Calculating drift for formation jumps

Every group that leaves the aircraft opens somewhere other than the point it exited over. Two things move it. Wind drive is the upper winds carrying the group for every second it spends in freefall, whether it moves or not. Intentional movement is the ground an angle or tracking group plans to cover.

Total displacement is wind drive plus intentional movement. Work out the wind first, because it acts on every group on the load, then add the movement plan on top.

Why nautical miles

Wind forecasts and aviation reports give wind speed in knots, and one knot is one nautical mile per hour. Work in those units and there is nothing to convert:

Drift (NM) = wind speed (kt) × time in freefall (hours)

The only step is turning freefall seconds into hours. Forty seconds is 0.011 hours, fifty seconds is 0.014 hours, and fifty-seven seconds is 0.016 hours. A 20 kt upper wind on a belly group for 57 seconds gives 20 × 0.016, about 0.32 NM of drift before anyone has flown anywhere.

Freefall time sets the drift

To compare groups, use one working band: exit at 14,500 ft and open around 4,500 ft, which is 10,000 ft of freefall. How long that takes depends on how the group falls.

• Belly falls slowest, at around 57 seconds for the band. It spends longest in the wind and drifts most.
• Angle groups sit in between, and the steepness moves the number. A flat angle takes around 53 seconds, close to belly. An intermediate angle takes around 50, and a steep angle around 45.
• Vertical freefly falls fastest, at around 40 seconds, so it drifts least.

Adjust the times for your own exit altitude, breakoff and group speed. The arithmetic stays the same.

The wind-drive table

This is the drift over the 14,500 ft to 4,500 ft band, using the times above. The grid-square column assumes a drop zone map gridded at 0.2 NM per square, rounded to the nearest half square. The drift applies to every group on the load, so draw it before any lane or sector.

Upper wind (kt)	Belly (~57 s)	Flat angle (~53 s)	Intermediate angle (~50 s)	Steep angle (~45 s)	Vertical freefly (~40 s)
10	0.16 NM (1.0 sq)	0.15 NM (0.5 sq)	0.14 NM (0.5 sq)	0.13 NM (0.5 sq)	0.11 NM (0.5 sq)
20	0.32 NM (1.5 sq)	0.29 NM (1.5 sq)	0.28 NM (1.5 sq)	0.25 NM (1.5 sq)	0.22 NM (1.0 sq)
30	0.47 NM (2.5 sq)	0.44 NM (2.0 sq)	0.42 NM (2.0 sq)	0.38 NM (2.0 sq)	0.33 NM (1.5 sq)

Use the average wind over the freefall band, from the winds-aloft forecast. The windsock shows the surface wind, and the direction aloft often differs from it, sometimes by a lot. Take the direction of the drift arrow from the forecast too.

Published figures land in the same range. Bryan Burke’s exit-order work puts a 70-second belly jump about 3,000 ft downwind in a 30 mph upper wind, and a 45-second freefly jump nearer 2,000 ft. Put his wind and times into the formula above and it gives both numbers almost exactly, which is a good check on the method.

Intentional movement on top

Once the wind is drawn, add the ground the group plans to cover. These are typical planning bands for a working jump on the same 0.2 NM grid:

Group	Typical planning band
Beginner angle, and advanced steep angle	0.5 to 1.0 squares (about 0.10 to 0.20 NM)
Intermediate angle	1.0 to 1.5 squares (about 0.20 to 0.30 NM)
Advanced flat tracking	2.5 to 3.0 squares (about 0.50 to 0.60 NM)

Use these for planning, and plan less when the conditions, the group’s experience or its spread call for it.

Beginners cover little ground because they fly slowly and conservatively. Advanced steep groups also cover little ground, because a steep angle puts more of the group’s speed into falling and less into horizontal range. Advanced flat-tracking groups cover the most ground, so they need the most airspace and the most carefully planned lane.

Putting it on the map

A drop zone aerial map gridded at 0.2 NM turns all this into something you can draw. Nautical miles become squares, and the whole load can see the squares and brief against them.

1. Draw the wind arrow first. Take its direction from the winds-aloft forecast and its length from the table.
2. Draw the movement arrow from the tip of the wind arrow. Take its direction from the jump plan and its length from the planning band.
3. The tip of the second arrow is the expected opening area. Check it against the landing area, other groups’ lanes, and anything you would not want to open over.

