• Exit Separation: Time Really Matters

    Posted on February 15, 2012 by melissa in Blog, Education.

    Ever wonder about Exit Separation? Why is it necessary? How can you tell if you’ve left enough time?

    I’ve got to be honest – Exit Separation was a mystery to me for the longest time. Ages ago, I was told to look out at the group before me, make sure they’re at a 45 degree angle and then you’re good to jump. Well, that’s a good back up plan, and it’s ALWAYS good to stick your head out of the plane, spot the DZ, clouds, air traffic AND watch the group before you jump; however, there’s much more to it than just looking out.

    Most importantly before exiting after a group, is to visually confirm there is enough separation between you and those that exited prior. This is important so that you do not encounter each other’s groups that have exited before you to ensure clear airspace for freefall separation and canopy openings.

    To roughly calculate Exit Separation, determine how fast the plane is traveling on jump run and the winds aloft forecast, to have a general idea of how fast the plane is traveling. With that information, you may determine:

    1. On days with strong uppers or a slower ground speed, you will need more time between groups
    2. On days with little to no uppers or a higher ground speed, requires less time between groups

    Trust me, I hate math so don’t let this intimidate you, I hope to have broken it down as simple as possible to illustrate this example!

    For this example, I’m going to use the Winds Aloft Forecast and Compass Rose used in the previous article, “Winds Aloft”; next I’m going to use a twin otter for our aircraft; lastly, we’re going to use the example as if the aircraft is running jump run into the wind (we’ll talk about other jump runs afterwards).

    In aviation, we use wind speed in knots, if you’d like to convert it to miles per hour, use this calculation: Knots * 1.2 = mph.

    First, we have to figure the airplane’s ground speed. How do we do that for the first load, when the pilot doesn’t even know his airspeed. We’re going to make a pretty darn good guess. So let’s use this scenario – on average, I would safely guess that a twin otter on jump run, without anything affecting its flight, would be about 80 kts. Next, we need to know the winds aloft at the altitude we’re jumping at. Since the Winds Aloft Forecast doesn’t have a 13,000-14,000 foot option, I’ll take the average wind speed from 12,000 and 18,000 on the forecast. In this example, the average wind speed is 46 kts. Now, take 80kts average airspeed minus 43kts average wind speed and we get 37 kts Ground Speed.

    Pilots (especially at Skydive Chicago) use a very calculated chart with Exit Separation time for each ground speed to ensure 1000 feet of horizontal separation between groups, but we don’t always have that luxury. So I’m going to give you my wisdom-chart of making a pretty darn good guess.

    I know that 100kts ground speed is super fast, I know 80kts is average and 60kts is slow. So let’s make a chart with that in mind:

    Aircraft Ground Speed              Time Between Groups

    100 kts                                             6 seconds

    (generally, in a twin otter, I don’t like to leave less than 5 seconds, but some situations may warrant, and the pilot should properly notify you beforehand)

    80 kts                                             7 – 9 seconds

    60 kts                                             10 + seconds

    Yes!! Now we at least have a guideline of how much time we should leave! So let’s go back to our example. We said that if we’re running jump run into the wind (which according to the forecast, should be South/South West), we should be going about 37 kts. If we go to the chart, we can see that we’ll be leaning towards a LONGER separation because we’ll be traveling across the ground much slower.

    Does that make sense? So, this is what I teach my students, especially if we’re on Load 1. Then, I’ll ask the pilot what our actual ground speed is to see how accurate of a guess we made, and to know what our separation should be.

    Now, there are drop zones who will only run jump run in certain directions. At Skydive City they will only run it North to South or visa versa – so this may mean a cross wind jump run. Usually cross wind jump runs may be offset to consider freefall drift as well, be a faster jump run and, it will change your best guess. So if you’re ever unsure, PLEASE ASK!!!

    If we’re at a larger drop zone where they’re running larger planes, jump runs can be strung out too far. It’s best to know Exit Separation so we can maximize the sky in order to safely have enough sky for each group and to generally avoid go-arounds.

    (The next topic will talk about Jump Run.)

    Try This At Home: Figure out the Winds Aloft Forecast for the day in your area (if you haven’t read or need to review, go to the “Winds Aloft” article). Ask your pilot what the general airspeed is of the aircraft you use at your drop zone (each aircraft will vary in speed). Take the average airspeed minus the average of winds and get your ground speed. Use the chart above and figure the best guess of Exit Separation.

    NOTE: Exit Separation is crucial in order to avoid other groups in freefall and to have a safe area in which to perform your jump. Everyone has a right to safe airspace and we need to do our best to know this information AND apply it. If EVER you are unsure, please at least ask an Instructor, Pilot or S&TA what Exit Separation should be. It will be different every day, for different airplanes, etc.

    Be safe out there and spread the good information!

     

     

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