Wind Drift & Exit Order Graphic

 This graphic is based directly on the output of John Kallend's freefall simulation program set to the following conditions:Aircraft speed of 85 mph (125 fps)Fast faller exiting firstSlow faller exiting second, 10 seconds laterWinds aloft of 35 mph (51 fps), ending at 8000 feetTo view a graphic of the same conditions but the exit order reversed, click here.This is what CAN happen under THESE conditions. It goes without saying that there are endless possibilities and variables besides those presented here.I believe it is important to note Kallend's remarks regarding the creation of his program: I tried to avoid anything that included the vagaries of human behavior, as far as possible. The two cases I consider could be considered as extrema. One is a belly flier who maintains constant attitude with respect to the relative wind, and falls at a speed achieving a 65 second freefall from 14000 ft. The second is a head down flier maintaining constant attitude and taking 50 seconds. I determined the ballistic coefficients by back-calculation (fancy term for "trial and error"). I don't think a belly flier will go less far in forward throw than my calculation, and I don't think a head downer will go farther. Someone who 'mixes it up' will go some intermediate distance.

Most of the belly fliers I know maintain an essentially constant attitude unless they funnel. I suspect the head downers may vary (I don't freefly).

Also missing are such things as systematic backsliding, freaking around, etc. I believe these will impose a random spread on the calculated paths."

-John Kallend, Ph.D.

Conclusion

Many people have assumed, based on the arguments posed by this graphic, that it is a case for making fast fallers always exit after slow fallers. This is not the purpose of the graphic. There are instances where, as a DZO, I would have the fast fallers exiting first. An example might be if I had a DZ on an island and the spot was critical; I might consider the risk caused by a premature deployment to be less than the risk posed by drowning skydivers. At a wide-open DZ (like Skydive Arizona), out landings are less of a hazard, and the horizontal separation ensured by putting fast fallers out last becomes the deciding factor.

The bottom line is that there is much more to consider than wind drift and fall rates. Every drop zone needs to adopt a policy that provides the best safety under the conditions and types of skydiving that take place. There is no single solution that works for everybody all the time.

Perhaps most importantly, skydivers need to be educated about the effects of wind drift, fall rate, and time. Time is the key to understanding freefall drift; it is a basic observation that a slow falling group will experience more drift because it is drifting for a longer period of time than a fast falling group. While they are in the same moving air mass, they are drifting horizontally at the same rate, but they experience different periods of time in that air mass (in the above example, 29 seconds for the fast faller and 36 seconds for the slow faller). In conditions where there are extremely high winds aloft, it is possible for a slow falling group, exiting well after a fast falling group, to open up several thousand feet downwind of the first group.

-Tim Wagner, D-10552

Further Resources:

Winsor Naugler's Another Look at Descent Kinematics

John Kallend's Interactive Java Freefall Simulation

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