Skydiving – Turbulence, Not Your Friend

There are two basic types of air flow, laminar and turbulent. Despite the fancy names, the ideas are very simple. Picture the smoke rising off the tip of a cigarette. Near the hot end the smoke flows smoothly upward. At a certain point, it starts curling and swirling. That’s turbulent flow.

Why should that little bit of fluid physics matter to skydivers? Because turbulent flow is one of the major contributors to certain high risk circumstances. Turbulence can turn your body in air unexpectedly and it can cause your canopy to misbehave.

That can lead to fatalities. More than one skydiver has had his main collapse because of turbulence when he was too low for the reserve to inflate fully before impact. Even when safely on the ground turbulent air flow can knock you off your feet before you detach the harness. Laminar flow that’s strong enough can too, but you are then being pushed in one direction. Turbulence tends to twist you, making you more unstable and more likely to fall.

In fact, turbulence is more likely closer to the ground in most cases. Because of boulders and other surface features, the air gets stirred up like water moving over rocks. But unlike water, turbulent air is invisible.

The possibility of turbulence is one reason skydivers always check the weather carefully before going up. Some conditions are more likely to result in significant turbulence than others. ‘Significant’ is an important qualifier because there is always a little bit of turbulent flow around.

Wind speed is one major factor. As the wind blows faster it’s much more likely to become turbulent, other things being equal. That’s why jet plane wings always experience turbulent flow in flight. As the air flows past the wing, it gets stirred up at the trailing edge. A skydiver’s body acts somewhat the same (though only approximately).

Other factors exist, too.

Temperature matters. Hot air is more likely to be turbulent than cool air, but what really matters is the temperature difference. When hot air meets cooler air, the chances for turbulence are increased. Pressure differences between one air mass and another contribute to the possibility of significant turbulence.

As a skydiver approaches the ground the problem of turbulence is increased. One reason is the obvious reduced time and distance for reacting to a problem. But the conditions themselves change as well. Any buildings near the landing point, for example, can generate turbulence like the rocks in the river.

Turbulence around buildings is a familiar phenomenon to anyone who has lived in a big city. But tall buildings also create turbulence at intervals above them, sometimes as much as ten times their height.

The size of this so-called ‘turbulent zone’ varies depending on lots of factors. But the mere fact that it can occur at heights of ten times the building means that a 20 foot building can cause turbulence 200 feet up. That’s far too low to deploy a reserve if your main canopy collapses. Yet, a 200 foot fall to any ground hard enough to support a building is extremely likely to be fatal.

Skydiving is a very safe activity, relatively speaking. The odds of a fatality are about 1 in 100,000. Knowing something about how, when and where turbulence is created can help you maximize your odds of a safe landing.

Skydiving – Tracking and Gliding

Tracking is a skydiving skill emphasized very early in training. It involves positioning the body to produce horizontal movement.

Usually, a skydiver begins by assuming the classic skydiving position commonly seen in films: face down, belly to Earth, slightly arched, arms and legs spread. That provides a free fall rate of approximately 120 mph (193 kph). To begin tracking, the skydiver changes position. The back is straightened, the arms are drawn in, the legs are brought closer together.

Depending on the air conditions and exact body weight, clothing, position assumed and more a number varies called the glide ratio. That’s the ratio of fall to horizontal drift. Skilled skydivers can achieve close to a 1:1 ratio. They cover a significant horizontal distance as they fall from, say, 10,000 feet (3,050 m) to where the canopy is deployed at about 2,500 feet (762 m).

Once the canopy is deployed, of course, the situation takes on a whole new character. Gliding now takes place.

Because of the type of material, and particularly because of the large surface area, shape and thinness of a modern nylon ram canopy, it’s possible to considerably extend the techniques.

The ram canopy is sewn into cells, strips of material that form individual curved sections across the large rectangle of the chute. With toggles that brake and handles that allow the rear left and right cells to be manipulated, it’s possible to glide for miles, often achieving speeds of 50-100 mph (80-161 kph).

It’s possible in this way to hit a drop zone target miles away. At the same time, gliding and braking can be so precisely done today that expert target jumpers regularly land within two inches of a central point.

