Emergency parachutes and safety
By John Holstein
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Despite all manufacturers of parachutes are claiming that it is dangerous
and life threatening to make a parachute jump, I believe that most pilots
feel a little more confident and safer wearing a parachute on their back.
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You can tell from history that some pilots have been forced to leave the
aircraft in the air and due to the parachute, they are alive and can tell
their story.
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In theory all soaring pilots are taught how to loosen the seat belts, how
to jettison the canopy, how to bail out as quickly as possible and how to
pull the parachute handle. But what happens after this is rarely described.
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Like the aircraft, the parachute is dependent on the aerodynamical rules to function properly.
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A parachute consists of 3 main items.
1. Harness with container, 2. Canopy, 3. Pilot chute.
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Most emergency parachutes are equipped with a round canopy. Like different types of aircraft, that can glide faster or further due to the wing profile, also parachutes are different, and all parachutes are unfortunately not functioning equal effectively.
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When a pilot for some reason decides to leave the aircraft in the air, it is seldom at high altitude nor with a lot of speed. Usually, it is close to the ground and at low speed. Therefore, it is always the time that is the crucial enemy. Obviously, it is very important that the parachute will open as quickly as possible. You cannot afford to waste 3, 4 or 5 seconds waiting for the canopy to open. The speed through the surrounding air is of paramount importance for the speed of the opening.
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After acceleration for a couple of seconds, a human body, in altitudes below 1000m, will reach a falling speed of approximately 60m per second. If the pilot chooses to leave the aircraft at the top of a wire start at app. 300m the time is very limited!
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For this reason, it really matters what canopy is inside the container. It must be a canopy that utilizes the aerodynamical forces as optimal as possible.
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The type of canopy fabric has the big word to have said. Not all parachute canopies offer full security to the user. Several factors come into play here, but most important of all is the permeability of the fabric. You can call it porosity. There is a value for how much air is able to penetrate the fabric under a certain load. This is measured in CFM, Cubic feet per square Feet per Minute, but you do not have to know this formula here.
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What we must know is which influence varying CFM has to the opening speed of the parachute.
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When the pilot bail out from the aircraft he or she has the same speed as the aircraft and will continue in the same direction as the aircraft for a short second before the vertical acceleration commence.
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When pulling the opening handle, the little spring-loaded pilot chute is released. It jumps away from the body and catches air immediately. With the bridle cord the pilot chute will pull out the top of the canopy. Then the whole canopy and the suspension lines in reverse order of how it is loaded into the container. See figure 1.
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When the whole system is stretched and the lines have opened the small safety diaper, which sits at the bottom of modern canopies, air begins to flow into the canopy. Due to the speed, more air flows into the canopy than can reach out through the top vent hole. This air collects inside the top of the canopy and flushes it out so that the upper end of the canopy forms a balloon or bulb dome. See figure 2.
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As the air collects in the top of the canopy, the dome becomes larger and larger, fig. 3, and the inside pressure becomes greater. By the whole systems way down through the ambient air,
the air on the outside must travel a longer and longer distance to get around
the onion dome. Thereby a negative pressure is created on the outside of
the canopy, which pulls the fabric apart. Suddenly, the opening at the bottom
becomes so large that the entire canopy fills with air and stops. Hereby all
the relative air that has been moved, is forced back on top of the canopy.
FIG. 4. Now the system is settling, and the canopy is filled with air. FIG. 5.
The principle is shown at the subsequent images.
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The entire process last shorter or longer time according to the porosity or
permeability of the fabric. Of course, if the canopy is very porous, a lot of air
can penetrate the fabric during the filling phase. Fig. 2+3.
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The more air that that is forced out through the fabric, the longer it takes
before the canopy becomes load bearing. In other words, a canopy with a
porosity of 200 CFM or more takes eerily much longer time to open than a
canopy with a porosity of 0 CFM.
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Therefore, it is of paramount importance to have all the old High-porous canopies replaced with new modern 0-porosity canopies.
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Another thing that porosity affects is the sink rate. It is obvious that a canopy with low or zero porosity is descending much slower than a canopy with high porosity. But this parameter is not so important.
The most important thing is the opening speed because the time is your worst enemy.
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Unfortunately, in Denmark we have no lifetime limit for emergency parachutes for pilots.
The authorities have left it to the interest groups to make rules for the use of parachutes in Denmark.
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Traditionally, it has been riggers and equipment experts from the Danish Parachute Association who have dealt with the approval of parachutes in Denmark.
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Therefore, as parachute technology became more and more advanced and round canopies were no longer used for sport parachuting, the number of materiel experts dealing with round parachutes became fewer and fewer.
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Especially since the turn of the millennium, parachute technology has advanced by leaps and bounds. The Danish Parachute Association has taken the consequence of this and introduced lifetime restrictions for sport parachutes. This is 20 and 25 years, respectively, depending on the type of gear.
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Unfortunately, emergency parachutes for pilots are not covered by these rules. It is really a pity because it is often the oldest canopies that have the highest porosity.
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In our close neighboring countries, the lifespan of emergency parachutes is 20 years. This means that used parachutes with an age of 18-19 years often end up in Denmark. When a German parachute has a few years of life left, it still has a certain commercial value for the owner if he sells it to a Danish pilot. therefore, old emergency parachutes continue to appear in Denmark. Many of them with high porosity of 200 CFM or more.
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Since the harness and container do not suffer quite as much from the same obsolescence symptoms as canopies, it can often pay to replace the old canopy with a new one in the existing harness.
