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The Skydiving Handbook - Chapter 4 (The Skydiving Universe)

We've already discussed your body's relation to the relative wind. Now let's look at your relationship to space and time. When you leave an airplane at our customary exit altitude of 12,500 feet above the ground, your accelerate from zero miles per hour vertical speed to approximately one hundred and ten miles an hour in about ten seconds. It doesn't seem too dramatic because the aircraft speed was already about 100 miles an hour, so you reall only gain ten miles per hour. At that point you reach terminal velocity, the speed at which the air pressure against your body balances the pull of gravity. Ignoring minor changes in body position, you will stay at that speed until something stops you - hopefully the deployment of your parachute! At terminal velocity you pass through one thousand feet every six seconds. If your parachute opens at 4,500 feet, that gives you about 52 seconds of freefall. (Ten seconds for the first thousand, six for each of the next thousand.) If your parachute did not open, you would now have a life expectancy of 27 seconds. Opening altitudes are based on allowing skydivers time to be sure that they do land under an open parachute. More experienced jumpers commonly open at about 2,500 feet because of their greater familiarity with equipment and emergency procedures. This gives them about 65 seconds of freefall from a 12,500-foot exit.

The main thing about altitude is that if you run out of it while in freefall, you die. However, since your fall rate is constant, your consumption of altitude is constant. This means that if you have plenty of altitude, relax, because only time can take it away from you. Time and altitude are directly related.

Loss of altitude awareness is a major contributor to skydiving fatalities. Always bear in mind that no distraction is worth dying for. Until your body's freefall clock has been programmed so that you know how long you've been in freefall, your only reference is your altimeter. Every time you do anything - intentionally or not - check your altitude. That way, you won't lose altitude awareness if a distraction such as a difficult maneuver or loose goggles comes along. Keep in mind that since you are consuming altitude (time) at a constant rate, you can't stop what you are doing, think it over, go back, and try again. In freefall, there are no time outs! That's why we try to do all of our freefall tasks carefully and deliberately, getting them right the first time. If you rush, you will actually lose time because the extra mistakes that result will slow you down. And when you consider the cost of freefall time, you'll appreciate the value of thorough ground preparation!

Besides our time reference (altitude), we also make use of space references. There are two types of space references, orientation to the ground and orientation to other skydivers. We'll call the ground reference heading. Heading is an imaginary line drawn from a point on the horizon directly in front of you through your center. You use this reference for tasks such as turns, backloops, or simply hovering in place. Eventually you will substitute the line of flight for a personal heading. The line of flight is the heading the aircraft was on when you left it. The advantage of using line of flight is that now all the skydivers on the airplane have the same heading reference, instead of each picking their own. This makes it much easier to coordinate group activities.

Your reference to the other skydivers is called the center point. The center point is that spot closest to all of the skydivers. When you are alone, it is in the middle of your body. With others, imagine a ball falling straight down around which everyone flies. In other words, four skydivers holding hands in a circle would have the center point in the middle of the ring. If they all backed up ten feet, it would still be in the same place because thjey are all still equally close to that point. In many ways, the center point of a formation is like the center of your box man discussed in the previous section. If one corner of a formation is low relative to the center point, the formation will turn in that direction. If two corners are low, it will slide in the direction of the low side.

By now you can see that while skydiving, you have to be aware of several different things: altitude, your own body position, your position relative to the ground, and your position relative to others. Initially this will seem like a lot to be aware of, so on your first few jumps you will concentrate almost entirely on altitude and your body position while your jumpmasters take care of the rest. When you are release to fly free, you will add your own heading, and eventually you will be able to monitor these, the formation center point, and the line of flight as easily as you monitor your speed, direction, location, and other traffic as you drive to the drop zone!



Test yourself:

1. "Temporal distortion" refers to the fact that in an emergency situation (losing control of your car, for example), the rush of chemicals to your brain can cause events to seemingly go into slow motion. Why would temporal distortion be extremely dangerous to a skydiver?

2. Why is ability to hold a heading considered essential to flying with other skydivers?

Proceed to Chapter 5 (After the Freefall)




By Bryan Burke on 2011-09-20 | Last Modified on 2012-10-26

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