The cause of
hard openings is a subject of much speculation. We must start any analysis with
the premise that “if a parachute opens soft one time then it is capable of
opening soft every time”. Assuming that, logic then says in order to repeat
soft openings we must control all variables involved. That is, all of the
variables that go into controlling opening shock must first be identified with
their particular variability.
To begin that
process we must first understand all about opening shock. When we talk about
hard openings we must delineate between types. There are openings which merely
deflate your lungs and there are openings which do serious physical damage to
you and or your gear. We will concentrate on the latter while trying to cover
the complete spectrum. Before we go into the technical part of this article,
something must be said about jumper perception of opening shocks. This is best
illustrated by relating a conversation with an old friend who was doing some
test jumping for another parachute company. He spoke about making a jump one
day with a data logger, to record force, and getting a normal opening shock.
Using the same pack job and again with a data logger made a jump the second day
and it slammed him. The data from both jumps showed the same relative force,
but the jumper felt the second jump was much harder. This is largely due to
preparedness or lack thereof, of the body and mind before deployment. It is due
to the way you feel on a given day and is not what we are looking for in terms
of analysis of potentially destructive openings.
Opening shock can be divided into basic components; “Snatch Force” and
“Inflation Force”. Snatch force is that point in the deployment when the mass
is accelerated to speed. This simply means when everything (lines, canopy, and
bridle) is stretched out with tension and load. When that load occurs you are
at snatch. This is also when the slider is beginning to take air. Hopefully the
slider won’t take air until just after snatch. This can be helped by rubber
banding the apex of the slider to a center B or C line attachment to
momentarily retain it. Inflation force, on the other hand, normally occurs
after snatch and has less force than snatch. It is when the canopy begins
inflation and commences the argument between the resistance of the slider and
the inflation forces spreading the canopy. This usually occurs in steps of
decreasing force.
There are
canopies which have a greater inflation force than snatch force. This is not to
say they occur out of order but that one part of the opening is greater than
the other part. Yes, those types of canopies are known to be hard openers but
do not necessarily produce the catastrophic type of openings we are trying to
define. What is described above is what should happen on a normal opening and does
in fact occur on most deployments. Allowing deviation from this sequence will
most likely cause higher than normal opening force. The one thing we must avoid
is allowing “Inflation” to occur before “Snatch” or to allow them to occur
simultaneously. When that happens, people get hurt. When inflation occurs
before snatch it is call a “canopy first” deployment. That is the way all
parachutes opened in the beginning, then the French invented the “Sleeve” and
created “line first” deployment. The D-bag is a later development replacing the
sleeve. Early squares did use a short sleeve.
Let us now look at the individual components and break down their function and
foibles. Canopy design: Some of the major canopy design variables are slider
size, nose angle, trim angle, cell size and brake setting. If one of them is
incorrect your first jump will be hard and you will know it. When this is the
case then all of the openings will be hard. The only thing you could do, in the
field, is change the slider size or sometimes an adjustment of the deployment
brakes can help. Some people have claimed that the line type has an effect. I
accept the fact that there may be an effect from line type or weave or the
friction of pulling the lines from the rubber bands but I don’t see it in data
acquired during many opening shocks recorded. If there is an effect it must be
minor.
Pilot Chute drag: Large high drag pilot chutes can cause some increase in
snatch. I know of cases where a smaller pilot chute did solve the problem.
However, this again is not the problem we are looking for. We want to know
about catastrophic openings, the kind that destroys. To find those variables
responsible for this phenomenon we must move on, defining functions and
variables.
Deployment
Bag design: The D-bag is the single most important component in this equation.
There is much conversation and disagreement about this subject. Here is my
take. The D-bag is used to prevent the canopy from taking any air or opening
prior to an orderly, properly sequenced line deployment. We use the suspension
lines stowed in rubber bands to keep the bag closed. There are other devices
designed to do the same job and the phenomenon described here is applicable to
all of those devices. If the bag is allowed to open prematurely, exposing the
canopy to inflation you will have a very hard “canopy first” opening. These
openings may vary in intensity depending on exactly when in the process of
deployment the canopy inflation begins. We want inflation to be the last thing
that happens. This malfunction is referred to as line strip/dump or “out of
sequence deployment”.
