FDS Research › Tools

Exit separation visualizer

A theoretical jump run viewed from the side: the airplane runs from left to right with groups leaving in sequence, coming off the hill, drifting in freefall, then pulling. We compare the distance between openings to the USPA’s guidance—at least 1,000 ft between solo jumpers, 1,500 ft between small groups. The tool defaults to 11-knot uppers—the mean freefall-column wind across the 48 American dropzones of our comparison, computed from decades of NOAA weather-balloon soundings. Setting this slider to zero isolates the hill; increasing the value makes clear how uppers affect the contrasting wind drift of different types of groups in freefall. This tool is a companion to the much simpler exit separation calculator. It does not apply to aircraft that do not have a jump run—helicopters and hot-air balloons require separation by other means. Canopy flight is not shown.

The model. Each group leaves the door at the aircraft’s ground speed and decelerates toward the wind’s speed (zero, in still air) with the natural time constant of freefall (terminal velocity divided by g—about 5 seconds a belly group, 5.5 a belly solo, 7 freefly, 8 head-down): that decay is the hill. Vertical speed builds to terminal velocity by the same physics. Trackers are shown diverting direction after the hill—indicated by a dashed line (drawn along the jump run). We don't measure the distance of angle or wingsuit groups to others as they are tasked with achieving separation through different means. Breakoff is not shown by default—jumpers track variable distances in variable directions, so each group falls as a single point unless the breakoff toggle is set (next section). Real breakoff is ideally perpendicular to jump run, but larger groups fan our radially from the formation’s center—and by geometry (or while avoiding each other) somebody often ends up tracking up jump run, spending part of the bought spacing before anyone opens. That is exactly why larger groups always require more separation. We assume here that jump run is flown directly into the wind—a standard convention; a crosswind component drifts all groups identically and changes none of the spacing, so only the along-run wind is modeled. The wind slider allows visualization of a downwind jump run if you slide it to the left—a real downwind run would also raise the ground speed, a slider the reader holds. Further intentional simplifications: one wind value for the whole freefall column, one shared pull altitude for sport groups—tandems pull at 5,500 ft, high pulls open 800 ft below exit altitude, and an AFF group opens twice: the student at 5,500, the instructor(s) continuing to 4,000—paths end at opening. While our visualization ends at opening, the effect of winds aloft does not and chaos is brought into the equation as jumpers open facing random directions and make different decisions about where to fly their canopies (if safety permits it is best to fly a canopy perpendicular to jump run after opening)—none of which this visualization attempts to model.

Breakoff characteristics. With Show breakoff on, each belly and freefly group is shown breaking off along jump run simply as a worst-case scenario. Our model uses a flat track at about 88 ft/s horizontal speed and 147 ft/s fall rate (100 mph—tracking away from a group slows fall rate when done correctly). The distance you aim to track determines the the tracking time, and the time in turn determines how high breakoff must be initiated. Every separation on this page then measures between nearest trackers, not group centers—which is another why larger groups need more separation. The comparison judges each pair against its own figure: 1,000 ft according to USPA guidelines where both are solo jumpers, 1,500 ft where a group is involved—and with the breakoff toggle on, every distance runs between nearest trackers (effectively solo jumpers at opening), so the 1,000 ft floor applies throughout.

A poorly understood subtlety. Two theoretically identical group types drift identically: whatever the uppers do to the first, they do to the second as well, so the distance between their opening points stays exactly what their spacing coming out the door provided—a nice result. Wind aloft reaches the separation between groups through only two means: through ground speed (the same wind that will push the jumpers slows the airplane over the ground first, such that each second waiting in the door buys less separation) and through fall-rate differences between groups (a slower faller stays in the wind longer than a faster one, so unequal groups drift unequal amounts—the main reason exit order matters at all). The tool makes this easy to visualize: put two belly groups in a row and slide the wind—the distance between their openings holds. Swap one for a freefly group and slide it again: the opening separation no longer matches what the door bought, and the mismatch grows with the wind.

Complications post-opening. As mentioned in the introduction, this tool does not visualize canopy flight as it is too unpredictable. Flying up jump run after opening, however, is an absolutely notorious problem when it comes to group separation and the cause of nearly all collisions between open canopies and jumpers still in freefall. It is always best practice if safety permits to fly perpendicular to jump run immediately after opening.

This is not a spotting tool: real conditions involve layered winds aloft, drift off jump run, canopy flight, and many other factors. Nothing here is a substitute for proper training by certified instructors. See, for example, the United States Parachute Association's Integrated Student Program, described in the Skydiver's Information Manual.