An Inconvenient Truth Regarding PLF'sBy Jack Guthrie on 2010-03-02
When I started skydiving round parachutes were the only parachutes available. The landing under a round canopy had a high straight down component. Although Para Commanders had a noticeable forward speed under no wind conditions, landing in winds over 10 mph were again straight down. Due to straight down landings most injuries were to the ankles because one tended to sit down on the ankles during a less than perfect timing of the landing techniques required by round parachutes. Use of the Parachute Landing Fall (PLF) greatly reduced the extent and severity of landing injuries. Virtually all successful skydivers were experts at PLFs because not every landing was executed perfectly, no matter how many jumps one had, and sooner or later one had to land a round reserve. In the classroom it required a block of at least two hours to teach landing the parachute because so much of the time was devoted to teaching an actual PLF. The complete PLF technique is not a natural, readily apparent procedure. During the actual jump about half of the students would perform a PLF and the other half wouldn’t no matter how much time and practice was spent on PLFs. The injury rate was quite high. About 1 in 20 first jump students would suffer some type of ankle injury.
With the advent of square parachutes, and particularly with the advent of placing students under square parachutes, the landing injuries changed. Each experienced person reading this article will realize that the injuries from landings that we see today are not limited to ankles. This is because the parachute is moving the jumper across the ground in almost all circumstances. However, there are cases were a person would be descending straight down even today. A high or rapid or deep flare by a student would be one example. In this case the PLF position for landing would definitely influence the ability of the student to walk away from their mistake.
In my opinion there are instances when use of a PLF for a botched landing is NOT beneficial to the jumper. A downwind landing is such an instance. The PLF introduces a roll and a square parachute introduces a large amount of speed. For the sake of this example, assume the wind is blowing at ten mph. The jumper has made a mistake and is landing downwind. He/she has the presence of mind to have the toggles at his/her ribs (half flight) for an air speed of ten mph and a ground speed of 20 mph. Please bear with me for the easy math.
20 mph = 20 mph X 5280 ft/hr = 105,600 ft/hr 105,600 ft/hr = 105,600 ft/hr divided by 60 minutes/hour = 1,760 ft/min 1,760 ft/min = 1,760 ft/min divided by 60 sec/min = 29.33 ft/second
At 20 mph ground speed a person is moving 29.33 ft per second across the ground. At half flight the downward speed (3 to 5 mph) would be somewhere between 4.4 ft/sec and 7.33 ft/sec. I believe that in this example the best outcome for the jumper would be to try to absorb as much of the initial downward velocity as possible with their legs (knees bent and pressed tightly together) and then NOT ATTEMPT A PLF.
The danger inherent to a high speed roll/tumble is a direct result of the laws of physics. An example using a cylinder will illustrate the principle. I am 52 inches around the shoulders so let’s talk about a perfectly round cylinder, 52 inches in circumference, moving in the air just above the ground and oriented perpendicular to the line of flight.
At touchdown the cylinder is going to pick up a rotational (angular) velocity based upon the speed across the ground (linear velocity) divided by the circumference of the cylinder. Excluding friction, which will slow the cylinder by scraping, the cylinder will initially roll across the ground at 29.33 ft/sec divided by 52” (circumference) per rotation.
52 inches divided by 12 inches per foot = 4.33 feet (circumference) 29.33 ft/sec divided by 4.33 ft / rotation = 6.77 ROTATIONS PER SECOND!!!
If this seems quite a lot, it is. However, this is why occupants of a rolling vehicle are thrown so far from the vehicle in a rollover car accident (buckle up?). In the rollover case the rotational speed (angular velocity) of the vehicle rolling is translated to the linear velocity of the thrown body. The thrown objects, including unbuckled occupants, are thrown hundreds of feet.
Returning to the skydiver, I am not a perfect cylinder. I am more of an imperfect ellipse (oval) seen from above. This is not to my benefit in a rolling landing as the ground (not perfectly smooth) will have a tendency to turn me into a round object by attempting to break off my protrusions (shoulders, arms, knees, etc.) This is complicated by the fact that the PLF was developed using round parachutes which did not have flare capability. As such the arms in a round canopy PLF were above the head, elbows bent and cushioning the head from the sides. This positioning essentially extends the arms along the long axis of the cylinder. With a square parachute our hands are down at our sides (hopefully) controlling the flare. During the rotation of a PLF, and particularly a rotation of 6.77 rpm, the hands and arms are thus exposed to quite a bit of trauma due to impact with the ground, more than once, as the roll proceeds.