Both tracking and gliding, because it can involve high speeds, may be dangerous. Students learn how to perform it safely through classroom instruction and practical experience in the air. Blacking out in the air or becoming disoriented is possible. Hitting the ground even softly at a horizontal speed of 50 mph (80 kph) would be fatal. Even 15 mph (24 kph) is about twice the speed of the fastest sprint.

To avoid injury, it’s important to incorporate in gliding and tracking practice techniques that allow the skydiver to slow his horizontal movement as he nears the ground. That typically starts at about 1,000 feet (305 m). At that level, the skydiver shifts his focus to spotting the dropzone target, reducing horizontal speed, and preparing for landing.

Also, jumpers rarely skydive in an area totally devoid of other jumpers. Part of the sport often involves cooperation with other skydivers. Here, that means practicing tracking and gliding when others are present. Awareness of their position and distance away is important at all times.

In some cases, skydivers will want to hook up. At others, they’ll want to ensure they have plenty of horizontal separation. That’s needed in order to deploy the canopy safely, to avoid one getting entangled with another leading to collapse. The more divers there are in a given area, the more critical that becomes.

Safety and excitement in skydiving are always in a delicate balance. Practicing tracking and gliding the right way can help achieve that proper mix.

Skydiving – The Parts of a Parachute

A modern parachute is a complex piece of equipment – and lucky for the skydiver that it is, too. All that complexity is there to serve a purpose: to bring skydivers to Earth safely while providing a quality experience in the air.

The most obvious part of a parachute is the canopy, so let’s start there.

Even here there’s more than meets the novice eye. Contemporary canopies are made of nylon in the shape of roughly a large rectangle. That much is obvious. But what the untrained eye may not notice at first glance is that within that large rectangle are many parts.

The entire canopy is made of cells, which are ribbed sections formed by gathering the nylon into long, curved strips with sturdy thread. The outer edges are not simply plain flat cloth, either. They curve inward gently in order to provide two functions.

One is to scoop an ample amount of air into the interior, to provide for good air resistance on the descent. That need is obvious. But it’s also true that too much air, gathered too quickly can be (or rather, used to be) a problem.

That’s why canopies from years ago used to have a hole in the top to let some air escape. Without that, it’s more likely that the lines would get fouled and the canopy collapse. What followed next is easy to predict. With modern materials and geometry that’s now extremely unlikely. Stabilizers at each side left and right, attached to the end cells, add still more safety and function to modern parachutes.

Now, on to the other components.

Nylon lines are attached to the canopy at regular intervals that not only attach the skydiver to the drag-producing canopy, but keep the chute and diver stable. To help keep them from getting tangled when the canopy is deployed a number of techniques and devices are employed.

Packing technique is critical in allowing the lines to draw out and deploy correctly. Certified riggers are well trained in that technique. Their job is made easier by a small device called a slider. The lines are threaded through this rectangular piece of nylon in such a way that as the canopy opens, it opens at the right rate. That prevents the gear from getting torn and the skydiver from being rudely jerked from 120 mph (193 kph) down to 10 mph (16 kph).

In order to deploy that canopy effectively a small pilot chute called a drogue is used. When the skydiver pulls the release, he’s pulling out this drogue. It fills with air and tugs on the lines attached to it and the canopy. That, in turn pulls the main out of something called the D-bag by means of bridle lines.

That D-bag is stuffed inside a larger bag called the container which holds it, the drogue, and all the grommets, straps that are needed to keep the whole assembly together. The container has straps, both leg straps and chest strap, that are attached to the skydiver for a comfortable and safe fit.

Once deployed, the skydiver can make use of front and rear risers and a steering toggle or brake in order to control the direction and, to an extent, the rate of descent.

Safely down, all parts intact. Excellent. Thank you, modern parachute designers.

Skydiving – The High Flying Sport

Skydiving or parachuting brings to mind familiar images from films and TV commercials. Who hasn’t seen one or a hundred people floating through the air, wind flapping against their jumpsuits, followed by a swoosh and a colorful canopy blooming?

The sport or hobby involves a myriad of skills needed to make that activity as safe as it has become. That skill is acquired slowly or quickly depending on the type of training involved.

Static line training, for example, takes a student through weeks or months of instruction and exercises to reach the point of solo freefalling. AFF (accelerated free falling), on the other hand, may take only days or a little longer to accomplish the same task.

