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BASE Jumping: Articles: Problems: Offheadings

Offheadings

by BASEwiki

Of all problems in BASE, the offheading is the most dangerous one. Granted, a total malfunction with no canopy coming out of the container is potentialy more deadly, but a lot easier to avoid.

Offheadings are not entirely avoidable. Many famous BASE jumpers have said there are two types of jumpers; those who have hit an object, and those who will.

Effect

An offheading happens any time your canopy opens up and ends up flying in a different direction than you intended. Worst case scenario you find yourself flying directly towards a solid object, often when you have the dreaded one-eighty.

However, even offheadings of lesser degrees (nineties, or hundred-twenties) can cause significant problems if not corrected quickly. The key is to avoid obstacle collision at all cost. Given a choice between hitting an object and then the ground, or hitting just the ground, the latter option is usually a better choice.

Cause

There are five major factors involved in an offheading:
  1. Packing
  2. Body Position
  3. Winds
  4. Pilotchute Dynamics
  5. Bad Luck

Different people attach different levels of importance to these factors. It is generally accepted that the first three factors are the most important, with body position being a strong leader.

Packing

Avoiding an offheading starts with your packjob. Make sure you keep everything symmetric while you pack. Whatever you do on the left side, do it on the right side too. One area to pay extra attention is the dressing of your risers. If one riser is loaded before the other one, you will induce a turn in that direction. More importantly, be careful when closing your rig. Many beginning jumpers pack perfectly symmetrical and then completely distort their symmetry during the closing sequence of the container. Especially on hard-to-close pin rigs, you can see people yank hard on the pull-up cord, distorting the packjob on the inside.

If you find yourself getting consistent offheadings in a particular direction, it is a good idea to consider what you do when you are closing your container.

Body Position

Body position is by many considered the greatest factor in heading control. Many jumpers that go off unstable, drop-a-shoulder, go head-low or come out of an aerial with assymetrical rotation end up getting a canopy that is not flying on heading.

There are two main factors that cause an unstable body position to result in an offheading. First, the canopy comes out of your container the way you put it in. That means that you may very well get an onheading opening relative to your container, but since you presented the container in an unintentional direction, your canopy will open that way. Secondly, if you load your risers unevenly, the riser that gets loaded sooner will initiate a turn in that direction.

Winds

Winds contribute greatly to the chance of getting an offheading. Generally, a canopy has a tendency to open into the wind. However, it can also give your inflating canopy momentum in a certain direction. This is especially deadly in crosswinds. If you find yourself on top of a solid object with a strong crosswind, reconsider your desire to jump. Admittedly the same goes for winds from other directions. A general rule of thumb is that solid objects should be only be jumped in zero wind conditions.

When jumping from an antenna, the wind can actually help you. By making sure you jump with a tailwind and in between the wires, you will be pushed away from the wires. This is discussed in more detail in the section on [[SiteAnalysis/Antennas | antennas.

Theoretically, a strong headwind can actually improve your heading performance. The wind will blow into the exposed center cell and the canopy will have a tendency to open into the wind. This knowledge is rarely useful since the drawbacks of a headwind if you do get an offheading are too great (the headwind will push you back into the object).

Basejumper dedicates an entire section to
  • Closing the wind considerations.

    Pilot Chute Dynamics

    The pilotchute is what extracts your canopy out of the container towards line-stretch. A moving pilotchute will transfer its movement to the canopy it’s attached to, at least to a certain extent.

    This problem is often referred to as the ‘oscillating pilotchute. Imagine a stowed pilotchute tossed out to your side. It will inflate and extend and come from the side towards the center. It carries momentum, continues to the other side and then bounces back. Essentially you see a pilotchute bouncing back and forth above the jumper. This movement is transferred to the canopy contributing to an offheading opening.

    There are two major causes for pilotchute oscillations:
    1. An assymetrical pilotchute, either because it’s manufactured that way or because it’s attached to the bridle that way. An assymmetrical pilotchute will spill more air on one side than on the other and start tipping over, until the other side spills more and tips to the other side, never quite finding an equilibrium. See the section on
      • Closing the pilotchutes for more information on how to detect and avoid this.
      • A pilotchute throw that is too strong to the side. Note that the force with which you throw your pilotchute is a delicate one. You want to make sure the pilotchute clears a potential burble and has a chance to inflate and do its work. At the same time, a throw that is too strong can contribute to offheadings. When in doubt, err on the side of throwing too hard. It is better to have an offheading canopy than no canopy at all.

    To combat pilotchute oscillations, manufacturers came up with vented pilotchutes that reduce oscillations. These vents let some amount of air spil through the top of the pilotchute, allowing the low pressure point above the pilotchute to be filled. Supposedly this results in pilotchutes that are more stable.

    Practice has shown vented pilotchutes to offer some advantage over non-vented pilotchutes, but the science of it is too fuzzy to come to strong conclusions. Some people prefer non-vented F-111 over vented ZP pilotchutes arguing that the F-111 is permeable over its entire surface, achieving what vents try to do more effectively.

    Bad Luck

    Remedy

    Todo, talk about avoiding, factors (body position, winds, packing, magic), dealing with.
    Turning around “quickly” needs to be qualified, look at three criteria:
    1. Altitude used
    2. Distance covered
    3. Time taken

Submitted by BASEwiki on 2007-06-13 | Last Modified on 2007-07-03

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