What one hopes for is the best case scenario where the jumper is rolling from shoulder to shoulder across the ground. The worst case scenario would be tumbling head to toe across the ground. Unless one is a trained gymnast this would expose the head and neck to several impacts with the ground. A real life roll/tumble would probably fall somewhere between these two extremes. At a rotational speed of 6.77 rpm a PLF would risk head and neck injury. At a rotational speed of 6.77 rpm, even a perfect PLF has great risk to shoulders, arms, hips and legs.
There is a solution to this downwind landing possibility. It is easy to train and easily understood by newcomers and experienced skydivers alike. The simplest way to prepare for as many landing scenarios as possible is to assume the PLF position, fly the parachute and slide on one hip in the event of a forward motion, high speed landing. I teach our students to envision holding a dollar bill between their knees and squeezing a quarter between their glutes (“feet and knees, dollar and a quarter”). Actually doing so puts one into the PLF position.
When the person discovers a downwind mistake has been made, he/she should assume this PLF position. At touch down the jumper should try to absorb as much as possible of the downward impact with their feet but lean back in the harness. Under no circumstances should the person allow themselves to be thrown head first. The jumper wants to stay on their feet as long as possible, tending to sitting down. As the person sits down he/she wants to transition (during the squat) onto a hip. One does not want to impact directly onto the butt. The spine will tolerate torsion (bending) but very little compression. Sitting down directly onto the butt could cause spinal problems on its own (disks and vertebrae). Absorbing the actual butt touchdown with the hip will allow the spine to flex. Hitting a rock with the tailbone while sliding across the ground could be quite painful as a likely result is a cracked tailbone. Hitting a rock with the hip while sliding across the ground might possibly bruise the hip, an easier recovery than any spinal injury. The jumper should perform a baseball slide into second base ensuring that they remain sliding feet first. The person will get dirty. Done properly, one may see damage to the leg strap cover on the hip, but a dirty/torn jumpsuit and/or a dirty/scraped hip cover will be all that one sees.
During a proper landing (into the wind) and under reasonable conditions, if a person is in a PLF position (feet and knees, dollar and a quarter) accompanied by a flare anywhere near half–flight, the parachute will lay the student down in the first half of a PLF. This is all that is necessary with a properly sized square canopy. The “lay down” is a result of the fact that most students are not true into the wind at touchdown and thus the square parachute almost always imparts some degree of forward AND sideward motion to the student.
The occurrence of downwind landings is relatively rare. However, bear in mind that most of these are done by a jumper off student status (off radio assistance). This person has received very little if any formal instruction since the first solo landing class. So the technique must be simple to learn, retain and execute months after the initial training.
Since I began teaching this concept, decades ago, I have not had a single jumper injury related to downwind landings when my advice was put into practice.
My motivation for writing this article comes from the words that I hear when traveling to other drop zones and the words that I read in articles such as “Incident Reports”, “…you should have done a PLF”. This is not always the case, particularly with today’s parachutes. A PLF is no longer a panacea for all conditions. I also want to point out that, in my opinion, the instructor showing a first jump student a PLF accomplishes nothing at all. Having each student perform a PLF on the ground is no better. For a person to learn a PLF requires repetitions by the student, MANY repetitions, from an elevated platform.
A person or publication telling anyone that he/she should have done a PLF, which the person has never actually learned, is not accomplishing what the student needed and the knowledge that the publication is trying to disseminate.
More articles in this category:
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- The Abort Zone - by Brian Germain (Posted: 2010-09-01)
- An Inconvenient Truth Regarding PLF's - by Jack Guthrie (Posted: 2010-03-02)
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- Big Canopies in Turbulence - by Brian Germain (Posted: 2009-06-03)
- Canopy Control - A DVD Review - by Eduardo Guillen (Posted: 2009-04-06)
- Canopy Formation Part II - by Peter A. Pfalzgraf (Posted: 2009-02-16)
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