BASE jumping grew out of skydiving. Although both share certain similarities, there are three main technical differences between the two. First, BASE jumps are generally made from much lower altitudes than skydives. Second, a BASE jump takes place in close proximity to the cliff or tower which provides the jump platform. Third, the BASE jumper generally has a lower airspeed than a skydiver throughout the jump, because a BASE jump starts with zero airspeed, and due to the limited altitude, a BASE jumper very seldom approaches the terminal velocity (airspeed) of a skydiver. All three factors have significant implications.

The BASE parachute system is designed to open very quickly at low airspeeds. Second, the cliff or tower presents a risk to the BASE jumper if, for example, the parachute opens facing backwards. An off-heading opening is not considered a problem in skydiving, but has caused fatal impact injuries in BASE jumping. Off heading opening resulting in object strike is the leading cause of serious injury and death in BASE jumping.

An experienced skydiver is recommended to deploy their parachute no lower than 2,000 feet (610 m). At that time, if they have already been in free-fall for at least 1,000 feet (305 m), the jumper is traveling 120 miles per hour (54 m/s), and is 11 seconds from the ground. Most BASE jumps are made from less than 2,000 feet (610 m). For example, a BASE jump from a 500 foot (152 m) object is about 5.6 seconds from the ground if the jumper remains in freefall. On such a jump, the parachute must open at about half the airspeed of the skydiver, and more quickly (ie. in a shorter distance fallen). Standard skydiving parachute systems are not designed for this situation. Many BASE jumpers use specially designed harnesses and parachute containers, with extra large pilot chutes, and jump with only one parachute - since, with a total freefall time of 5.6 seconds, there would be no time to use a reserve parachute. In these systems, the actual parachute canopy should also be specifically manufactured for BASE jumping, however skydiving parachutes with some modifications (primarily the addition of a tail pocket for stowing suspension lines) are occasionally used by those unable to purchase appropriate equipment. The rest of the system is almost always specifically designed for BASE use. Standard skydiving equipment can only be used on relatively high BASE jumps. If modified, by removing the bag and slider, stowing the lines in a tail pocket, and fitting a large pilot chute, standard skydiving gear can be used for lower BASE jumps, but is then prone to kinds of malfunction which are rare in normal skydiving (such as "line-overs" and broken lines).

The vast majority of people who try BASE jumping are those that have already learned to skydive. It is important to know how to safely fly and land a parachute, and this is best learned on airplane skydives, from higher deployment altitudes, over large fields that provide room for error in learning how to land. Most BASE jumping venues have very small areas in which to land. A beginner skydiver, after parachute deployment, may have 3 minutes or more of a parachute ride to the ground. A BASE jump from 500 feet (152 m) will have a parachute ride of about 10 to 15 seconds.

One way to make a parachute open very quickly is to use a static line or direct bag. These devices form an attachment between the parachute and the jump platform, which stretches out the parachute and suspension lines as the jumper falls, before separating and allowing the parachute to inflate. This method enables the very lowest jumps (below 200 ft) to be made, although most BASE jumpers are more motivated to make higher jumps involving free fall.

In parachuting, height is safety, and by making lower altitude jumps, BASE jumpers give up the safety margins built into skydiving (such as the option of using a reserve parachute if there are problems deploying the main chute). The lower airspeed of a BASE jump is also a risk factor. Skydivers use the air flow to stabilise their position, allowing the parachute to deploy cleanly. BASE jumpers, falling at lower speeds, have less aerodynamic control, and may tumble. The attitude of the body at the moment of jumping determines the stability of flight in the first few seconds, before sufficient airspeed has built up to enable aerodynamic stability. On low BASE jumps, parachute deployment takes place during this early phase of flight, so if a poor "exit" leads into a tumble, the jumper may not be able to correct this before the opening. If the parachute is deployed while the jumper is tumbling, there is a high risk of entanglement or malfunction. Beginner BASE jumpers often make the error of rotating forwards by jumping with a swimming-pool type of diving motion, leading to an involuntary forward loop. Better technique is to exit without any rotational motion.

On higher BASE jumps, those which allow a free fall of five seconds or more, it may be necessary to use freefall tracking technique to move away from the jump object (especially on cliff jumps). Jump platforms providing an overhang, such as arch bridges or naturally overhung cliffs, are more forgiving in this respect and so are more suitable for beginner BASE jumpers.

Via: WIkipedia
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