Freefall Drift and How It Affects the Exit Spot – Part 3


This post is all about how to combine the previous two posts (reading winds aloft forecasts and calculating freefall drift) to start off with the best spot possible. This post assumes that you are on the first load of the day.

I like to cheat:

I start by looking at the ground/surface winds and deciding where I think my ideal holding area might be; then I check the winds aloft forecast.  If I notice that the winds at 3000′ are much stronger or are coming from a different direction then I may decide to alter my ideal parachute opening point.

Is that really cheating:

Not really…maybe I should have said I like to simplify the process.  However, you can also calculate how far your parachute will travel from your opening altitude to the ground, thus making the whole process much more numbers involved.  But, I don’t.

What to do when there are dastardly winds:

  • Example 1: The wind on the ground is relatively light (5 knots) but I notice that the winds aloft forecast says that the winds at 3000′ are 20 knots.  I will change my ideal holding point to be further upwind than I would if the winds were lower.  However, I probably won’t make my ideal opening point as far upwind as I would if both the ground winds and the winds at 3000′ were high.  Why not? The winds aloft forecast may be wrong and/or I don’t know at what altitude the winds will begin calming down.  The wind may die down at 800′ or 2800’…I won’t know until someone jumps.  Unless I see clouds racing by I’ll probably split the difference and say that my ideal opening spot will be similar to a day with moderate winds.  Notice, that I may also choose to elongate my downwind leg in case the winds do not die down until a low altitude.
left-handed landing area with moderate winds

This diagram shows how I might change my holding area when wind speeds are low on the ground, but higher at 3000. Dashed line shows a holding area for low winds while the dash-dot line shows a holding area for high winds.

 


 

  • Example 2: The wind on the ground is from the west, but the wind at 3000′ is from the south. Whenever you have winds that don’t line up nicely at the different altitudes, it can cause problems. In this example our winds are off by 90 degrees from each other.  If the winds at 3000′ are moderate to strong then I may think about changing the shape from a normal holding area (above left) to what I have shown below to account for the difference.
left-handed landing area with skewed holding area

A left-handed landing pattern for when the ground winds are coming from the west, but the winds at 3000 are more from the south.

 


I think I know where my holding area should be:

Now we calculate the amount of drift and the direction of drift that we expect in freefall.  Once we have those numbers we can find our ideal exit spot, like we did in the last post on calculating freefall drift.

Realistically, this takes quite a bit of experience to perfect 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 and many times you don’t know what they are unless you are using some sort of drift indicator.  If you are new to the sport then 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.

What happens at larger DZs:

You will rarely ever have the 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 kind pilot to go around.  If you don’t have the luxury of a kind pilot 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 is not a good enough spot while you were on the ground.

We will use a simple example for this.  There are moderate winds coming from the west, and we expect to drift 0.3 miles to the southeast.  Check out the aerial photo and decide if you would exit the plane at points A, B, C, and D.

 

hypothetical aircraft exit pointsPoint A is to the west of the airport, but our freefall drift will push us further to the southeast.  Upon opening, we will be further south than we would like, but we should be able to land on the airport without a problem.

Point B is already on the south side of the airport and we will open even further south.  We can probably make it back to the south side of the airport, but will have to be careful landing between a runway and a taxiway.

Point C is to the southeast of the airport and we will drift even further in that direction. With the winds coming from the south I would not want to exit the plane at this spot. Notice that very few alternate landing areas exist between our projected opening point and the airport.

If we exit the aircraft at point D, then we will probably open just east of the airport.  Even though we will be fighting the wind to make it back to the airport, we have assumed only moderate winds and should be able to land at the airport.  If the winds happen to be higher than expected there is a large field available to land in across the road.

Of all these scenarios, points B and C are most concerning, and one should do their best to avoid point D as well.  Putting our analysis together I have put a yellow oval around the area that would give the best opening positions. Notice the points A and D define the acceptability boundaries. For those experienced jumpers reading this, it is a bit on the conservative side.

acceptable exit points

And now, a confession:

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

Hope you enjoyed reading about freefall drift and how it impacts your exit point.  Have questions? Leave a comment and we will answer them.

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