In static line, the student jumps out of the plane with a line pre-attached to the container which, as the name suggests contains the parachute or canopy. Exiting at about 3,500 feet above the ground, the line opens the chute and the student’s rate of descent drops from about 120 mph (193 kph) to around 10 mph (16 kph). They’re often part of a group of a dozen or more students guided by several instructors.

In AFF training, students receive much more one-on-one training. A ground school that takes 6 hours gets them up to speed on the basics of simple physics, equipment and other things they need to know. That happens in Static training too. Then, they will have an instructor accompany them on a jump.

In standard training, a tandem jump may be the first one or two jumps. The student simply holds onto the front of the instructor (though they do have their own pack) and the instructor will deploy his or her canopy, bringing both gently down. In AFF training, the teacher only holds the hand of the student until they are assured the student can safely deploy their chute.

What happens next is very much the same in either case.

At around 2,500-3,000 feet the skydiver pulls a small handle to deploy a drogue chute. This is called a BOC or bottom of container arrangement. The drogue is a small chute to which is attached cords that are in turn connected to the main canopy. Rapidly moving air catches inside the drogue as it exits the container. That pulls the lines out, which pull the main canopy free of the part of the container called the D-bag.

The main canopy begins to fill with air. But it’s important to prevent it from doing so too quickly. That would decelerate the skydiver from 120 mph (193 kph) to 10 mph (16 kph) too quick and possibly tear the gear not to mention injuring the skydiver. So, a small nylon fabric slider gradually slides down the lines, allowing the canopy to open more slowly. That provides a gentler deceleration.

As the skydiver descends to Earth, he or she moves into a horizontal glide and walks along as easily as they would trot across a meadow. Only in special circumstances will they do the pound and roll popular in old war movies.

Getting down safely isn’t guaranteed, but appearances aside, skydiving is one of the safest activities around. At the same time, the adrenaline of flying through the air at 120 mph (193 kph), then gliding gently to the ground, provides the thrill that keeps skydivers coming back for more.

Skydiving – Tandem Skydiving

There are several ways to get initiated into the exciting activity of skydiving. One of the most common is something called a Tandem Jump.

As the term suggests, this is a technique in which a pair of skydivers exit the plane and descend together. The student is at all times attached by a harness to a more experienced skydiver, the tandem instructor. The instructor completely controls the jump, the free fall and the canopy release. He or she will pilot the pair horizontally and vertically through the entire jump. The student is, so to speak, just along for the ride.

But that passive position is the perfect beginning for some. It allows the novice to get acquainted with skydiving with minimal risk, minimal anxiety and minimal training. Other jump programs require several hours of ground instruction.

That instruction includes discussion on skydiving physics, parachute construction and behavior, and much else. It’s a lot to absorb, but the information is vital to having a safe, yet still enjoyable first-time skydive.

Tandem jumping, by contrast, gets students into the air more quickly. Though there’s still often some amount of training on the ground. Since the tandem instructor and student are tied together, there’s a certain amount of cooperation needed to exit the plane safely and descend properly.

During that discussion, instructors will outline how the jump will proceed.

When the pair exit the plane, the tandem instructor immediately deploys a small chute called a drogue. Typically about 4 feet wide (1.2m), it helps slow the descent of the pair to normal free fall speed, about 120 mph (193 kph). Without it, the combined weight of student and instructor would result in terminal velocity of about 200 mph (321 kph) or higher. That’s much faster than a first time skydiver should experience. It also would introduce a safety factor when deploying the main canopy.

At the appropriate elevation above ground level (about 3,000 feet – 914 m), the tandem instructor pulls a cord that allows the drogue to operate as in normal jumps. It pulls the canopy out of the D-bag, inside the container. That canopy is larger in the case of tandem jumps. It’s designed to support the weight of two men with a large safety factor.

The pair then descend at the normal rate, about 10 mph (16 kph), until they touch down. The instructor then releases the student from the harness and he or she throws up. Yes, it sounds odd, but many first time jumpers will vomit from the adrenaline of their first jump. Not always, but it’s not uncommon either. Even more oddly, they’re often eager to jump again right way. That’s the strength of attraction of skydiving.

