Starting with this issue, John Perryman will write a regular column in SAM. Combine the lifetime skiing skills of a national ski patroller, senior test examiner and amateur racer with the technical disciplines of a mechanical engineer and throw in the communicating skills that have made him a regular columnist in Ski and Ski Business Magazines and senior faculty member of the Skiing Mechanics & Managers Workshops . . . and you have, among other things, one of the most widely retained experts in skiing accident litigation.
I have been asked, on several occasions, to contribute my technical opinions regarding skiing safety at mountain resorts around the country. This is because many skiers seem to take their problems to court these days, and areas as well as shops and manufacturers have legitimate concerns about litigation problems. The question as to where prudent care leaves off and skier control responsibility begins is not only a legal and moral one, but it is also an engineering question in many instances. While I don’t profess to be a legal expert or a moralist, I can relate some engineering facts based on litigation experiences that may be helpful to all interested readers. This will be the essence of the columns I will submit to SAM.
To begin with, there have been cases involving the padding and/or lack thereof of obstacles such as lift towers, trees, telephone poles, etc. I say “obstacles” because, from an engineering point of view and as a skier, one can categorize many things as obstacles without singling one out. For instance, I have heard people refer to lift towers as “obstacles” but not include any natural object such as a tree, or exposed rock. To me, an obstacle is anything that may inhibit the continuance of a skier on a straight path through it. Thus, a buried rock or stump is not an obstacle if one’s skis can easily pass over it without undue disturbance. On the other hand, a barrier or buffer may itself become an obstacle.
Now, the “obstacle” becomes a “hazard” only if it is not foreseeable by a skier in time to control his path away from it. I leave it to the lawyers, through witnesses, to clarify the differences between an alleged “hazard” and an “obstacle.” That is precisely what many cases are about.
However, when an obstacle (whether a hazard or not) is struck by a skier at a given speed, we have an engineering problem. It is somewhat analagous to the automobile bumper that reduces repair costs. The bumper only helps if the car hits an object at a relatively low speed. Certainly a car hitting a brick wall at 30 mph is not going to be saved by a special bumper.
Objects on, or adjacent to ski trails are generally not hazardous to skiers who ski under control. But, what happens if one loses control and slides into an object? Aside from the fact that he may be skiing beyond his capability (a common male trend), the injury likely to be sustained when he hits the object is a function of his deceleration after impact. As has been said many times, “It’s not how fast you hit the wall, it’s how quickly you slow down that counts.” How fast you slow down is called deceleration.
Forces on the body are the result of a mass times an acceleration, or deceleration. Your mass is your weight divided by gravity, and your deceleration is the rate at which you go from full speed at impact to zero speed at rest after impact. Well, your mass is easy to calculate by simply dividing your body weight (with full equipment) by gravity, or 32.2 feet per second. For instance, a 160 lb. skier has a mass of 160/32.2 equals 4.97 Slugs.
If we assume that the 4.97 Slug skier hits a pad that is 4 inches thick, (.33 ft.), and which is attached to a rigid structure, then the deceleration is based on stopping the impact speed in 4 inches, the most the pad could possibly compress. Now, let us assume that the impact speed is 20 mph, or 29.3 feet per second. The deceleration from 29.3 fps to 0 fps in 4 inches compression is calculable and is 1288 ft. per sec. per sec. Thus, the 160 lb. skier hitting a 4 inch pad at 20 mph would develop a force of F equals 4.97 x 1288 equals 6400 pounds. If the man hit the obstacle with the 4 inch pad on one small part of his body, such as his head, it is easily understood how he could sustain a significant injury. If he distributed that force over, let’s say a one foot by one foot area of his body, such as the small of his back, the pressure against him at that location would be 6400 divided by 144 sq. in. equals 44.4 psi.
To put this into more meaningful terms, let’s assume that any padding on an obstacle should result in controlling the reaction forces on the body of a skier hitting it to no more than his own body weight. That is, we will permit a 1 G force to act on the skier in the hopes that injury may be averted. It is no guaranty, of course, but 1 G is a nominal force we can use for illustrative purposes.
The force on the body can be shown to be 0.4 times the body weight times the square of his speed divided by the compression, or thickness of the padding, where force is in pounds, weight is in pounds, speed is in mph and pad thickness is in inches.
Now we can tabulate the skier’s speed versus the padding thickness which limits the forces against the body to that of his own weight, 1 G, or double his weight, 2 G.
| Skier Speed (mph) | Pad Thickness — 1G Limit | Pad Thickness — 2G Limit |
|---|---|---|
| 0 | 0 | 0 |
| 2 | 1.6 | .8 |
| 4 | 6.4 | 3.2 |
| 6 | 14.4 | 7.2 |
| 8 | 25.6 | 12.8 |
| 10 | 40 | 20 |
| 20 | 160 | 80 |
| 30 | 360 | 180 |
| 40 | 640 | 320 |
| 50 | 1000 | 500 |
The conclusions one may reach are that obstacle padding in the order of 2 to 4 inches of thickness can only keep loads on the colliding body to no more than the body weight at speeds below 4 mph. Out-of-control skiers who are likely to hit obstacles on or off the trail are probably going in excess of 10 mph to be “out-of-control.” This means that padding in the order of 40 inches of compressible thickness would be required to control impact forces to no more than one G.
Injuries produced by collisions at very low speeds (under 5 mph) are not generally as traumatic as those resulting from high speed loss of control, as in the case of the 160 lb. skier above. So from an engineering viewpoint, the merits of padding on all obstacles as if they were hazards without it is highly questionable.
Of course, there are those who consider hay bales as a better answer than padding. But, hay bales are not extremely compressive or they wouldn’t be bales in the first place. They also have a tendency to retain water and freeze. So, a 24-inch hay bale certainly cannot have 24 inches of compressibility.
A logical solution to the dilemma is for the skier to ski under control commensurate with his ability. If he or she can’t handle the trees in the glade, stay away from it. The same is true of any other on or off trail obstacle. An obstacle that is fun for me is perhaps a problem for another skier. The ski area must cater to all kinds of skiers. He cannot “pave” an entire mountain for a few while the rest of his customers drive up to the more “challenging” area farther on.