The lines being released from the bag prematurely and allowing the canopy out
is, in and of itself not the complete cause of these catastrophic openings. The
slider, being allowed to fall down the lines away from the bottom skin allowing
the initial inflation to occur simultaneously or prior to snatch is the cause.
The amount of inflation is dependent on the resistance of the slider and the
geometric size of the bottom of the canopy, at that instant, which is created
by the trapezoid defined by the bottom plate of the canopy, the slider, and the
lines between them. In other words the more the slider is allowed to drop the
more deadly the shock will be as more surface area is exposed. This may be
ameliorated by retaining (rubber banding), the slider. Some manufacturers have
used a snap to hold the slider to the canopy in place of a rubber band. While
either will help in most cases neither is a panacea. These devices are designed
to release upon inflation. If the canopy begins rapid inflation just off your
back it will spread causing the slider to release and fall quickly down the
lines before you reach the end of the lines. You really don’t want your slider
to beat you to the end of the lines.
What causes
the lines to release from the bag allowing the canopy to get out prematurely?
Inertia is the answer. Newton said “Bodies at rest tend to stay at
rest; bodies in motion tend to stay in motion. A body is a unit of mass. Lines
stows are divided into 3 units of mass. They are; the bights, 2 each, and the
span or the part of the lines between the stows or bights. Each of these
components, separated by the stow rubber bands, is a unit of mass within itself.
The mass of the 2 bights must equal the mass of the span. In terms of
percentage the span should make up 50% of the entire line stow from side to
side. We want 25% in each bight so as to equalizes the tendency of the span to
“drop out” or “Dump” or “Strip” or pull the bights out of their stows, upon
extraction from the container, by overcoming their mass. The combined mass of
the bights equalize the downward force of the span portion of the lines as they
try to stay in the pack tray upon bag extraction. If the mass of the span is
greater than the mass of the bights then it will pull the bights out of the
stows upon extraction from the container. If the span is, say 70% of the entire
line stow from side to side that only leaves 30% to be divided by each bight or
15% per side. The 70% in the middle will easily overcome the 15% on each
side.
Some people
double wrap the stow bands to help keep the line bights retained. That is a
“brute force” method of keeping the lines in place and doesn’t always work. I
am sure most of us have seen pictures of the span portion of the lines drooping
in an arc as the bag is extracted. That droop is trying to pull the bights out
of the rubber bands and open your bag. This is a line dump in process. Note the
stretch of the rubber bands and that the canopy has already escaped the locking
stows. Note also how the canopy is “slumped” in the bag. All of this is caused
by inertia. This is a tandem with a collapsed drogue and large rubber bands
which should produce a reduced Snatch. How much force do you think it takes to
stretch those rubber bands? How well do you think this bag would retain the
lines?
Most severe openings occur at high speeds. The higher the Dynamic Pressure or
“Q” the more things drag and the greater the differential of mass inertia. In
order to endure these forces the deployment must be smooth and progress evenly
throughout the process. The spread of your arms in freefall will do more to
change the nature of the opening than any normal pack job. In other words slow
down when you deploy; it will soften the shock. It is important to pack
consistently from jump to jump. It is also important to utilize the same body
position with the same speed if you want repetitive openings. The shoulders
should level to the horizon, with the body rotating from flat to vertical,
through an axis defined by the shoulders, during deployment. This allows for
air flow over the back to sweep away any burble and it positions the jumper,
sitting into the saddle of the harness. Use your arms to take some of the
opening load by grabbing the risers with your hands during inflation. This will
make you ready to take command of your canopy sooner. Be careful not to get
your fingers between the risers, they bite. Never look over your shoulder to
clear a burble. Look straight up over your head and keep your shoulders level.
Uneven shoulders result in uneven line deployment. Looking over your shoulder
usually causes you to drop a shoulder making your body into a propeller. While
your body might not start to rotate, the air above you does, and it spins your
bag causing line twists.
In conclusion I would like to mention the new trend in D-bags which is to
reduce or eliminate the stows. I have seen some that look like they might be OK
and I have seen some that make me run away. The most telling comment I have
heard about them is that they don’t work so well on tandems. If you understand
all I have said previously then you know why.