Freefall Drift: Navigating Your Exit Spot

Understanding freefall drift is crucial for accurate skydiving spotting, especially given how much winds aloft can vary throughout the year. From my experience jumping in Florida, summer often sees relatively consistent winds at jump run altitude (barring tropical weather). However, winter months here can bring significantly higher and more unpredictable winds. This increased wind not only impacts your selected skydiver exit spot, dictating where you need to leave the aircraft to account for drift, but also, in conjunction with the aircraft's ground speed on jump run, influences the amount of separation needed between freefall groups. To consistently land on the airport, mastering the calculation of freefall drift is absolutely essential.

How to Read the Winds Aloft Forecast

To begin with, you'll have to get comfortable reading the winds aloft forecast. It's important to remember the word forecast – this isn't a live reading of what's actually happening in your area. The winds aloft report is a prediction, created four times a day by supercomputers running complex mathematical models. Fortunately, these predictions are usually quite close to what you can expect, as the computers compile data from pilot reports and weather balloons (which are typically sent up from major airports). This ongoing data collection helps ensure their reliability. However, there are times when they can be quite a bit off, so always be aware and ready to adjust. This guide will help you learn how to read aviation weather for skydivers.

You can access the winds aloft report through several dedicated apps or the reliable Aviation Weather Center. These reports typically include timestamps indicating when the data was taken (e.g., 1200Z) and the period for which it is valid (e.g., from 1400Z to 2100Z). For convenience, this time is often presented in UTC (Coordinated Universal Time), also known as Zulu time. If you're new to using UTC, a handy UTC to US time zone conversion chart can be a useful tool to translate it to your specific US time zone.

First, locate your nearest airport. For this example, I'll use data for JAX (Jacksonville airport), as it's closest to my location, focusing only on altitudes that pertain to skydivers (3,000′ to 12,000′ MSL), though reports often include data up to 39,000 feet. The altitudes are shown in parentheses:

1023 (3000′) 0919+14 (6000′) 0917+09 (9000′) 1020+03 (12000′)

To read this forecast, you need to know that the first two numbers correspond to the wind direction (remember to add a zero to the end of those first two numbers), and the next two numbers indicate the wind speed in knots. Any numbers after the wind speed indicate the temperature in degrees Celsius.

For example:

  • At 3,000′ MSL: The wind forecast shows that the winds are from 100 degrees at 23 knots.

  • At 6,000′ MSL: The winds are coming from 90 degrees at 19 knots, and the temperature is +14 degrees Celsius.

  • At 9,000′ MSL: Winds are from 90 degrees at 17 knots, and the temperature is +9 degrees Celsius.

  • At 12,000′ MSL: Winds are from 100 degrees at 20 knots, and the temperature is +3 degrees Celsius.

And now you know how to read the winds aloft report!

Calculating Freefall Drift: Your Skydiver's Trajectory

Now, we get to estimate how far you will drift in freefall. Notice the word estimate – this is not an exact calculation, as we use generalizations in our number crunching. But, it will give you a good starting point when you are deciding where to leave the plane on the first load of the day. So, what do we do with the winds aloft information?

First, we average the wind speed and then we will average the wind direction. For our example using the JAX data, the average wind speed will be the sum of the wind speeds from 3,000′ to 12,000′ divided by the total number of readings. In our case, that's (23 + 19 + 17 + 20) / 4. This gives us an average wind speed of 19.75 knots. The conversion of knots to the more well-known miles per hour is 1.15 mph per knot. So, 19.75 knots is equivalent to approximately 22.7 mph.

Once we have figured out our average wind speed, we can now think about our freefall drift distance. In general, we are in freefall for approximately 1 minute. Since our average wind speed has been converted to miles per hour, we can translate that into miles per minute: 22.7 (miles/hour) * (1 hour / 60 min) = 0.38 miles/min. This calculation makes it seem like during our freefall we will drift approximately just over a third of a mile.

But that is not the end of the story.

What is Forward Throw?

When we exit the aircraft, we do not immediately begin falling straight down. Forward throw is due to your body wanting to continue moving at the same speed and direction as the plane you just left. It takes several seconds before the relative wind slows you down and you begin to just fall. All this means is that in light to moderate winds, you will be thrown forward about 0.2 miles upon exit before being significantly subjected to drift. In heavier winds, you will experience little to no forward throw. This is a key part of understanding forward throw in skydiving.

So, in our example, our calculated drift will be adjusted by this forward throw. If we take our 0.38 miles of drift and account for a 0.2-mile forward throw, our effective drift will be 0.38 miles - 0.2 miles = 0.18 miles.

In Which Direction Am I Drifting?

To figure out which direction we are drifting, we calculate the average of the wind directions. In our example using the JAX data, we would find the average with this equation: (100 + 90 + 90 + 100) / 4 = 95.

Since we know that the average wind direction is from 95 degrees (coming from the east), then we will drift in freefall to the west. The wind will be blowing us from the east to the west, helping to determine your skydiver drift direction.

Where Should I Exit the Aircraft?

Based on these calculations, you should select an exit point roughly 0.2 miles to the east of your intended opening point. This is a vital step in skydiver exit point calculation.

Selecting a Good Spot for the First Load of the Day

The "Wind Dummy Load"

This term refers to the brave skydivers on the first load of the day, whose jump helps confirm the ideal spot. It's essentially the first jump load helping everyone else fine-tune their desired exit spot.

As we have already discussed, we can read the winds aloft forecast and calculate the amount of freefall drift that we might expect, but selecting a good spot for skydiving is not an exact science. Nothing can truly beat sending up the first load, observing where everyone opens, and adjusting the spot from there.