Tandem jumps are often less expensive than Static Line or AFF (accelerated free fall) training. But most who gain an interest will quickly graduate to one or the other. A few jumps later, the student is going solo in one of the most thrilling activities possible: skydiving.

Skydiving – Skydiving Techniques

Beyond the classic face down, belly-toward-Earth flying position, there are many skydiving techniques that it’s great to know. The more you know, the more fun you can have for those few precious seconds during freefall.

Back flying is one of the earlier ones you might want to explore and practice. One of the best ways to do that is by using a wind tunnel at your local skydiving school. Unlike conventional wind tunnels used for testing jets and cars, the skydiving wind tunnel points vertically. It’s used to provide artificial lift to simulate freefall conditions.

In back flying, you are (as the name suggests) on your back. In the wind tunnel you can’t move very far horizontally because the room is relatively small. But you can practice the starting position, maintaining the position and moving out of it all in a very safe environment. Even if the wind failed completely (which is extremely rare), you’re only a few feet off the ground and won’t endure much of a thud when you hit.

You can practice the correct amount of arch and exercise turning. You’ll get used to the sensation of wind on your back. That helps judge what it feels like when it isn’t there. Put your hands above your head and let the wind carry you up. The head should remain inline with the torso, looking straight up. Assume the position you would when sitting in a chair and keep the legs wide to create a stable position.

The third basic position, beyond belly flat and back flying, is the headdown position. Here, the body is straight, all in one plane. Keep tension in the butt in order to keep the hips from rotating away from that single plane. Tense the thighs to maintain that same uncurved position.

The head is straight down toward the Earth, with the foot flexed up, making a right angle with the legs. This isn’t usually achievable in the wind tunnel because of the force required to keep you from falling down. Your head, shoulders and feet don’t present a large enough surface area to create large lift. You should feel a force on your feet like a big dog was sitting on them. Flex your toes upward to keep the tension on the feet and calves.

Now you can practice (in the air) moving from one position to the other. From a belly flat to a back fly to a headdown to a belly flat. Running through this routine provides practice in control of your body in the air. That builds confidence and can reduce any sense of panic when the wind suddenly flips you out of position.

They’re also great fun! When you’re on your back carrying a video camera you can get some great shots of other skydivers above and around you. You can perform exercises hooking up with other skydivers. And you can get the most out of the exciting experience called skydiving.

Skydiving – Skydiving Gear

Skydiving looks so simple. You open the canopy, you drift down. What could be simpler? In fact, there is an amazing variety of gear used in even the most elementary jumps.

The parachute is the most obvious, of course. But even here we find lots of complexity.

In the average rig there’s the container itself, which holds several straps. The straps can be attached in various ways and the specifics of how they’re arranged affect the experience. Individual tastes vary.

Inside the container there are several component parts. The main canopy is housed in a unit called a D-bag and attached via lines to a smaller pilot chute called a drogue. Alongside there is a reserve chute that can be deployed in case the main fails. Once deployed there are hand toggles used to steer and brake.

On the way down, it’s always helpful to know how far Above Ground Level (AGL) you are and, sometimes, how fast you’re descending. It can be useful to know which direction you’re going and the speed and direction in which the wind is blowing.

AGL is measured by a familiar device called an altimeter. Most are calibrated automatically on the ground. Other things being equal, the higher up you are the lower the air pressure. The altimeter senses these pressure levels and calculates the elevation accordingly. Most are digital today and some contain radio transceivers.

Since altimeters don’t measure the height above the ground directly (say by bouncing a laser beam off the Earth and back to a receiver, one technique used to measure the distance to the Moon), they can be off by several meters. Most are only approximate.

Another important, and now standard piece of gear, is the AAD or automatic activation device. A very sophisticated hand held computer, the device has a projectile that’s used to automatically cut a strap to deploy the skydiver’s parachute if he or she fails to do so by the appropriate height.

Measuring elevation, descent rate and other factors, the AAD is designed with elaborate algorithms to prevent deploying a canopy that is already open. That can happen for example, if the main is correctly deployed but the AAD misjudges the situation and deploys the reserve accidentally. Today, such events are very rare, but they do happen.

In normal circumstances, a novice or even advanced skydiver may black out, get tangled or for some other reason be unable to activate their parachute. The AAD then becomes a lifesaver and has been so more than once.