The leader flies this plan by looking at the ground, and needs to see it for the whole jump. Keep movement jumps well clear of cloud, and have a plan B for days when cloud rules them out.

Worked examples on a 0.2 NM grid

A belly 4-way on a 20 kt day. No planned movement. The table gives 0.32 NM, which is 1.5 grid squares downwind. Whoever spots the load needs that arrow drawn before they can pick the exit point.

An intermediate angle group, same day. Wind drive is 1.5 squares. The plan adds 1.0 to 1.5 squares of movement across the wind line. On the board that is two arrows at an angle, and the opening area sits about 2 squares from the exit point along the diagonal. The group briefs the opening area from the map and plans the canopy flight from there.

A flat-tracking group running downwind, same day. Wind drive of 1.5 squares plus 3.0 squares of movement in the same direction makes 4.5 squares, or 0.9 NM. Running downwind nearly doubles the footprint. When the map allows, take the lane across the wind or into it. The leader picks the direction on the ground and draws it for the whole load to see.

A vertical group on a 30 kt day. No planned movement, and the table still gives 1.5 squares. A fast fall rate makes the drift smaller, and some always remains. On a windy day even a group falling straight down opens well away from where it exited.

Building the table for your own drop zone

The reference numbers work anywhere, and the map work is local. Grid your aerial map at 0.2 NM, or note what your existing grid measures. Set the freefall band from your usual exit and opening altitudes.

Time the band for each discipline from your own jumps or the estimates above. Convert the seconds to hours, multiply by the forecast wind, and round to the half square. Redraw the wind arrow every load, because the forecast changes through the day.

Related pages: Exit order and separation, What a movement leader owns, and Reading a movement jump briefing.

Jurisdiction notes: who regulates what here

Drift planning itself is airmanship guidance. The things around it are regulated and vary by country: exit separation practice, movement-jump approval, who may lead, and drop zone board procedures.

In Australia these sit under the Australian Parachute Federation’s Operational Regulations, with the APF Freefly and Angles Guide as the national educational reference. In the United States, section 5-10 of the United States Parachute Association Skydiver’s Information Manual covers movement jumps and section 5-2 covers spotting, with drop zones adding local policy on top.

Some drop zones add rules of their own. Hunter Valley (Newcastle Sport Parachute Club), for example, rules out movement jumps in or near cloud and limits a load to two movement groups unless the drop zone safety officer approves more.

Other countries have their own national-body equivalents. Your drop zone’s rules and the day’s load organiser always take precedence over anything on this page.

References and further reading

National-body guidance:

• United States Parachute Association, Skydiver’s Information Manual: section 5-2 on spotting gives the standard method of vector-averaging the winds at several altitudes to set jump run and freefall drift, and section 5-10 covers movement jumps.
• Australian Parachute Federation, Freefly and Angles Guide (APF006): the Australian educational guide, covering movement-jump planning and how movement groups fit into a load.
• British Skydiving: the Tracking Progression Manual covers movement-jump progression and flight-path planning in the UK system.
• Newcastle Sport Parachute Club, Hunter Valley angle and movement jump guidelines: movement groups per load, cloud, and the leader’s navigation skills.

Numbers and analysis:

• Bryan Burke, The Horizontal Flight Problem (Skydive Arizona, 2013): what moving groups do to a load’s separation, including why angle dives fall at freefly speed while covering less ground than a flat track.
• Bryan Burke, Exit Order for Free Flyers: drift figures by discipline, the numbers checked against the table on this page.
• John Kallend, freefall separation study (Illinois Institute of Technology, 2000): the physics simulation behind current exit-separation practice, including the case where the lower winds oppose the uppers.

Drop zone policies and practical guides:

• Skydive Arizona, tracking and angle flying policy: a drop zone policy built around lane planning and winds aloft.
• Skydive Spaceland Houston, movement jump policies: a published drop zone rule set for tracking and tracing dives.
• Christy West, Ten Tips for Safe Tracking Dives (Skydive Mag, 2018): planning advice that ties the flight-path angle to the day’s uppers.

Tools:

• winDZaloft: a free tool that uses the vector-average drift method and draws the result over a satellite map of your drop zone.
• AXIS exit separation calculator: exit timing from ground speed and winds, from AXIS Flight School.

---

Advisory, educational content. Wind drift numbers are planning estimates, and your drop zone’s procedures and the day’s load organiser set the rules that apply. If the forecast or the drop zone board disagrees with this page, go with the board.