Calculating drift for formation jumps

Every group that leaves the aircraft opens somewhere other than the point it exited over. The displacement has two parts. Wind drive is the passive part: the upper winds carry the group for every second it is in freefall, whether it means to move or not. Intentional movement is the deliberate part: the ground an angle or tracking group covers on purpose.

Total displacement is wind drive plus intentional movement. The order matters: 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. Keeping the whole calculation in those units means there is nothing to convert:

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

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

Freefall time sets the drift, and discipline sets the freefall time

Use a common working band so groups can be compared: exit at 14,500 ft, opening area around 4,500 ft, which is 10,000 ft of freefall. How long that takes depends on how the group falls:

• Belly falls slowest, around 57 seconds for the band. Longest exposure to the wind, most drift.
• Angle groups sit in between, and steepness moves the number. A flat angle runs close to belly time, around 53 seconds. An intermediate angle around 50. A steep angle around 45.
• Vertical freefly falls fastest, around 40 seconds. Least exposure, least drift.

Adjust the times for your own exit altitude, breakoff and group speed. The arithmetic does not change.

The wind-drive table

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

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 rather than the windsock. The direction aloft often differs from the surface wind, sometimes by a lot, so the drift arrow on the map takes its direction from the forecast too.

Published figures land in the same bracket. 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. Feed his wind and times into the formula above and it reproduces both numbers almost exactly, which is a good check on the method.

Intentional movement on top

After the wind is drawn, add the ground the group plans to cover on purpose. 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)

Treat these as planning bands rather than performance targets. Conditions, experience and group spread can all argue for less.

The table’s shape has a pattern. Beginners cover little ground because they fly slowly and conservatively. Advanced steep groups also cover little ground, because a steep angle trades horizontal range for vertical speed. The big movers are advanced flat-tracking groups, which is why they need the most airspace and the most careful lane.

Putting it on the map

A drop zone aerial map gridded at 0.2 NM turns all of this into something you can draw. Nautical miles become squares, and squares are what a load can see and brief against. The habit that works:

1. Draw the wind arrow first: direction from the winds-aloft forecast, length from the table.
2. Draw the movement arrow from the tip of the wind arrow: direction from the jump plan, 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 do not want to open over.

Worked examples on a 0.2 NM grid

A belly 4-way on a 20 kt day. No intentional movement. The table gives 0.32 NM, so the group opens about a mile and a half of grid squares downwind: 1.5 squares. Anyone spotting for the load needs that arrow drawn before the exit point makes sense.

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 the canopy plan starts from there.

A flat-tracking group that runs downwind, same day. Wind drive 1.5 squares plus 3.0 squares of movement in the same direction is 4.5 squares, which is 0.9 NM. Choosing to run downwind nearly doubles the footprint, which is the argument for taking the lane across the wind or into it when the map allows. Direction is a leader’s decision, made on the ground, drawn for the whole load to see.

A vertical group on a 30 kt day. Zero intentional movement, and the table still says 1.5 squares. Fast fall rate shrinks the number and never removes it. On a windy day even the groups flying straight down are opening well away from the point they exited over.

Building the table for your own drop zone

The reference numbers travel well, 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 standard exit and opening altitudes. Time the band for each discipline from your own jumps or the estimates above, convert seconds to hours, multiply by the forecast wind, and round to the half square. Redo the wind arrow every load: the forecast changes through the day.

Jurisdiction notes: who regulates what here

Drift planning itself is airmanship guidance. The things wrapped 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, United States Parachute Association Skydiver’s Information Manual section 5-10 covers movement jumps and section 5-2 covers spotting, with drop zones layering local policy on top.

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

Published treatments of drift, separation and movement-jump planning.

National-body guidance:

• United States Parachute Association, Skydiver’s Information Manual: section 5-2 on spotting carries 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 load integration.
• British Skydiving: the Tracking Progression Manual covers movement-jump progression and flight-path planning in the UK system.

Quantitative treatments:

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

Drop-zone policies and practical guides:

• Skydive Arizona, tracking and angle flying policy: a mature drop-zone-level 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): practical planning advice that ties the flight-path angle to the day’s uppers.

Tools:

• winDZaloft: a free tool that implements 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.

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

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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. When the forecast and the board disagree with a table on the internet, believe the board.