Other gear is common and more important than appears at first glance.

A helmet is often used to protect against wind and possible head trauma when hitting the ground. Unlike old movies, it’s very rare for a skydiver to have to hit the ground and roll. But it’s possible for a gust of wind, a trip or other event to put a skydiver’s head on the ground. At such times, a helmet is a must. Two-way radios are often incorporated to allow in-the-air and air-to-ground communication.

Skydiving suits are common and they come in a variety of styles. They differ in how and where straps are attached, what type of flight and speed characteristics their design results in and a wide array of other factors.

Gear up and get ready for a great adventure.

Skydiving – Reserve Deployment

No one wants to have to deploy their reserve chute. And, fortunately, it’s rare to have to do so. But hang around a busy dropzone for a few days and you’re almost certain to see at least one instance. Far from the dire emergency it may sound, reserve deployment is a normal part of skydiving, even though it’s infrequent.

A main canopy can fail to deploy properly, collapse or encounter other problems.

Lines can get tangled to the point that the slider doesn’t slide down properly, leaving the canopy in an irregular shape. Turbulence or inappropriate use of the controls can cause a drop in air pressure or low tension on the lines, leading to an unstable main. When that instability or collapse is unrecoverable, deploying the reserve is a must.

In rare cases, a canopy will rip. A small tear is minor. A modern ram canopy is composed of many curved cells that hold air under pressure. Losing part of one doesn’t affect the rate of descent much. At a certain point, however, it’s wise to let it go and deploy the reserve.

When the reserve has to be deployed, a procedure called a cut-away is performed. The main is released, disconnecting it from the harness. That’s done using the cut-away handle (mounted on the harness), which detaches both main risers. It flies away, to be picked up later. One hopes! Canopies run around $1,500. A second later, the reserve is deployed.

In some cases, performing a cut-away on the main automatically deploys the reserve. In some designs, a reserve static line is attached that pulls the reserve out as the main flies away. In others, the skydiver pulls a second handle to deploy the reserve manually under controlled conditions.

That second method can be desirable if there’s still plenty of air and the skydiver wants to reposition him or herself to avoid an obstacle. Turbulence can be caused near mountains, for example, and it may be good to track away before deploying the reserve.

Beyond mechanical problems, whether natural or man-made, skydivers can panic especially when inexperienced. They can black out. That can produce a situation in which a canopy has to be deployed automatically. That’s the job of the AAD (automatic activation device).

The AAD is a compact computer that automatically senses elevation above ground level, air speed and other variables – not least of which is whether the main canopy has already been deployed. When the skydiver reaches 750ft – 229m, it will activate a projectile called a cutter that cuts a strap and causes the canopy to be deployed automatically. That device can be used on the main or the reserve.

Deploying a reserve is often a pain. The main has to be recovered. Gear attached to the reserve that flies away when it’s deployed has to be retrieved. A certified rigger may have to inspect the chute, leading to delays in going back up again and extra charges. At the very least, the skydiver will often miss the dropzone and have an unpleasant walk ahead.

But everyone is glad the reserve is there and is happy to use it when necessary. The alternative is just so much worse.

Skydiving – Instant Opening Static and IAD

Tandem and AFF (accelerated free fall) skydiving methods get a lot of attention. They are, it’s true, two very common ways of instructing students in the art of falling out of a plane with a parachute. But there’s a traditional method that is still very frequently employed – Instant Opening.

Instant Opening skydiving comes in two basic types that accomplish the same purpose: Static Line and Instructor Assisted Deployment. In either case the result is the same: the student has his or her parachute deployed without action on their part.

In the second case, as the student exits the plane, an instructor ‘pulls the ripcord’. That extracts a small pilot chute called a drogue, which fills with air and extracts the main canopy a few seconds later. That delay provides some minimal level of near free fall.

The Static Line method, by contrast, is familiar to anyone who has seen an old WWII movie. The skydiver or the instructor attaches a mechanism (such as a simple clip) to a line or hook inside the plane. As the student leaves the plane, the attached line pulls the parachute open automatically.

Jumps made by either method will often be done from about 3,500-4,000 feet (1,067-1,220m), though in some cases it’s higher. Usually, those higher elevations will occur as the student progresses through more jumps. One reason for the technique being used in the military, where it was invented, is that it allows for rapid chute deployment to accommodate low level jumps, though.

