoid a canopy collision. When you factor in high breakoff altitudes, fast trackers and fast-flying parachutes, suddenly the bare minimum of 900 feet between groups does not provide much separation. With so much at stake, doubling that distance is not unreasonable.
If you want to ensure at least 1,600 feet between two smaller-sized groups, that could mean waiting as little as eight seconds (200 feet per second x 8 seconds = 1,600 feet) if you have a 120-knot ground speed or as much as 16 seconds (100 feet per second x 16 seconds = 1,600 feet) between exits with a 60-knot ground speed. And if the upper winds are even higher, it reduces the ground speed to a crawl, so the time between groups will need to increase even more. Adding time between groups reduces the chances of a collision between jumpers in different groups, so use conservative estimates on the first load of the day. You can always cut the time between groups on subsequent loads.
For airplanes that are flying into the wind for jump run and are carrying multiple groups, the slower-falling groups will drift farther downwind than the faster-falling groups. So, putting the slower groups out first (belly flyers before freeflyers) helps to ensure that they maintain adequate separation throughout the freefall. Once the freeflyers are out, solo students generally go next, followed by tandems. But remember, the drift from the upper winds will push the slower-falling jumpers (such as students) farther, so additional time is required between a freefly exit and a student exit to help ensure that the student group does not drift into the freefly group’s airspace.
Jump runs flown crosswind will lessen the chance that groups will have separation issues due to freefall drift from the upper winds but can create congested airspace at lower altitudes as everyone descends toward the main landing area. And there is more to consider: Larger groups require more time between exits. Some jumpers will track farther than others, which means their groups will require more separation.
Whew! That’s a lot to keep track of, but we haven’t even considered the other groups that take up a whole bunch of sky! What about wingsuit flyers, tracking groups and angle flyers? These groups need to fly a pattern that takes them laterally away from the rest of the jumpers before turning to fly parallel to them. Depending on drop zone policy, these groups exit the plane first or last or somewhere in between. But it is important that they fly far enough perpendicular to jump run to provide a safe horizontal distance from the others on the load. In some cases, it may not be possible to ensure that everyone has a reasonable chance at finding clear airspace, so one of the groups may need to manifest for a different load. Having several groups of wingsuit flyers, a tracking group and an angle group all on the same airplane is just asking for someone to have a canopy collision. Common sense needs to come into play when it comes to organizing multiple groups on large jump planes.
The airspace above our drop zones continues to become more complex as we add disciplines with different freefall speeds and horizontal capabilities. Planning for every skydiver to have clear airspace for a safe deployment requires a thorough understanding of how all the pieces of the puzzle fit together. It’s not rocket science, but knowing your math sure does help!