Conclusion 1 (cont.)

To achieve 1,200 ft separation between groups that have the same fall rate, popular methods based on groundspeed work UNLESS the lower winds are opposite the uppers, in which case extra spacing is needed.

Why, then, do the common groundspeed (or ground-distance) based methods work? Basically, because the windspeed at opening altitude is usually small compared to the airspeed of the jumpship, and can be fairly safely ignored. If the lower level winds are opposite the uppers, then extra spacing is needed.

We now need to use a freefall trajectory simulation program to investigate these and other effects. This can be downloaded from Since it is an interactive program, you can put in typical jump plane speeds from your DZ, (70kts IAS might be typical of an Otter) and check out the effects of fall rate, windspeeds, etc. I suggest that you try various combinations of upper and lower windspeeds to check out the previous conclusions.