As the skydiver descends, they’ll typically be in two-way radio contact with an instructor on the ground who will provide directions on landing.

Novice skydivers may want to select Static or IAD for any number of reasons.

Ground instruction in AFF can take nearly all day, with class lasting 6-8 hours. With Static or IAD, that time is often shaved down to as little as four hours. Since the chute is deployed for the student there’s less material to cover on safety, what actions the skydiver has to take to deploy the chute and more. It’s still necessary to cover many things, such as how to deploy the reserve canopy if needed, landing and many other topics. So instruction in Instant Opening still takes a few hours.

The alternative to Static or IAD may be a Tandem jump. But in that method, the student is strapped to an instructor by a harness. The instructor deploys the chute and controls every aspect of the skydive. That leaves the student with little to worry about, but also little to do. For some, that brings peace of mind, while still enjoying the thrill of a dive. For others, it reduces the value of the experience. Instruction required is minimal.

AFF is an often chosen alternative. Here the student receives a lot of ground instruction, but that’s followed by minimal hand holding (literally and figuratively) by the instructor. In some schools, about 8 jumps are required before the student graduates to solo free fall. With Static or IAD, the free fall is minimal but the skydive is solo from the very first jump.

Prices and the experiences vary for every kind of jump. Each individual is different and will choose according to his or her own personality and the goals they bring to the exciting activity of skydiving.

But Static Line or IAD has a big advantage. You get up in the air quickly and you’re on your own from the minute you exit the plane.

Skydiving – How To Deploy

In broad outline, how a modern ram canopy parachute deploys is pretty much the same, barring emergencies. The skydiver extracts a pilot chute that fills with air and tugs on attached lines. Those lines are also attached to the main canopy that is pulled from the D-bag in the backpack. It in turn fills with air. Simple.

But within that broad outline many variations are possible. Those details, heavily dependent on the skydiver himself, can influence how the canopy behaves. That behavior has a big effect on the skydiver himself.

That behavior starts before you ever get in the air. How the chute is packed plays a role. That’s why it’s advisable as students move from first-timer to intermediate that they should learn to pack their own chutes. The more you know, the more you can connect how one thing influences another. Hands-on experience is one of the best teachers.

Once in the air there are many new factors that come into play. Height is one, since air pressure and temperature, wind speed and direction, and others are different at different elevations.

But usually even more important is how the skydiver uses his or her own body. Body orientation and configuration has a huge effect on the terminal velocity. Altering your body position can change your terminal velocity from 120 mph to 150 mph (193 kph to 241 kph) very easily. The speed you’re traveling when you deploy greatly affects the forces on you at release. That has a large effect on whether canopy release is a gentle and gradual slow down or a very rude jerk. Orientation also affects whether the forces at work are distributed across the body or concentrated in one area.

Canopy design helps minimize those effects. Under a wide range of circumstances a canopy will deploy slowly no matter what you do. That leads to a more gradual deceleration from the initial terminal velocity to the new one, say from 140 mph (225 kph) to 10 mph (16 kph) within five seconds.

But canopy design can only do so much.

If you pack your rig in a certain way, extra airspeed can produce an opening that results in a painful yank as you decelerate. A better packing results in a smoother, gentler decrease.

Where you look when you start to deploy affects your body configuration and position, which in turn affects how your canopy and harness produce forces on you. Looking over your shoulder, for example, almost invariably tilts your shoulders. But a flat body surface toward the Earth produces a different result than one in which your body is angled, providing less wind resistance.

If you look down at the ground as your canopy deploys, your body tends to tilt forward. That increases your airspeed and produces a smaller angle with the ground. That increased airspeed can lead to a bigger jerk, as discussed above. And the angle produces a larger rotation of your body as the flying canopy moves you into the sitting position. That angle and rotation causes more leg swing and a higher force on your shoulders.

Looking toward the horizon instead, tends to keep your head from moving down. That leads to a more even distribution of the force of opening. Instead of being concentrated at the shoulders, it’s distributed across the entire harness. That produces a smoother, gentler, more gradual opening with less swing.

Think about your speed and body position. Practice proper orientation. You’ll have a safer, more comfortable deployment.