My Approach: Simplifying Spotting

I prefer to simplify the spotting process. I start by looking at the ground/surface winds and deciding where I think my ideal canopy holding area might be; then I check the winds aloft forecast. If I notice that the winds at 3,000′ are much stronger or are coming from a different direction, then I may decide to alter my ideal parachute opening point. While you could also calculate how far your parachute will travel from your opening altitude to the ground, making the whole process much more numbers involved, I generally don't go that deep on every jump.

What to Do When There Are Challenging Winds:

Dealing with highly variable or strong winds requires a bit more nuance in your spotting strategy. Here are a couple of common scenarios:

Example 1: Light Ground Winds, Stronger Upper Winds Imagine the wind on the ground is relatively light (say, 5 knots), but you notice that the winds aloft forecast indicates the winds at 3,000′ are significantly stronger (e.g., 20 knots). In this situation, you'll want to change your ideal holding point to be further upwind than you would if the winds were uniformly lower. You'd visualize your landing pattern shifted more "into the wind" from your target. However, you probably won't make your ideal opening point as far upwind as you would if both the ground winds and the winds at 3,000′ were high.

Why not? The winds aloft forecast might be off, and/or you don’t know at what exact altitude the winds will begin calming down. The wind may die down at 800′ or 2,800′—you won’t know until someone jumps and you can observe their descent. Unless you see clouds racing by (a clear sign of strong upper winds!), I'd probably split the difference and aim for an ideal opening spot similar to a day with moderate winds throughout. Additionally, you may choose to elongate your downwind leg in your skydiving landing pattern in case the winds do not die down until a very low altitude, giving yourself more room to work with.

Example 2: Cross-Directional Winds Now, consider a scenario where the wind on the ground is from the west, but the wind at 3,000′ is coming from the south. Whenever you have winds that don’t line up nicely at different altitudes, it can cause problems for your skydive landing pattern. In this example, your winds are off by 90 degrees from each other. If the winds at 3,000′ are moderate to strong, you might think about changing the shape of your typical landing area from a normal rectangular holding pattern to something more "L-shaped" or curved to account for the difference. For instance, if you're planning a left-handed landing pattern with ground winds from the west, but the 3,000' winds are more from the south, you might need to stretch your base leg or adjust your turn points significantly to manage the sideways push from the upper winds, essentially creating a crosswind landing pattern.

Practical Spotting in the Real World

Now that you know where your ideal canopy holding area should be located, the next step is to calculate the amount and direction of drift that you expect in freefall. Once you have those numbers, you can find your ideal exit spot, like we did in the previous section on calculating freefall drift.

Realistically, perfecting skydiving spotting takes quite a bit of experience, and even then, experienced jumpers can be quite a bit off – hence the reason that the first load is also known as the "dummy load" on windy days. There are a lot of variables at play, and many times you don’t know what they are unless you are using some sort of drift indicator (like watching other jumpers or ground-based flags/indicators). If you are new to the sport, it’s a great idea to have a reputable coach, instructor, or D-license holder to confer with and confirm that your plan is a good one. Don't hesitate to ask for help with skydiving wind calculations!

What Happens at Larger Dropzones: The "Good Enough" Spot

At larger dropzones, you will rarely ever have the absolutely ideal spot. Everyone needs to know what is "good enough." I find that it is easier to think in terms of what is not good enough. At what point should you look down from the plane and say, "not good enough," and ask the pilot to go around? If you don’t have the luxury of a pilot who can go around (or if you're too far into jump run), then you may need to make a quick decision regarding possibly landing off the airport or riding the plane down. Depending on your DZ’s location, it may be infinitely wiser to ride that plane back down versus risking an off-airport landing. But, it’s much easier to make that decision in the plane when you have already thought about what constitutes a "not good enough" spot while you were on the ground.

Let's use a simple example for this. Imagine moderate winds coming from the west, and you expect to drift 0.2 miles to the southeast (using our earlier calculations). Consider these hypothetical exit points relative to an airport:

  • Point A: Located to the west of the airport. Your freefall drift will push you further to the southeast. Upon opening, you will be further south than you would like, but you should still be able to land on the airport without a problem. This might be an acceptable spot.

  • Point B: Already on the south side of the airport, and you will open even further south due to drift. You can probably make it back to the south side of the airport, but you'll have to be very careful landing between a runway and a taxiway. This is pushing the limits.

  • Point C: To the southeast of the airport, and you will drift even further in that direction. With the winds coming from the south pushing you, you would not want to exit the plane at this spot. Very few alternate landing areas exist between your projected opening point and the airport, making this a risky exit.

  • Point D: If you exit the aircraft at this point, just east of the airport, you will probably open just east of the airport. Even though you will be fighting the wind to make it back to the airport, assuming moderate winds, you should be able to land at the airport. Critically, if the winds happen to be higher than expected, there is a large field available to land in, across the road, making it a safer "boundary" spot.

Of all these scenarios, points B and C are most concerning, and one should do their best to avoid point D as well if possible. Putting our analysis together, the best opening positions would define an area, let's say, like a yellow oval encompassing ideal conditions. Points A and D might define the acceptability boundaries. For those experienced jumpers reading this, this example is a bit on the conservative side, prioritizing safety.

A Practical Perspective on Spotting:

I don't usually calculate freefall drift anymore unless I am teaching students. After doing several of these calculations, you will start to notice patterns. For instance, if your average wind speed is 30 mph, you will likely drift about 0.25 miles (assuming jump run is into the wind). An average wind speed of 60 mph? You will drift closer to a mile, and you should probably be much more concerned about skydiving exit separation! But, that is another topic for another time.

I hope this guide has provided valuable insights into skydiving freefall drift, winds aloft forecasting, and how it impacts your exit point and landing strategy. Stay safe and happy jumping!